I have a matrix, say A, of the Eigen library and I want to fill its columns from multiple threads, i.e. the threads call A.col(j) = xj. Each column j will be written exactly once and only by one thread. So no two threads ever write to the same column but two different columns could be written at the same time.
I found a short paragraph in Eigen's docs about multi-threaded code saying Eigen::initParallel() should be called before using Eigen in threaded environments. However, it does not make a statement about using Eigen in a scenario as above.
Is Eigen safe to use in the way described above? Thank you in advance!
There way you describe is safe, regardless of using Eigen or not, because no two threads ever write to the same memory location (i.e., the columns in your array.) There are no race conditions because no two threads access that same memory location if you implement it as you say you will. This will not be thread safe if there is an attempt to write to the same memory location.
One thread could even read or write to its assigned column many times without worrying about thread safety, and even another row on the same column, should you like — but just so long as nothing else is accessing that column at the same time.
I’m not sure it’s best practice or not, or if it’s better to use smart pointers or not. Either way, the process you describe is thread safe.
Related
I have a situation where I have a legacy multi-threaded application I'm trying to move to a linux platform and convert into C++.
I have a fixed size array of integers:
int R[5000];
And I perform a lot of operations like:
R[5] = (R[10] + R[20]) / 50;
R[5]++;
I have one Foreground task that mostly reads the values....but on occasion can update one. And then I have a background worker that is updating the values constantly.
I need to make this structure thread safe.
I would rather only update the value if the value has actually changed. The worker is constantly collecting data and doing calculation and storing the data whether it changes or not.
So should I create a custom class MyInt which has the structure and then include an array of mutexes to lock for updating/reading each value and then overload the [], =, ++, +=, -=, etc? Or should I try to implement anatomic integer array?
Any suggestions as to what that would look like? I'd like to try and keep the above notation for doing the updates...but I get that it might not be possible.
Thanks,
WB
The first thing to do is make the program work reliably, and the easiest way to do that is to have a Mutex that is used to control access to the entire array. That is, whenever either thread needs to read or write to anything in the array, it should do:
the_mutex.lock();
// do all the array-reads, calculations, and array-writes it needs to do
the_mutex.unlock();
... then test your program and see if it still runs fast enough for your needs. If so, you're done; that's all you need to do.
If you find that the program isn't fast enough due to contention on the mutex, you can start trying optimizations to make things faster. For example, if you know that your threads' operations will only need to work on local segments of the array at one time, you could create multiple mutexes, and assign different subsets of the array to each mutex (e.g. mutex #1 is used to serialize access to the first 100 array items, mutex #2 for the second 100 array items, etc). That will greatly decrease the chances of one thread having to wait for the other thread to release a mutex before it can continue.
If things still aren't fast enough for you, you could then look in to having two different arrays, one for each thread, and occasionally copying from one array to the other. That way each thread could safely access its own private array without any serialization needed. The copying operation would need to be handled carefully, probably using some sort of inter-thread message-passing protocol.
So I was trying to optimize an array operation in Julia, but noticed that I was getting a rather large error on my matrix occasionally. I also noticed that there existed the possibility of concurrently writing to the same index of a SharedArray in Julia. I was wondering if Julia can safely handle it. If not, how may I able able to handle it?
Here is a basic example of my issue
for a list of arbitrary x,y indexes in array J
j[x,y] += some_value
end
Can Julia handle this case or, like C, will there exist the possibility of overwriting the data. Are their atomic operations in Julia to compensate ffor this?
Shared arrays deliberately have no locking, since locking can be expensive. The easiest approach is to assign non-overlapping work to different processes. However, you might search to see whether someone has written a locking library, or have a go at it yourself: https://en.wikipedia.org/wiki/Mutual_exclusion
I have two threads running. They share an array. One of the threads adds new elements to the array (and removes them) and the other uses this array (read operations only).
Is it necessary for me to lock the array before I add/remove to/from it or read from it?
Further details:
I will need to keep iterating over the entire array in the other thread. No write operations over there as previously mentioned. "Just scanning something like a fixed-size circular buffer"
The easy thing to do in such cases is to use a lock. However locks can be very slow. I did not want to use locks if their use can be avoided. Also, as it came out from the discussions, it might not be necessary (it actually isn't) to lock all operations on the array. Just locking the management of an iterator for the array (count variable that will be used by the other thread) is enough
I don't think the question is "too broad". If it still comes out to be so, please let me know. I know the question isn't perfect. I had to combine at least 3 answers in order to be able to solve the question - which suggests most people were not able to fully understand all the issues and were forced to do some guess work. But most of it came out through the comments which I have tried to incorporate in the question. The answers helped me solve my problem quite objectively and I think the answers provided here are quite a helpful resource for someone starting out with multithreading.
If two threads perform an operation on the same memory location, and at least one operation is a write operation, you have a so-called data race. According to C11 and C++11, the behaviour of programs with data races is undefined.
So, you have to use some kind of synchronization mechanism, for example:
std::atomic
std::mutex
If you are writing and reading from the same location from multiple threads you will need to to perform locking or use atomics. We can see this by looking at the C11 draft standard(The C++11 standard looks almost identical, the equivalent section would be 1.10) says the following in section 5.1.2.4 Multi-threaded executions and data races:
Two expression evaluations conflict if one of them modifies a memory
location and the other one reads or modifies the same memory location.
and:
The execution of a program contains a data race if it contains two
conflicting actions in different threads, at least one of which is not
atomic, and neither happens before the other. Any such data race
results in undefined behavior.
and:
Compiler transformations that introduce assignments to a potentially
shared memory location that would not be modified by the abstract
machine are generally precluded by this standard, since such an
assignment might overwrite another assignment by a different thread in
cases in which an abstract machine execution would not have
encountered a data race. This includes implementations of data member
assignment that overwrite adjacent members in separate memory
locations. We also generally preclude reordering of atomic loads in
cases in which the atomics in question may alias, since this may
violate the "visible sequence" rules.
If you were just adding data to the array then in the C++ world a std::atomic index would be sufficient since you can add more elements and then atomically increment the index. But since you want to grow and shrink the array then you will need to use a mutex, in the C++ world std::lock_guard would be a typical choice.
To answer your question: maybe.
Simply put, the way that the question is framed doesn't provide enough information about whether or not a lock is required.
In most standard use cases, the answer would be yes. And most of the answers here are covering that case pretty well.
I'll cover the other case.
When would you not need a lock given the information you have provided?
There are some other questions here that would help better define whether you need a lock, whether you can use a lock-free synchronization method, or whether or not you can get away with no explicit synchronization.
Will writing data ever be non-atomic? Meaning, will writing data ever result in "torn data"? If your data is a single 32 bit value on an x86 system, and your data is aligned, then you would have a case where writing your data is already atomic. It's safe to assume that if your data is of any size larger than the size of a pointer (4 bytes on x86, 8 on x64), then your writes cannot be atomic without a lock.
Will the size of your array ever change in a way that requires reallocation? If your reader is walking through your data, will the data suddenly be "gone" (memory has been "delete"d)? Unless your reader takes this into account (unlikely), you'll need a lock if reallocation is possible.
When you write data to your array, is it ok if the reader "sees" old data?
If your data can be written atomically, your array won't suddenly not be there, and it's ok for the reader to see old data... then you won't need a lock. Even with those conditions being met, it would be appropriate to use the built in atomic functions for reading and storing. But, that's a case where you wouldn't need a lock :)
Probably safest to use a lock since you were unsure enough to ask this question. But, if you want to play around with the edge case of where you don't need a lock... there you go :)
One of the threads adds new elements to the array [...] and the other [reads] this array
In order to add and remove elements to/from an array, you will need an index that specifies the last place of the array where the valid data is stored. Such index is necessary, because arrays cannot be resized without potential reallocation (which is a different story altogether). You may also need a second index to mark the initial location from which the reading is allowed.
If you have an index or two like this, and assuming that you never re-allocate the array, it is not necessary to lock when you write to the array itself, as long as you lock the writes of valid indexes.
int lastValid = 0;
int shared[MAX];
...
int count = toAddCount;
// Add the new data
for (int i = lastValid ; count != 0 ; count--, i++) {
shared[i] = new_data(...);
}
// Lock a mutex before modifying lastValid
// You need to use the same mutex to protect the read of lastValid variable
lock_mutex(lastValid_mutex);
lastValid += toAddCount;
unlock_mutex(lastValid_mutex);
The reason this works is that when you perform writes to shared[] outside the locked region, the reader does not "look" past the lastValid index. Once the writing is complete, you lock the mutex, which normally causes a flush of the CPU cache, so the writes to shared[] would be complete before the reader is allowed to see the data.
Lock? No. But you do need some synchronization mechanism.
What you're describing sounds an awful like a "SPSC" (Single Producer Single Consumer) queue, of which there are tons of lockfree implementations out there including one in the Boost.Lockfree
The general way these work is that underneath the covers you have a circular buffer containing your objects and an index. The writer knows the last index it wrote to, and if it needs to write new data it (1) writes to the next slot, (2) updates the index by setting the index to the previous slot + 1, and then (3) signals the reader. The reader then reads until it hits an index that doesn't contain the index it expects and waits for the next signal. Deletes are implicit since new items in the buffer overwrite previous ones.
You need a way to atomically update the index, which is provided by atomic<> and has direct hardware support. You need a way for a writer to signal the reader. You also might need memory fences depending on the platform s.t. (1-3) occur in order. You don't need anything as heavy as a lock.
"Classical" POSIX would indeed need a lock for such a situation, but this is overkill. You just have to ensure that the reads and writes are atomic. C and C++ have that in the language since their 2011 versions of their standards. Compilers start to implement it, at least the latest versions of Clang and GCC have it.
It depends. One situation where it could be bad is if you are removing an item in one thread then reading the last item by its index in your read thread. That read thread would throw an OOB error.
As far as I know, this is exactly the usecase for a lock. Two threads which access one array concurrently must ensure that one thread is ready with its work.
Thread B might read unfinished data if thread A did not finish work.
If it's a fixed-size array, and you don't need to communicate anything extra like indices written/updated, then you can avoid mutual exclusion with the caveat that the reader may see:
no updates at all
If your memory ordering is relaxed enough that this happens, you need a store fence in the writer and a load fence in the consumer to fix it
partial writes
if the stored type is not atomic on your platform (int generally should be)
or your values are un-aligned, and especially if they may span cache lines
This is all dependent on your platform though - hardware, OS and compiler can all affect it. You haven't told us what they are.
The portable C++11 solution is to use an array of atomic<int>. You still need to decide what memory ordering constraints you require, and what that means for correctness and performance on your platform.
If you use e.g. vector for your array (so that it can dynamically grow), then reallocation may occur during the writes, you lose.
If you use data entries larger than is always written and read atomically (virtually any complex data type), you lose.
If the compiler / optimizer decides to keep certain things in registers (such as the counter holding the number of valid entries in the array) during some operations, you lose.
Or even if the compiler / optimizer decides to switch order of execution for your array element assignments and counter increments/decrements, you lose.
So you certianly do need some sort of synchronization. What is the best way to do so (for example it may be worth while to lock only parts of the array), depends on your specifics (how often and in what pattern do the threads access the array).
I'm working on some code that has a global array that can be accessed by two threads for reading writing purposes.
There will be no batch processing where a range of indexes are read or written, so I'm trying to figure out if I should lock the entire array or only the array index I am currently using.
The easiest solution would be to consider the array a CS and put a big fat lock around it, but can I avoid this and just lock an index?
Cheers.
Locking one index implies that you can keep track of which thread is accessing what part of the array. Keeping track of this information, which is shared between the reading and the writing thread, implies that you have one lock around this information. So, you still end up with a global lock.
In this situation, I think that the most efficient approaches are:
- using a reader/writer lock
- or dividing the big array into a few subsets, each subset using a distinct lock.
If this is C++ i suggest you to use STL containers. std::vector or something else which suits your job. They are fast, easy to use, no memory leaks.
If you want to do it all by your self, then of course one method will be to use a single mutex ( which is bad ).
or you can use some reader writer thingy for the whole array.
I think its not feasible to make each element of an array thread safe with its own lock!! that would eat your memory. Check the link and there are 3 solutions with different out comes. Test them out and use the best for your case. ( don't think like "ok i think my program needs the readers preference algorithm". try using it in your system and decide. because we really cant assume such things sometimes )
There is no way of knowing what will be optimal unless you profile under realistic running conditions. I would suggest implementing an array-like class, where you can lock a varying number of elements in groups. Then you fine-tune the size of these groups.
Another option would be to enqueue all read/write operations using an active object. This would make all access sequential, and means you could use a non-concurrent array type to store the data. It would require some sort of concurrent queue data structure under the hood.
this is a beginner question i guess but i couldn't find the answer to this particular question:
i have a standard (c++) vector v of size 10 and type int.
is it safe to have a thread alter all the even positions (v.at(0) = x; v.at(2) = y; etc.)
and another thread alter all the values for the odd positions (v.at(1) = a; v.at(3)=b;etc.) at the same time?
so, no changing of size, no push_back() etc. during the lifetime of these 2 threads.
if it is not safe, would the use of an array be a better way to do this?
thanks for your help.
vector does not provide any thread-safety guarantees, so technically the answer would be no.
In practice, you should be able to get away with it... until the day that someone (possibly you) makes a small change in one corner of the program and all hell breaks loose. I wouldn't feel comfortable doing this in any non-trivial program.
From MSDN: Thread Safety in the Standard C++ Library
For reads to the same object, the object is thread safe for reading:
From one thread at a time when no writers on other threads.
From many threads at a time when no writers on other threads.
For writes to the same object, the object is thread safe for writing from one thread when no readers on other threads
For reads to different objects of the same class, the object is thread safe for reading:
From one thread at a time.
From one thread at a time when no writers on other threads.
From many threads at a time.
From many threads at a time when no writers on other threads.
For writes to different objects of the same class, the object is thread safe for writing:
From one thread when no readers on other threads.
From many threads.
So from the above, Theorotically, NO, it won't be threadsafe.
Theoretically: No.
Practically: Yes (According how all well known STLs are implemented)
It's machine-dependent. If you have a vector<char> the processor might not be able to load v[i] and v[i+1] in separate words. You may have cache consistency problems.
Both the compiler and processor might re-order instructions, which can break your program even if the above doesn't apply. This is why C++0x has a memory model.
The scenario you describe will be tread-safe - you effectively manipulate individual elements of a fixed size array (since vector size will not change during those operations), so you don't need any additional synchronization in the first place unless you manipulate any element from more than one thread (which is not you case).