CODEWITHSUNDEEP
c++
Vaibhav
Vaibhav

Reputation: 215

Arena in Malloc Function

I am using malloc_stats() to print malloc related statistics in which I am finding "Arena 0" for some programs and "Arena 0 and Arena 1" for some other programs.

What do these arenas represent?

Upvotes: 6

Views: 5232

Answers (4)

Vaibhav
Vaibhav

Reputation: 215

The heap code resides inside the glibc component, and is packaged in the libc.so.x shared library. 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.

Upvotes: 6

MSalters
MSalters

Reputation: 180020

In certain malloc implementations, an "arena" is a pool of memory from which individual allocations are made. The algorithms to determine which arena is used will differ between implementations, so it's not possible for us to explain why you see a difference. One common factor is allocation size.

Upvotes: 2

Bertrand Marron
Bertrand Marron

Reputation: 22210

Everything is there: http://www.gnu.org/software/libc/manual/html_node/Statistics-of-Malloc.html

int arena
This is the total size of memory allocated with sbrk by malloc, in bytes.

Upvotes: 0

dimba
dimba

Reputation: 27631

See link text. It looks like heap is a collection of arenas ("sub-heaps") to handle memory allocation between several threads, thus reducing contention.

Upvotes: 2

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