I would like to use some C++ std compliant memory management in form of a class derived from std::allocator, but able to allocate chunks of memory and releasing & freeing them in smaller parts. I only found boost::pool, but this is not std compliant in the above sense. Is there anything more useful around or do I have to code this myself?
(Note that the std::allocator is often useless for allocating many small objects, i.e. when using a std::list.)
EDIT to clarify.
Say, I want to use a std::list of many small objects, then implementations of std::allocator which allocate each object using ::new cause significant overhead in run time (but also memory I think). It is much more efficient to allocate big chunks of objects and hand them out one by one. For this, I need a std-compliant allocator (doesn't need to be derived from std::allocator, but must implement the same concept) that can be used with any std library container and provides the required memory management, ideally allowing me to tell it how many objects I am likely to individually allocate.
GCC provides a few extension allocators as alternatives to std::allocator.
You haven't really said what your requirements are, so it's not possible to say if any of them would be suitable for you.
Edit following OP's edit:
Say, I want to use a std::list of many small objects, then implementations of std::allocator which allocate each object using ::new cause significant overhead in run time (but also memory I think).
Why also memory? The overhead of additional pointers in each std::list node will be present whether the memory comes from new or a custom allocator. Do you just mean the bookkeeping done by the heap to track all the small allcoations?
It is much more efficient to allocate big chunks of objects and hand them out one by one.
Have you measured it?
If you don't want the overhead of allocating lots of separate nodes are you sure std::list is the right container? What about vector or deque?
boost::stable_vector is still node-based but has less per-node memory overhead than std::list.
A boost::flat_map<int, T> isn't node-based and could be used instead of std::list<T>
Allocators are tricky and not always the best answer to (real or perceived) problems.
Related
I want to write a custom memory manager/allocator for learning. I'm tempted to have a master allocator that requests n bytes of ram from the heap (via new). This would be followed by several allocator... Adaptors? Each would interface with the master, requesting a block of memory to manage, these would be stack, linear, pool, slab allocators etc each managing allocations from their slice of the master pool allocator.
The problem I have is whether I should write custom allocator_traits to interface with these for the various STL containers; or if I should just ignore the adaptor idea and simply overload new and delete to use the custom pool allocator/manager, the master one.
What I'm interested in understanding is what tangible benefit I would gain from having separate allocators for STL containers? It seems like the default std::allocator calls new and delete as needed so if I overload those to instead request from my big custom memory pool, I'd get all the benefit without the kruft of custom std::allocator code.
Or is this a matter where certain types of allocator models, like using a stack allocator for a std::deque would work better than the default allocator? And if so, wouldn't the normal stl implementation already specialise the default allocator for the various container types, or otherwise be optimised in the calls to the default allocator?
If it matters at all, I'm using C++20 via GCC 10+
If you want to replace the global allocator, including in every library you are using, you don't have to use std::allocator.
std allocators let you do things like create temporary allocation pools. Suppose you have some data structures you can guarantee will not outlive a certain scope, and you know that (whatever is allocated) 90%+ will remain allocated to the end of the scope.
A relatively simple std allocator could hand outmemory, never recycle it, and clean it up at the end of the scope much faster than any global new or delete operator could.
Whenever you have special knowledge of the contents and lifetime patterns of a container, you could hand-tune an allocator for that specific container. The standard allocator cannot. Sometimes when you are willing to make compromises that the std containers are not, you can patch their behavior with a custom allocator.
std::deque cannot efficiently use a stack allocator, because it cannot presume you'll mainly use it as a stack. You might use it mainly a queue. A stack allocator when you use it mainly as a queue would be a disaster; but if you used it 90%+ as a stack, a stack allocator could be much faster at the cost of modest memory overhead (and if 99%+, a stack allocator that handles the exceptional case and cleans up the non-stack based operations).
Finally, allocators can permit you to distinguish between kinds of containers. You might want the memory for your document (persistent) state to be allocated in one region of memory, and your "scratch" non-persistent data to be allocated elsewhere.
And yes, using a std allocator is something you should consider not doing. Optimization is fungible, and tweaking low level memory allocation is something you can work on after you have made the rest of the system more efficient and functional. Only when you have something that works, isn't fast enough, and you have identified new/delete as a fundamental bottle neck you can't design around should you say "ok, time to replace allocation!"
Use Case: Security Software needs to shred memory on delete, 'cause it cannot afford to let sensitive data remain somewhere in the physical RAM, optionally accessible by later instantiated processes. The delete operators of standard run-times won't do this expensive operation. Overwriting the heap operators might lead to linker problems with libraries depending on the runtime versions of those.
Answering the two questions in-order:
Should I write custom allocator_traits to interface my allocators for the various STL containers?
Yes, for easy manipulations. Pretty soon in the implementation, situations such as controlling memory overlaps would arise. For example, while stress-testing the implementation at full capacity of individual allocators and figuring out an algorithm for re-allocation. In this regard, you would need to specialize the allocator_traits class for the allocators rather than implement its member types from scratch using new and delete operators.
The reason allocator_traits is used is because it facilitates easy handling of certain rules that need to be respected. Such rules occur all across memory management. [Refer here for three such rules during allocator construction.]
What tangible benefit I would gain from having separate allocators for STL containers?
Absolute control of how the master allocator assigns, re-assigns, copies, moves, and destructs memory (with added controls over quantifying/enhancing performance). Pretty cool, isn't it! If the default std allocator is used, you would loose this control and rely on a (albeit very good) default implementation of memory management.
I was looking into how custom containers are created, such as eastl's container and several other models and I see that they all use an "allocator", much like std::vector does with std::allocator. Which got me thinking, why do new implementations of a vector container use an allocator when they typically have an underlying memory management override for new and delete?
Being able to replace operator new() and operator delete() (and their array versions) at program level may be sufficient for small program. If you have programs consisting of many millions lines of code, running many different threads this isn't at all suitable. You often want or even need better control. To make the use of custom allocators effective, you also need to be able to allocate subobjects using the same objects as the outer allocator.
For example, consider the use of memory arena to be used when answering a request in some sort of a server which is probably running multiple threads. Getting memory from operator new() is probably fairly expensive because it involves allocating a lock and finding a suitable chunk of memory in a heap which is getting more and more fragmented. To avoid this, you just want to allocate a few chunks of memory (ideally just one but you may not know the needed size in advance) and put all objects there. An allocator can do this. To do so, you need to inform all entities allocating memory about this chunk of memory, i.e. you need to pass the allocator to everything possibly allocating memory. If you allocate e.g. a std::vector<std::string, A> the std::string objects should know about the allocator: just telling the std::vector<std::string, A> where and how to allocate memory isn't enough to avoid most memory allocations: you also need to tell it to the std::string (well, actually the std::basic_string<char, std::char_traits<char>, B> for a suitable allocator type B which is related to A).
That is, if you really mean to take control of your memory allocations, you definitely want to pass allocators to everything which allocates memory. Using replaced versions of the global memory management facilities may help you but it is fairly constrained. If you just want to write a custom container and memory allocation isn't much of your concern you don't necessarily need to bother. In big systems which are running for extensive periods of time memory allocation is one of the many concerns, however.
Allocators are classes that define memory models to be used by Standard Library containers.
Every Standard Library container has its own default allocator, However the users of the container can provide their own allocators over the default.
This is for additional flexibility.
It ensures that users can provide their own allocator which provides an alternate form of memory management(eg: Memory Pools) apart from the regular heap.
If you want to produce a standard-compatible container then the answer is of course yes... allocators are described in the standard so they are required.
In my personal experience however allocators are not that useful... therefore if you are developing a container for a specific use to overcome some structural limitation of the standard containers then I'd suggest to forget about allocators unless you really see a reason for using them.
If instead you are developing a container just because you think you can do better than the standard vector then my guess is that you are wasting your time. I don't like the allocator idea design (dropping on the type something that shouldn't be there) but luckily enough they can be just ignored. The only annoyance with allocators when you don't need them (i.e. always) is probably some more confusion in error messages.. that however are a mess anyway.
Has anyone seen an allocator that calls mlock(2) to prevent an STL container's contents from being swapped to disk?
There is afaik only one tricky part to writing such an allocator, namely minimizing the number of mlocked pages by clustering allocations to be mlocked. Ergo, one should probably start by modifying some shared memory allocator?
If I wanted to implement this (which is difficult to imagine, because I find it hard to believe it's the right solution to any problem :^), I'd try to do it by using a boost::pool_allocator (which provides a standard library compatible Allocator from a pool) and then - I forget the details; think it'll involve the RequestedSize template argument to singleton_pool and a user_allocator ? - there will be some way of having that sit on top of a pool which requests bigger chunks of memory by the mechanism of your choice which in your case would be allocation of mlocked pages.
According to C++ Primer 4th edition, page 755, there is a note saying:
Modern C++ programs ordinarily ought to use the allocator class
to allocate memory. It is safer and more flexible.
I don't quite understand this statement.
So far all the materials I read teach using new to allocate memory in C++.
An example of how vector class utilize allocator is shown in the book.
However, I cannot think of other scenarios.
Can anyone help to clarify this statement? and give me more examples?
When should I use allocator and when to use new? Thanks!
For general programming, yes you should use new and delete.
However, if you are writing a library, you should not!
I don't have your textbook, but I imagine it is discussing allocators in the context of writing library code.
Users of a library may want control over exactly what gets allocated from where. If all of the library's allocations went through new and delete, the user would have no way to have that fine-grained level of control.
All STL containers take an optional allocator template argument. The container will then use that allocator for its internal memory needs. By default, if you omit the allocator, it will use std::allocator which uses new and delete (specifically, ::operator new(size_t) and ::operator delete(void*)).
This way, the user of that container can control where memory gets allocated from if they desire.
Example of implementing a custom allocator for use with STL, and explanation: Improving Performance with Custom Pool Allocators for STL
Side Note: The STL approach to allocators is non-optimal in several ways. I recommend reading Towards a Better Allocator Model for a discussion of some of those issues.
Edit in 2019: The situation in C++ has improved since this answer was written. Stateful allocators are supported in C++11, and that support was improved in C++17. Some of the people involved in the "Towards a Better Allocator Model" were involved in those changes (eg: N2387), so that's nice (:
The two are not contradictory. Allocators are a PolicyPattern or StrategyPattern used by the STL libraries' container adapters to allocate chunks of memory for use with objects.
These allocators frequently optimize memory allocation by allowing
* ranges of elements to be allocated at once, and then initialized using a placement new
* items to be selected from secondary, specialized heaps depending on blocksize
One way or another, the end result will (almost always) be that the objects are allocated with new (placement or default)
Another vivid example would be how e.g. boost library implements smartpointers. Because smartpointers are very small (with little overhead) the allocation overhead might become a burden. It would make sense for the implementation to define a specialized allocator to do the allocations, so one may have efficient std::set<> of smartpointers, std::map<..., smartpointer> etc.
(Now I'm almost sure that boost actually optimizes storage for most smartpointers by avoiding any virtuals, therefore the vft, making the class a POD structure, with only the raw pointer as storage; some of the example will not apply. But then again, extrapolate to other kinds of smartpointer (refcounting smartpointers, pointers to member functions, pointers to member functions with instance reference etc. etc.))
Looking at vector, I realized that I have never used the second argument when creating vectors.
std::vector<int> myInts; // this is what I usually do
std::vector<int, ???> myOtherInts; // but is there a second argument there?
Looking at the link above it says that it is for:
Allocator object to be used instead of constructing a new one.
or, as for this one:
Allocator: Type of the allocator object used to define the storage allocation model. By default, the allocator class template for type T is used, which defines the simplest memory allocation model and is value-independent.
I guess it has to do with something with memory management. However, I am not sure how to use that.
Any pointers regarding this?
The default allocator, std::allocator<>, will handle all allocations made by std::vector<> (and others). It will make new allocations from the heap each time a new allocation is needed.
By providing a custom allocator, you can for instance allocate a big chunk of memory up front and then slice it up and hand out smaller pieces when separate allocations are needed. This will increase the allocation speed dramatically, which is good for example in games, at the cost of increased complexity as compared to the default allocator.
Some std type implementations have internal stack-based storage for small amounts of data. For instance, std::basic_string<> might use what is called a small string optimization, where only strings longer than some fixed length, say 16 characters (just an example!), gets an allocation from the allocator, otherwise an internal array is used.
Custom allocators are rarely used in general case. Some examples of where they can be useful:
Optimization for a specific pattern of allocations. For example, a concurrent program can pre-allocate a large chunk of memory via standard means at the beginning of task execution and then shave off pieces off it without blocking on the global heap mutex. When task is completed, entire memory block can be disposed of. To use this technique with STL containers, a custom allocator can be employed.
Embedded software, where a device has several ranges of memory with different properties (cached/noncached, fast/slow, volatile/persistent etc). A custom allocator can be used to place objects stored in an STL container in a specific memory region.
Maybe this will help: http://www.codeguru.com/cpp/cpp/cpp_mfc/stl/article.php/c4079
You may try google for: stl allocator.
Allocators (STL) help you to manage memory for your objects in vector class. you may use the custom allocator for different memory model( etc).
Hi you can find example of custom allocator http://www.codeproject.com/KB/cpp/allocator.aspx