What is the meaning of the term arena in relation to memory?

I'm reading a book on memory as a programming concept. In one of the later chapters, the author makes heavy use of the word arena, but never defines it. I've searched for the meaning of the word and how it relates to memory, and found nothing. Here are a few contexts in which the author uses the term:

> "The next example of serialization incorporates a strategy called > memory allocation from a specific arena." > > "...this is useful when dealing with memory leaks or when allocating > from a specific arena." > > "...if we want to deallocate the memory then we will deallocate the > whole arena."

The author uses the term over 100 times in one chapter. The only definition in the glossary is:

> allocation from arena - Technique of allocating an arena first and then > managing the allocation/deallocation within the arena by the program > itself (rather then by the process memory manager); used for > compaction and serialization of complex data structures and objects, > or for managing memory in safety-critical and /or fault-tolerant > systems.

Can anyone define arena for me given these contexts?

An arena is just a large, contiguous piece of memory that you allocate once and then use to manage memory manually by handing out parts of that memory. For example:

char * arena = malloc(HUGE_NUMBER);

unsigned int current = 0;

void * my_malloc(size_t n) { current += n; return arena + current - n; }

The point is that you get full control over how the memory allocation works. The only thing outside your control is the single library call for the initial allocation.

One popular use case is where each arena is only used to allocate memory blocks of one single, fixed size. In that case, you can write very efficient reclamation algorithms. Another use case is to have one arena per "task", and when you're done with the task, you can free the entire arena in one go and don't need to worry about tracking individual deallocations.

Each of those techniques is very specialized and generally only comes in handy if you know exactly what you're doing and why the normal library allocation is not good enough. Note that a good memory allocator will already do lots of magic itself, and you need a decent amount of evidence that that's not good enough before you start handling memory yourself.

http://www.linuxjournal.com/article/4681">I'll go with this one as a possible answer.

•Memory Arena (also known as break space)--the area where dynamic runtime memory is stored. The memory arena consists of the heap and unused memory. The heap is where all user-allocated memory is located. The heap grows up from a lower memory address to a higher memory address.

I'll add http://en.wikipedia.org/wiki/Region-based_memory_management">Wikipedia's synonyms: region, zone, arena, area, or memory context.

Basically it's memory you get from the OS, and divvy out, then can be freed all at once. The advantage to this is that repeated small calls to malloc() could be costly (Every memory allocation has a performance cost: the time it takes to allocate the memory in your program’s logical address space and the time it takes to assign that address space to physical memory) where as if you know a ball park you can get yourself a big chunk of memory then hand it out to your variables as/how you need it.

Think of it as a synonym for 'heap'. Ordinarily, your process only has one heap/arena, and all memory allocation happens from there.

But, sometimes you have a situation where you would to group a series of allocations together (e.g. for performance, to avoid fragmentation, etc.). In that case, it's better to allocate a new heap/arena, and then for any allocation, you can decide which heap to allocate from.

For example, you might have a particle system where lots of objects of the same size are being frequently allocated and deallocated. To avoid fragmenting memory, you could allocate each particle from a heap which is only used for those particles, and all other allocations would come from the default heap.

From http://www.bozemanpass.com/info/linux/malloc/Linux_Heap_Contention.html:

> The libc.so.x shared library contains the glibc component and the heap > code resides inside it. The current implementation of the heap uses > multiple independent sub-heaps called arenas. Each arena has its own > mutex for concurrency protection. Thus if there are sufficient arenas > within a process' heap, and a mechanism to distribute the threads' > heap accesses evenly between them, then the potential for contention > for the mutexes should be minimal. It turns out that this works well > for allocations. In malloc(), a test is made to see if the mutex for > current target arena for the current thread is free (trylock). If so > then the arena is now locked and the allocation proceeds. If the mutex > is busy then each remaining arena is tried in turn and used if the > mutex is not busy. In the event that no arena can be locked without > blocking, a fresh new arena is created. This arena by definition is > not already locked, so the allocation can now proceed without > blocking. Lastly, the ID of the arena last used by a thread is > retained in thread local storage, and subsequently used as the first > arena to try when malloc() is next called by that thread. Therefore > all calls to malloc() will proceed without blocking.

You can also refer to this link:

http://www.codeproject.com/Articles/44850/Arena-Allocator-DTOR-and-Embedded-Preallocated-Buf