Related
I have really simple question. I have simple type variable (like int). I have one process, one writer thread, several "readonly" threads. How should I declare variable?
volatile int
std::atomic<int>
int
I expect that when "writer" thread modifies value all "reader" threads should see fresh value ASAP.
It's ok to read and write variable at the same time, but I expect reader to obtain either old value or new value, not some "intermediate" value.
I'm using single-CPU Xeon E5 v3 machine. I do not need to be portable, I run the code only on this server, i compile with -march=native -mtune=native. Performance is very important so I do not want to add "synchronization overhead" unless absolutely required.
If I just use int and one thread writes value is it possible that in another thread I do not see "fresh" value for a while?
Just use std::atomic.
Don't use volatile, and don't use it as it is; that doesn't give the necessary synchronisation. Modifying it in one thread and accessing it from another without synchronisation will give undefined behaviour.
If you have unsynchronized access to a variable where you have one or more writers then your program has undefined behavior. Some how you have to guarantee that while a write is happening no other write or read can happen. This is called synchronization. How you achieve this synchronization depends on the application.
For something like this where we have one writer and and several readers and are using a TriviallyCopyable datatype then a std::atomic<> will work. The atomic variable will make sure under the hood that only one thread can access the variable at the same time.
If you do not have a TriviallyCopyable type or you do not want to use a std::atomic You could also use a conventional std::mutex and a std::lock_guard to control access
{ // enter locking scope
std::lock_guard lock(mutx); // create lock guard which locks the mutex
some_variable = some_value; // do work
} // end scope lock is destroyed and mutx is released
An important thing to keep in mind with this approach is that you want to keep the // do work section as short as possible as while the mutex is locked no other thread can enter that section.
Another option would be to use a std::shared_timed_mutex(C++14) or std::shared_mutex(C++17) which will allow multiple readers to share the mutex but when you need to write you can still look the mutex and write the data.
You do not want to use volatile to control synchronization as jalf states in this answer:
For thread-safe accesses to shared data, we need a guarantee that:
the read/write actually happens (that the compiler won't just store the value in a register instead and defer updating main memory until
much later)
that no reordering takes place. Assume that we use a volatile variable as a flag to indicate whether or not some data is ready to be
read. In our code, we simply set the flag after preparing the data, so
all looks fine. But what if the instructions are reordered so the flag
is set first?
volatile does guarantee the first point. It also guarantees that no
reordering occurs between different volatile reads/writes. All
volatile memory accesses will occur in the order in which they're
specified. That is all we need for what volatile is intended for:
manipulating I/O registers or memory-mapped hardware, but it doesn't
help us in multithreaded code where the volatile object is often
only used to synchronize access to non-volatile data. Those accesses
can still be reordered relative to the volatile ones.
As always if you measure the performance and the performance is lacking then you can try a different solution but make sure to remeasure and compare after changing.
Lastly Herb Sutter has an excellent presentation he did at C++ and Beyond 2012 called Atomic Weapons that:
This is a two-part talk that covers the C++ memory model, how locks and atomics and fences interact and map to hardware, and more. Even though we’re talking about C++, much of this is also applicable to Java and .NET which have similar memory models, but not all the features of C++ (such as relaxed atomics).
I'll complete a little bit the previous answers.
As exposed previously, just using int or eventually volatile int is not enough for various reason (even with the memory order constraint of Intel processors.)
So, yes, you should use atomic types for that, but you need extra considerations: atomic types guarantee coherent access but if you have visibility concerns you need to specify memory barrier (memory order.)
Barriers will enforce visibility and coherency between threads, on Intel and most modern architectures, it will enforce cache synchronizations so updates are visible for every cores. The problem is that it may be expensive if you're not careful enough.
Possible memory order are:
relaxed: no special barrier, only coherent read/write are enforce;
sequential consistency: strongest possible constraint (the default);
acquire: enforce that no loads after the current one are reordered before and add the required barrier to ensure that released stores are visible;
consume: a simplified version of acquire that mostly only constraint reordering;
release: enforce that all stores before are complete before the current one and that memory writes are done and visible to loads performing an acquire barrier.
So, if you want to be sure that updates to the variable are visible to readers, you need to flag your store with a (at least) a release memory order and, on the reader side you need an acquire memory order (again, at least.) Otherwise, readers may not see the actual version of the integer (it'll see a coherent version at least, that is the old or the new one, but not an ugly mix of the two.)
Of course, the default behavior (full consistency) will also give you the correct behavior, but at the expense of a lot of synchronization. In short, each time you add a barrier it forces cache synchronization which is almost as expensive as several cache misses (and thus reads/writes in main memory.)
So, in short you should declare your int as atomic and use the following code for store and load:
// Your variable
std::atomic<int> v;
// Read
x = v.load(std::memory_order_acquire);
// Write
v.store(x, std::memory_order_release);
And just to complete, sometimes (and more often that you think) you don't really need the sequential consistency (even the partial release/acquire consistency) since visibility of updates are pretty relative. When dealing with concurrent operations, updates take place not when write is performed but when others see the change, reading the old value is probably not a problem !
I strongly recommend reading articles related to relativistic programming and RCU, here are some interesting links:
Relativistic Programming wiki: http://wiki.cs.pdx.edu/rp/
Structured Deferral: Synchronization via Procrastination: https://queue.acm.org/detail.cfm?id=2488549
Introduction to RCU Concepts: http://www.rdrop.com/~paulmck/RCU/RCU.LinuxCon.2013.10.22a.pdf
Let's start from int at int. In general, when used on single processor, single core machine this should be sufficient, assuming int size same or smaller than CPU word (like 32bit int on 32bit CPU). In this case, assuming correctly aligned address word addresses (high level language should assure this by default) the write/read operations should be atomic. This is guaranteed by Intel as stated in [1] . However, in C++ specification simultaneous reading and writing from different threads is undefined behaviour.
$1.10
6 Two expression evaluations conflict if one of them modifies a memory location (1.7) and the other one accesses or modifies the same memory location.
Now volatile. This keyword disables almost every optimization. This is the reason why it was used. For example, sometimes when optimizing the compiler can come to idea, that variable you only read in one thread is constant there and simply replace it with it's initial value. This solves such problems. However, it does not make access to variable atomic. Also, in most cases, it is simply unnecessary, because use of proper multithreading tools, like mutex or memory barrier, will achieve same effect as volatile on it's own, as described for instance in [2]
While this may be sufficient for most uses, there are other operations that are not guaranteed to be atomic. Like incrementation is a one. This is when std::atomic comes in. It has those operations defined, like here for mentioned incrementations in [3]. It is also well defined when reading and writing from different threads [4].
In addition, as stated in answers in [5], there exists a lot of other factors that may influence (negatively) atomicity of operations. From loosing cache coherency between multiple cores to some hardware details are the factors that may change how operations are performed.
To summarize, std::atomic is created to support accesses from different threads and it is highly recommended to use it when multithreading.
[1] http://www.intel.com/Assets/PDF/manual/253668.pdf see section 8.1.1.
[2] https://www.kernel.org/doc/Documentation/volatile-considered-harmful.txt
[3] http://en.cppreference.com/w/cpp/atomic/atomic/operator_arith
[4] http://en.cppreference.com/w/cpp/atomic/atomic
[5] Are C++ Reads and Writes of an int Atomic?
The other answers, which say to use atomic and not volatile, are correct when portability matters. If you’re asking this question, and it’s a good question, that’s the practical answer for you, not, “But, if the standard library doesn’t provide one, you can implement a lock-free, wait-free data structure yourself!” Nevertheless, if the standard library doesn’t provide one, you can implement a lock-free data structure yourself that works on a particular compiler and a particular architecture, provided that there’s only one writer. (Also, somebody has to implement those atomic primitives in the standard library.) If I’m wrong about this, I’m sure someone will kindly inform me.
If you absolutely need an algorithm guaranteed to be lock-free on all platforms, you might be able to build one with atomic_flag. If even that doesn’t suffice, and you need to roll your own data structure, you can do that.
Since there’s only one writer thread, your CPU might guarantee that certain operations on your data will still work atomically even if you just use normal accesses instead of locks or even compare-and-swaps. This is not safe according to the language standard, because C++ has to work on architectures where it isn’t, but it can be safe, for example, on an x86 CPU if you guarantee that the variable you’re updating fits into a single cache line that it doesn’t share with anything else, and you might be able to ensure this with nonstandard extensions such as __attribute__ (( aligned (x) )).
Similarly, your compiler might provide some guarantees: g++ in particular makes guarantees about how the compiler will not assume that the memory referenced by a volatile* hasn’t changed unless the current thread could have changed it. It will actually re-read the variable from memory each time you dereference it. That is in no way sufficient to ensure thread-safety, but it can be handy if another thread is updating the variable.
A real-world example might be: the writer thread maintains some kind of pointer (on its own cache line) which points to a consistent view of the data structure that will remain valid through all future updates. It updates its data with the RCU pattern, making sure to use a release operation (implemented in an architecture-specific way) after updating its copy of the data and before making the pointer to that data globally visible, so that any other thread that sees the updated pointer is guaranteed to see the updated data as well. The reader then makes a local copy (not volatile) of the current value of the pointer, getting a view of the data which will stay valid even after the writer thread updates again, and works with that. You want to use volatile on the single variable that notifies the readers of the updates, so they can see those updates even if the compiler “knows” your thread couldn’t have changed it. In this framework, the shared data just needs to be constant, and readers will use the RCU pattern. That’s one of the two ways I’ve seen volatile be useful in the real world (the other being when you don’t want to optimize out your timing loop).
There also needs to be some way, in this scheme, for the program to know when nobody’s using an old view of the data structure any longer. If that’s a count of readers, that count needs to be atomically modified in a single operation at the same time as the pointer is read (so getting the current view of the data structure involves an atomic CAS). Or this might be a periodic tick when all the threads are guaranteed to be done with the data they’re working with now. It might be a generational data structure where the writer rotates through pre-allocated buffers.
Also observe that a lot of things your program might do could implicitly serialize the threads: those atomic hardware instructions lock the processor bus and force other CPUs to wait, those memory fences could stall your threads, or your threads might be waiting in line to allocate memory from the heap.
Unfortunately it depends.
When a variable is read and written in multiple threads, there may be 2 failures.
1) tearing. Where half the data is pre-change and half the data is post change.
2) stale data. Where the data read has some older value.
int, volatile int and std:atomic all don't tear.
Stale data is a different issue. However, all values have existed, can be concieved as correct.
volatile. This tells the compiler neither to cache the data, nor to re-order operations around it. This improves the coherence between threads by ensuring all operations in a thread are either before the variable, at the variable, or after.
This means that
volatile int x;
int y;
y =5;
x = 7;
the instruction for x = 7 will be written after y = 5;
Unfortunately, the CPU is also capable of re-ordering operations. This can mean that another thread sees x ==7 before y =5
std::atomic x; would allow a guarantee that after seeing x==7, another thread would see y ==5. (Assuming other threads are not modifying y)
So all reads of int, volatile int, std::atomic<int> would show previous valid values of x. Using volatile and atomic increase the ordering of values.
See kernel.org barriers
I have simple type variable (like int).
I have one process, one writer thread, several "readonly" threads. How
should I declare variable?
volatile int
std::atomic
int
Use std::atomic with memory_order_relaxed for the store and load
It's quick, and from your description of your problem, safe. E.g.
void func_fast()
{
std::atomic<int> a;
a.store(1, std::memory_order_relaxed);
}
Compiles to:
func_fast():
movl $1, -24(%rsp)
ret
This assumes you don't need to guarantee that any other data is seen to be written before the integer is updated, and therefore the slower and more complicated synchronisation is unnecessary.
If you use the atomic naively like this:
void func_slow()
{
std::atomic<int> b;
b = 1;
}
You get an MFENCE instruction with no memory_order* specification which is massive slower (100 cycles more more vs just 1 or 2 for the bare MOV).
func_slow():
movl $1, -24(%rsp)
mfence
ret
See http://goo.gl/svPpUa
(Interestingly on Intel the use of memory_order_release and _acquire for this code results in the same assembly language. Intel guarantees that writes and reads happen in order when using the standard MOV instruction).
Here is my attempt at bounty:
- a. General answer already given above says 'use atomics'. This is correct answer. volatile is not enough.
-a. If you dislike the answer, and you are on Intel, and you have properly aligned int, and you love unportable solutions, you can do away with simple volatile, using Intel strong memory ordering gurantees.
TL;DR: Use std::atomic<int> with a mutex around it if you read multiple times.
Depends on how strong guarantees you want.
First volatile is a compiler hint and you shouldn't count on it doing something helpful.
If you use int you can suffer for memory aliasing. Say you have something like
struct {
int x;
bool q;
}
Depending on how this is aligned in memory and the exact implementation of CPU and memory bus it's possible that writing to q will actually overwrite x when the page is copied from the cpu cache back to ram. So unless you know how much to allocate around your int it's not guaranteed that your writer will be able to write without being overwritten by some other thread.
Also even if you write you depend on the processor for reloading the data to the cache of other cores so there's no guarantee that your other thread will see a new value.
std::atomic<int> basically guarantees that you will always allocate sufficient memory, properly aligned so that you don't suffer from aliasing. Depending on the memory order requested you will also disable a bunch of optimizations, like caching, so everything will run slightly slower.
This still doesn't grantee that if your read the var multiple times you'll get the value. The only way to do that is to put a mutex around it to block the writer from changing it.
Still better find a library that already solves the problem you have and it has already been tested by others to make sure it works well.
There are a couple of questions on this site asking whether using a volatile variable for atomic / multithreaded access is possible: See here, here, or here for example.
Now, the C(++) standard conformant answer is obviously no.
However, on Windows & Visual C++ compiler, the situation seems not so clear.
I have recently answered and cited the official MSDN docs on volatile
Microsoft Specific
Objects declared as volatile are (...)
A write to a volatile object (volatile write) has Release semantics;
a reference to a global or static object? that occurs before a write to
a volatile object in the instruction sequence will occur before that
volatile write in the compiled binary.
A read of a volatile object (volatile read) has Acquire semantics; a reference to a
global or static object? that occurs after a read of volatile memory in the
instruction
sequence will occur after that volatile read in the compiled binary.
This allows volatile objects to be used for memory locks and releases in multithreaded applications.
[emphasis mine]
Now, reading this, it would appear to me that a volatile variable will be treated by the MS compiler as std::atomic would be in the upcoming C++11 standard.
However, in a comment to my answer, user Hans Passant wrote "That MSDN article is very unfortunate, it is dead wrong. You can't implement a lock with volatile, not even with Microsoft's version. (...)"
Please note: The example given in the MSDN seems pretty fishy, as you cannot generally implement a lock without atomic exchange. (As also pointed out by Alex.) This still leaves the question wrt. to the validity of the other infos given in this MSDN article, especially for use cases like here and here.)
Additionally, there are the docs for The Interlocked* functions, especially InterlockedExchange with takes a volatile(!?) variable and does an atomic read+write. (Note that one question we have on SO -- When should InterlockedExchange be used? -- does not authoritatively answer whether this function is needed for a read-only or write-only atomic access.)
What's more, the volatile docs quoted above somehow allude to "global or static object", where I would have thought that "real" acquire/release semantics should apply to all values.
Back to the question
On Windows, with Visual C++ (2005 - 2010), will declaring a (32bit? int?) variable as volatile allow for atomic reads and writes to this variable -- or not?
What is especially important to me is that this should hold (or not) on Windows/VC++ independently of the processor or platform the program runs on. (That is, does it matter whether it's a WinXP/32bit or a Windows 2008R2/64bit running on Itanum2?)
Please back up your answer with verifiable information, links, test-cases!
Yes they are atomic on windows/vc++ (Assuming you meet alignment requirements etc or course)
However for a lock you would need an atomic test and set, or compare and exchange instuction or similar, not just an atomic update or read.
Otherwise there is no way to test the lock and claim it in one indivisable operation.
EDIT: As commented below, all aligned memory accesses on x86 of 32bit or below are atomic anyway. The key point is that volatile makes the memory accesses ordered. (Thanks for pointing this out in the comments)
As of Visual C++ 2005 volatile variables are atomic. But this only applies to this specific class of compilers and to x86/AMD64 platforms. PowerPC for example may reorder memory reads/writes and would require read/write barriers. I'm not familar what the semantics are for gcc-class compilers, but in any case using volatile for atomics is not very portable.
reference, see first remark "Microsoft Specific": http://msdn.microsoft.com/en-us/library/12a04hfd%28VS.80%29.aspx
A bit off-topic, but let's have a go anyway.
... there are the docs for The Interlocked* functions, especially InterlockedExchange which takes a volatile(!) variable ...
If you think about this:
void foo(int volatile*);
Does it say:
the argument must be a pointer to a volatile int, or
the argument may as well be a pointer to a volatile int?
The latter is the correct answer, since the function can be passed both pointers to volatile and non-volatile int's.
Hence, the fact that InterlockedExchangeX() has its argument volatile-qualified does not imply that it must operate on volatile integers only.
The point is probably to allow stuff like
singleton& get_instance()
{
static volatile singleton* instance;
static mutex instance_mutex;
if (!instance)
{
raii_lock lock(instance_mutex);
if (!instance) instance = new singleton;
}
return *instance;
}
which would break if instance was written to before initialization was complete. With MSVC semantics, you are guaranteed that as soon as you see instance != 0, the object has finished being initialized (which is not the case without proper barrier semantics, even with traditional volatile semantics).
This double-checked lock (anti-)pattern is quite common actually, and broken if you don't provide barrier semantics. However, if there are guarantees that accesses to volatile variables are acquire + release barriers, then it works.
Don't rely on such custom semantics of volatile though. I suspect this has been introduced not to break existing codebases. In any way, don't write locks according to MSDN example. It probably doesn't work (I doubt you can write a lock using just a barrier: you need atomic operations -- CAS, TAS, etc -- for that).
The only portable way to write the double-checked lock pattern is to use C++0x, which provides a suitable memory model, and explicit barriers.
under x86, these operations are guaranteed to be atomic without the need for LOCK based instructions such as Interlocked* (see intel's developer manuals 3A section 8.1):
basic memory operations will always be carried out atomically:
•
Reading or writing a byte
• Reading or writing a word aligned on a
16-bit boundary
• Reading or writing a doubleword aligned on a 32-bit boundary
The Pentium processor (and newer processors since) guarantees
that the following additional memory operations will always be carried
out atomically:
• Reading or writing a quadword aligned on a 64-bit
boundary
• 16-bit accesses to uncached memory locations that fit
within a 32-bit data bus
The P6 family processors (and newer
processors since) guarantee that the following additional memory
operation will always be carried out atomically:
• Unaligned 16-, 32-,
and 64-bit accesses to cached memory that fit within a cache line
This means volatile will only every serve to prevent caching and instruction reordering by the compiler (MSVC won't emit atomic operations for volatile variables, they need to be explicitly used).
As demonstrated in this answer I recently posted, I seem to be confused about the utility (or lack thereof) of volatile in multi-threaded programming contexts.
My understanding is this: any time a variable may be changed outside the flow of control of a piece of code accessing it, that variable should be declared to be volatile. Signal handlers, I/O registers, and variables modified by another thread all constitute such situations.
So, if you have a global int foo, and foo is read by one thread and set atomically by another thread (probably using an appropriate machine instruction), the reading thread sees this situation in the same way it sees a variable tweaked by a signal handler or modified by an external hardware condition and thus foo should be declared volatile (or, for multithreaded situations, accessed with memory-fenced load, which is probably a better a solution).
How and where am I wrong?
The problem with volatile in a multithreaded context is that it doesn't provide all the guarantees we need. It does have a few properties we need, but not all of them, so we can't rely on volatile alone.
However, the primitives we'd have to use for the remaining properties also provide the ones that volatile does, so it is effectively unnecessary.
For thread-safe accesses to shared data, we need a guarantee that:
the read/write actually happens (that the compiler won't just store the value in a register instead and defer updating main memory until much later)
that no reordering takes place. Assume that we use a volatile variable as a flag to indicate whether or not some data is ready to be read. In our code, we simply set the flag after preparing the data, so all looks fine. But what if the instructions are reordered so the flag is set first?
volatile does guarantee the first point. It also guarantees that no reordering occurs between different volatile reads/writes. All volatile memory accesses will occur in the order in which they're specified. That is all we need for what volatile is intended for: manipulating I/O registers or memory-mapped hardware, but it doesn't help us in multithreaded code where the volatile object is often only used to synchronize access to non-volatile data. Those accesses can still be reordered relative to the volatile ones.
The solution to preventing reordering is to use a memory barrier, which indicates both to the compiler and the CPU that no memory access may be reordered across this point. Placing such barriers around our volatile variable access ensures that even non-volatile accesses won't be reordered across the volatile one, allowing us to write thread-safe code.
However, memory barriers also ensure that all pending reads/writes are executed when the barrier is reached, so it effectively gives us everything we need by itself, making volatile unnecessary. We can just remove the volatile qualifier entirely.
Since C++11, atomic variables (std::atomic<T>) give us all of the relevant guarantees.
You might also consider this from the Linux Kernel Documentation.
C programmers have often taken volatile to mean that the variable
could be changed outside of the current thread of execution; as a
result, they are sometimes tempted to use it in kernel code when
shared data structures are being used. In other words, they have been
known to treat volatile types as a sort of easy atomic variable, which
they are not. The use of volatile in kernel code is almost never
correct; this document describes why.
The key point to understand with regard to volatile is that its
purpose is to suppress optimization, which is almost never what one
really wants to do. In the kernel, one must protect shared data
structures against unwanted concurrent access, which is very much a
different task. The process of protecting against unwanted
concurrency will also avoid almost all optimization-related problems
in a more efficient way.
Like volatile, the kernel primitives which make concurrent access to
data safe (spinlocks, mutexes, memory barriers, etc.) are designed to
prevent unwanted optimization. If they are being used properly, there
will be no need to use volatile as well. If volatile is still
necessary, there is almost certainly a bug in the code somewhere. In
properly-written kernel code, volatile can only serve to slow things
down.
Consider a typical block of kernel code:
spin_lock(&the_lock);
do_something_on(&shared_data);
do_something_else_with(&shared_data);
spin_unlock(&the_lock);
If all the code follows the locking rules, the value of shared_data
cannot change unexpectedly while the_lock is held. Any other code
which might want to play with that data will be waiting on the lock.
The spinlock primitives act as memory barriers - they are explicitly
written to do so - meaning that data accesses will not be optimized
across them. So the compiler might think it knows what will be in
shared_data, but the spin_lock() call, since it acts as a memory
barrier, will force it to forget anything it knows. There will be no
optimization problems with accesses to that data.
If shared_data were declared volatile, the locking would still be
necessary. But the compiler would also be prevented from optimizing
access to shared_data within the critical section, when we know that
nobody else can be working with it. While the lock is held,
shared_data is not volatile. When dealing with shared data, proper
locking makes volatile unnecessary - and potentially harmful.
The volatile storage class was originally meant for memory-mapped I/O
registers. Within the kernel, register accesses, too, should be
protected by locks, but one also does not want the compiler
"optimizing" register accesses within a critical section. But, within
the kernel, I/O memory accesses are always done through accessor
functions; accessing I/O memory directly through pointers is frowned
upon and does not work on all architectures. Those accessors are
written to prevent unwanted optimization, so, once again, volatile is
unnecessary.
Another situation where one might be tempted to use volatile is when
the processor is busy-waiting on the value of a variable. The right
way to perform a busy wait is:
while (my_variable != what_i_want)
cpu_relax();
The cpu_relax() call can lower CPU power consumption or yield to a
hyperthreaded twin processor; it also happens to serve as a memory
barrier, so, once again, volatile is unnecessary. Of course,
busy-waiting is generally an anti-social act to begin with.
There are still a few rare situations where volatile makes sense in
the kernel:
The above-mentioned accessor functions might use volatile on
architectures where direct I/O memory access does work. Essentially,
each accessor call becomes a little critical section on its own and
ensures that the access happens as expected by the programmer.
Inline assembly code which changes memory, but which has no other
visible side effects, risks being deleted by GCC. Adding the volatile
keyword to asm statements will prevent this removal.
The jiffies variable is special in that it can have a different value
every time it is referenced, but it can be read without any special
locking. So jiffies can be volatile, but the addition of other
variables of this type is strongly frowned upon. Jiffies is considered
to be a "stupid legacy" issue (Linus's words) in this regard; fixing it
would be more trouble than it is worth.
Pointers to data structures in coherent memory which might be modified
by I/O devices can, sometimes, legitimately be volatile. A ring buffer
used by a network adapter, where that adapter changes pointers to
indicate which descriptors have been processed, is an example of this
type of situation.
For most code, none of the above justifications for volatile apply.
As a result, the use of volatile is likely to be seen as a bug and
will bring additional scrutiny to the code. Developers who are
tempted to use volatile should take a step back and think about what
they are truly trying to accomplish.
I don't think you're wrong -- volatile is necessary to guarantee that thread A will see the value change, if the value is changed by something other than thread A. As I understand it, volatile is basically a way to tell the compiler "don't cache this variable in a register, instead be sure to always read/write it from RAM memory on every access".
The confusion is because volatile isn't sufficient for implementing a number of things. In particular, modern systems use multiple levels of caching, modern multi-core CPUs do some fancy optimizations at run-time, and modern compilers do some fancy optimizations at compile time, and these all can result in various side effects showing up in a different order from the order you would expect if you just looked at the source code.
So volatile is fine, as long as you keep in mind that the 'observed' changes in the volatile variable may not occur at the exact time you think they will. Specifically, don't try to use volatile variables as a way to synchronize or order operations across threads, because it won't work reliably.
Personally, my main (only?) use for the volatile flag is as a "pleaseGoAwayNow" boolean. If I have a worker thread that loops continuously, I'll have it check the volatile boolean on each iteration of the loop, and exit if the boolean is ever true. The main thread can then safely clean up the worker thread by setting the boolean to true, and then calling pthread_join() to wait until the worker thread is gone.
volatile is useful (albeit insufficient) for implementing the basic construct of a spinlock mutex, but once you have that (or something superior), you don't need another volatile.
The typical way of multithreaded programming is not to protect every shared variable at the machine level, but rather to introduce guard variables which guide program flow. Instead of volatile bool my_shared_flag; you should have
pthread_mutex_t flag_guard_mutex; // contains something volatile
bool my_shared_flag;
Not only does this encapsulate the "hard part," it's fundamentally necessary: C does not include atomic operations necessary to implement a mutex; it only has volatile to make extra guarantees about ordinary operations.
Now you have something like this:
pthread_mutex_lock( &flag_guard_mutex );
my_local_state = my_shared_flag; // critical section
pthread_mutex_unlock( &flag_guard_mutex );
pthread_mutex_lock( &flag_guard_mutex ); // may alter my_shared_flag
my_shared_flag = ! my_shared_flag; // critical section
pthread_mutex_unlock( &flag_guard_mutex );
my_shared_flag does not need to be volatile, despite being uncacheable, because
Another thread has access to it.
Meaning a reference to it must have been taken sometime (with the & operator).
(Or a reference was taken to a containing structure)
pthread_mutex_lock is a library function.
Meaning the compiler can't tell if pthread_mutex_lock somehow acquires that reference.
Meaning the compiler must assume that pthread_mutex_lock modifes the shared flag!
So the variable must be reloaded from memory. volatile, while meaningful in this context, is extraneous.
Your understanding really is wrong.
The property, that the volatile variables have, is "reads from and writes to this variable are part of perceivable behaviour of the program". That means this program works (given appropriate hardware):
int volatile* reg=IO_MAPPED_REGISTER_ADDRESS;
*reg=1; // turn the fuel on
*reg=2; // ignition
*reg=3; // release
int x=*reg; // fire missiles
The problem is, this is not the property we want from thread-safe anything.
For example, a thread-safe counter would be just (linux-kernel-like code, don't know the c++0x equivalent):
atomic_t counter;
...
atomic_inc(&counter);
This is atomic, without a memory barrier. You should add them if necessary. Adding volatile would probably not help, because it wouldn't relate the access to the nearby code (eg. to appending of an element to the list the counter is counting). Certainly, you don't need to see the counter incremented outside your program, and optimisations are still desirable, eg.
atomic_inc(&counter);
atomic_inc(&counter);
can still be optimised to
atomically {
counter+=2;
}
if the optimizer is smart enough (it doesn't change the semantics of the code).
For your data to be consistent in a concurrent environment you need two conditions to apply:
1) Atomicity i.e if I read or write some data to memory then that data gets read/written in one pass and cannot be interrupted or contended due to e.g a context switch
2) Consistency i.e the order of read/write ops must be seen to be the same between multiple concurrent environments - be that threads, machines etc
volatile fits neither of the above - or more particularly, the c or c++ standard as to how volatile should behave includes neither of the above.
It's even worse in practice as some compilers ( such as the intel Itanium compiler ) do attempt to implement some element of concurrent access safe behaviour ( i.e by ensuring memory fences ) however there is no consistency across compiler implementations and moreover the standard does not require this of the implementation in the first place.
Marking a variable as volatile will just mean that you are forcing the value to be flushed to and from memory each time which in many cases just slows down your code as you've basically blown your cache performance.
c# and java AFAIK do redress this by making volatile adhere to 1) and 2) however the same cannot be said for c/c++ compilers so basically do with it as you see fit.
For some more in depth ( though not unbiased ) discussion on the subject read this
The comp.programming.threads FAQ has a classic explanation by Dave Butenhof:
Q56: Why don't I need to declare shared variables VOLATILE?
I'm concerned, however, about cases where both the compiler and the
threads library fulfill their respective specifications. A conforming
C compiler can globally allocate some shared (nonvolatile) variable to
a register that gets saved and restored as the CPU gets passed from
thread to thread. Each thread will have it's own private value for
this shared variable, which is not what we want from a shared
variable.
In some sense this is true, if the compiler knows enough about the
respective scopes of the variable and the pthread_cond_wait (or
pthread_mutex_lock) functions. In practice, most compilers will not try
to keep register copies of global data across a call to an external
function, because it's too hard to know whether the routine might
somehow have access to the address of the data.
So yes, it's true that a compiler that conforms strictly (but very
aggressively) to ANSI C might not work with multiple threads without
volatile. But someone had better fix it. Because any SYSTEM (that is,
pragmatically, a combination of kernel, libraries, and C compiler) that
does not provide the POSIX memory coherency guarantees does not CONFORM
to the POSIX standard. Period. The system CANNOT require you to use
volatile on shared variables for correct behavior, because POSIX
requires only that the POSIX synchronization functions are necessary.
So if your program breaks because you didn't use volatile, that's a BUG.
It may not be a bug in C, or a bug in the threads library, or a bug in
the kernel. But it's a SYSTEM bug, and one or more of those components
will have to work to fix it.
You don't want to use volatile, because, on any system where it makes
any difference, it will be vastly more expensive than a proper
nonvolatile variable. (ANSI C requires "sequence points" for volatile
variables at each expression, whereas POSIX requires them only at
synchronization operations -- a compute-intensive threaded application
will see substantially more memory activity using volatile, and, after
all, it's the memory activity that really slows you down.)
/---[ Dave Butenhof ]-----------------------[ butenhof#zko.dec.com ]---\
| Digital Equipment Corporation 110 Spit Brook Rd ZKO2-3/Q18 |
| 603.881.2218, FAX 603.881.0120 Nashua NH 03062-2698 |
-----------------[ Better Living Through Concurrency ]----------------/
Mr Butenhof covers much of the same ground in this usenet post:
The use of "volatile" is not sufficient to ensure proper memory
visibility or synchronization between threads. The use of a mutex is
sufficient, and, except by resorting to various non-portable machine
code alternatives, (or more subtle implications of the POSIX memory
rules that are much more difficult to apply generally, as explained in
my previous post), a mutex is NECESSARY.
Therefore, as Bryan explained, the use of volatile accomplishes
nothing but to prevent the compiler from making useful and desirable
optimizations, providing no help whatsoever in making code "thread
safe". You're welcome, of course, to declare anything you want as
"volatile" -- it's a legal ANSI C storage attribute, after all. Just
don't expect it to solve any thread synchronization problems for you.
All that's equally applicable to C++.
This is all that "volatile" is doing:
"Hey compiler, this variable could change AT ANY MOMENT (on any clock tick) even if there are NO LOCAL INSTRUCTIONS acting on it. Do NOT cache this value in a register."
That is IT. It tells the compiler that your value is, well, volatile- this value may be altered at any moment by external logic (another thread, another process, the Kernel, etc.). It exists more or less solely to suppress compiler optimizations that will silently cache a value in a register that it is inherently unsafe to EVER cache.
You may encounter articles like "Dr. Dobbs" that pitch volatile as some panacea for multi-threaded programming. His approach isn't totally devoid of merit, but it has the fundamental flaw of making an object's users responsible for its thread-safety, which tends to have the same issues as other violations of encapsulation.
According to my old C standard, “What constitutes an access to an object that has volatile- qualified type is implementation-defined”. So C compiler writers could have choosen to have "volatile" mean "thread safe access in a multi-process environment". But they didn't.
Instead, the operations required to make a critical section thread safe in a multi-core multi-process shared memory environment were added as new implementation-defined features. And, freed from the requirement that "volatile" would provide atomic access and access ordering in a multi-process environment, the compiler writers prioritised code-reduction over historical implemention-dependant "volatile" semantics.
This means that things like "volatile" semaphores around critical code sections, which do not work on new hardware with new compilers, might once have worked with old compilers on old hardware, and old examples are sometimes not wrong, just old.
I'm writing a C++ app.
I have a class variable that more than one thread is writing to.
In C++, anything that can be modified without the compiler "realizing" that it's being changed needs to be marked volatile right? So if my code is multi threaded, and one thread may write to a var while another reads from it, do I need to mark the var volaltile?
[I don't have a race condition since I'm relying on writes to ints being atomic]
Thanks!
C++ hasn't yet any provision for multithreading. In practice, volatile doesn't do what you mean (it has been designed for memory adressed hardware and while the two issues are similar they are different enough that volatile doesn't do the right thing -- note that volatile has been used in other language for usages in mt contexts).
So if you want to write an object in one thread and read it in another, you'll have to use synchronization features your implementation needs when it needs them. For the one I know of, volatile play no role in that.
FYI, the next standard will take MT into account, and volatile will play no role in that. So that won't change. You'll just have standard defined conditions in which synchronization is needed and standard defined way of achieving them.
Yes, volatile is the absolute minimum you'll need. It ensures that the code generator won't generate code that stores the variable in a register and always performs reads and writes from/to memory. Most code generators can provide atomicity guarantees on variables that have the same size as the native CPU word, they'll ensure the memory address is aligned so that the variable cannot straddle a cache-line boundary.
That is however not a very strong contract on modern multi-core CPUs. Volatile does not promise that another thread that runs on another core can see updates to the variable. That requires a memory barrier, usually an instruction that flushes the CPU cache. If you don't provide a barrier, the thread will in effect keep running until such a flush occurs naturally. That will eventually happen, the thread scheduler is bound to provide one. That can take milliseconds.
Once you've taken care of details like this, you'll eventually have re-invented a condition variable (aka event) that isn't likely to be any faster than the one provided by a threading library. Or as well tested. Don't invent your own, threading is hard enough to get right, you don't need the FUD of not being sure that the very basic primitives are solid.
volatile instruct the compiler not to optimize upon "intuition" of a variable value or usage since it could be optimize "from the outside".
volatile won't provide any synchronization however and your assumption of writes to int being atomic are all but realistic!
I'd guess we'd need to see some usage to know if volatile is needed in your case (or check the behavior of your program) or more importantly if you see some sort of synchronization.
I think that volatile only really applies to reading, especially reading memory-mapped I/O registers.
It can be used to tell the compiler to not assume that once it has read from a memory location that the value won't change:
while (*p)
{
// ...
}
In the above code, if *p is not written to within the loop, the compiler might decide to move the read outside the loop, more like this:
cached_p=*p
while (cached_p)
{
// ...
}
If p is a pointer to a memory-mapped I/O port, you would want the first version where the port is checked before the loop is entered every time.
If p is a pointer to memory in a multi-threaded app, you're still not guaranteed that writes are atomic.
Without locking you may still get 'impossible' re-orderings done by the compiler or processor. And there's no guarantee that writes to ints are atomic.
It would be better to use proper locking.
Volatile will solve your problem, ie. it will guarantee consistency among all the caches of the system. However it will be inefficiency since it will update the variable in memory for each R or W access. You might concider using a memory barrier, only whenever it is needed, instead.
If you are working with or gcc/icc have look on sync built-ins : http://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/Atomic-Builtins.html
EDIT (mostly about pm100 comment):
I understand that my beliefs are not a reference so I found something to quote :)
The volatile keyword was devised to prevent compiler optimizations that might render code incorrect in the presence of certain asynchronous events. For example, if you declare a primitive variable as volatile, the compiler is not permitted to cache it in a register
From Dr Dobb's
More interesting :
Volatile fields are linearizable. Reading a volatile field is like acquiring a lock; the working memory is invalidated and the volatile field's current value is reread from memory. Writing a volatile field is like releasing a lock : the volatile field is immediately written back to memory.
(this is all about consistency, not about atomicity)
from The Art of multiprocessor programming, Maurice Herlihy & Nir Shavit
Lock contains memory synchronization code, if you don't lock, you must do something and using volatile keyword is probably the simplest thing you can do (even if it was designed for external devices with memory binded to the address space, it's not the point here)
I have a multi-R/W lock class that keeps the read, write and pending read , pending write counters. A mutex guards them from multiple threads.
My question is Do we still need the counters to be declared as volatile so that the compiler won't screw it up while doing the optimization.
Or does the compiler takes into account that the counters are guarded by the mutex.
I understand that the mutex is a run time mechanism to for synchronization and "volatile" keyword is a compile time indication to the compiler to do the right thing while doing the optimizations.
Regards,
-Jay.
There are 2 basically unrelated items here, that are always confused.
volatile
threads, locks, memory barriers, etc.
volatile is used to tell the compiler to produce code to read the variable from memory, not from a register. And to not reorder the code around. In general, not to optimize or take 'short-cuts'.
memory barriers (supplied by mutexes, locks, etc), as quoted from Herb Sutter in another answer, are for preventing the CPU from reordering read/write memory requests, regardless of how the compiler said to do it. ie don't optimize, don't take short cuts - at the CPU level.
Similar, but in fact very different things.
In your case, and in most cases of locking, the reason that volatile is NOT necessary, is because of function calls being made for the sake of locking. ie:
Normal function calls affecting optimizations:
external void library_func(); // from some external library
global int x;
int f()
{
x = 2;
library_func();
return x; // x is reloaded because it may have changed
}
unless the compiler can examine library_func() and determine that it doesn't touch x, it will re-read x on the return. This is even WITHOUT volatile.
Threading:
int f(SomeObject & obj)
{
int temp1;
int temp2;
int temp3;
int temp1 = obj.x;
lock(obj.mutex); // really should use RAII
temp2 = obj.x;
temp3 = obj.x;
unlock(obj.mutex);
return temp;
}
After reading obj.x for temp1, the compiler is going to re-read obj.x for temp2 - NOT because of the magic of locks - but because it is unsure whether lock() modified obj. You could probably set compiler flags to aggressively optimize (no-alias, etc) and thus not re-read x, but then a bunch of your code would probably start failing.
For temp3, the compiler (hopefully) won't re-read obj.x.
If for some reason obj.x could change between temp2 and temp3, then you would use volatile (and your locking would be broken/useless).
Lastly, if your lock()/unlock() functions were somehow inlined, maybe the compiler could evaluate the code and see that obj.x doesn't get changed. But I guarantee one of two things here:
- the inline code eventually calls some OS level lock function (thus preventing evaluation) or
- you call some asm memory barrier instructions (ie that are wrapped in inline functions like __InterlockedCompareExchange) that your compiler will recognize and thus avoid reordering.
EDIT: P.S. I forgot to mention - for pthreads stuff, some compilers are marked as "POSIX compliant" which means, among other things, that they will recognize the pthread_ functions and not do bad optimizations around them. ie even though the C++ standard doesn't mention threads yet, those compilers do (at least minimally).
So, short answer
you don't need volatile.
From Herb Sutter's article "Use Critical Sections (Preferably Locks) to Eliminate Races" (http://www.ddj.com/cpp/201804238):
So, for a reordering transformation to be valid, it must respect the program's critical sections by obeying the one key rule of critical sections: Code can't move out of a critical section. (It's always okay for code to move in.) We enforce this golden rule by requiring symmetric one-way fence semantics for the beginning and end of any critical section, illustrated by the arrows in Figure 1:
Entering a critical section is an acquire operation, or an implicit acquire fence: Code can never cross the fence upward, that is, move from an original location after the fence to execute before the fence. Code that appears before the fence in source code order, however, can happily cross the fence downward to execute later.
Exiting a critical section is a release operation, or an implicit release fence: This is just the inverse requirement that code can't cross the fence downward, only upward. It guarantees that any other thread that sees the final release write will also see all of the writes before it.
So for a compiler to produce correct code for a target platform, when a critical section is entered and exited (and the term critical section is used in it's generic sense, not necessarily in the Win32 sense of something protected by a CRITICAL_SECTION structure - the critical section can be protected by other synchronization objects) the correct acquire and release semantics must be followed. So you should not have to mark the shared variables as volatile as long as they are accessed only within protected critical sections.
volatile is used to inform the optimizer to always load the current value of the location, rather than load it into a register and assume that it won't change. This is most valuable when working with dual-ported memory locations or locations that can be updated real-time from sources external to the thread.
The mutex is a run-time OS mechanism that the compiler really doesn't know anything about - so the optimizer wouldn't take that into account. It will prevent more than one thread from accessing the counters at one time, but the values of those counters are still subject to change even while the mutex is in effect.
So, you're marking the vars volatile because they can be externally modified, and not because they're inside a mutex guard.
Keep them volatile.
While this may depend on the threading library you are using, my understanding is that any decent library will not require use of volatile.
In Pthreads, for example, use of a mutex will ensure that your data gets committed to memory correctly.
EDIT: I hereby endorse tony's answer as being better than my own.
You still need the "volatile" keyword.
The mutexes prevent the counters from concurrent access.
"volatile" tells the compiler to actually use the counter
instead of caching it into a CPU register (which would not
be updated by the concurrent thread).