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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?
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With arena allocation, new objects are allocated out of a large piece of preallocated memory called the arena. Objects can all be freed at once by discarding the entire arena, ideally without running destructors of any contained object (though an arena can still maintain a "destructor list" when required).
What is a Chunk? Glibc's malloc is chunk-oriented. It divides a large region of memory (a "heap") into chunks of various sizes. Each chunk includes meta-data about how big it is (via a size field in the chunk header), and thus where the adjacent chunks are.
When you free memory, malloc takes that memory block off the chain... and may or may not return that memory to the operating system. If it does, than accessing that memory will probably fail, as the OS will take away your permissions to access that location.
The malloc_stats() function prints (on standard error) statistics about memory allocated by malloc(3) and related functions. For each arena (allocation area), this function prints the total amount of memory allocated and the total number of bytes consumed by in-use allocations.
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.
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