[Bug libc/13137] New: malloc M_ARENA_MAX / MALLOC_ARENA_MAX limit not working correctly

["heuler at infosim dot net" <sourceware-bugzilla at sourceware dot org>]

http://sourceware.org/bugzilla/show_bug.cgi?id=13137

             Bug #: 13137
           Summary: malloc M_ARENA_MAX / MALLOC_ARENA_MAX limit not
                    working correctly
           Product: glibc
           Version: 2.12
            Status: NEW
          Severity: normal
          Priority: P2
         Component: libc
        AssignedTo: drepper.fsp@gmail.com
        ReportedBy: heuler@infosim.net
    Classification: Unclassified


We have an huge application which makes very heavy use of threads (typical 500
- 2000 threads running in parallel for task handling). In one setup around 5 GB
of physical memory are used. 
The same application on Windows (using MS VC++) uses only 60% with exactly the
same configuration. The reason for that is the glibc malloc() implementation
using multiple arenas to prevent thread contention. Using malloc_info() we
found out that 2 - 3 GB of memory are unused (free) in the memory management.
As that free memory is distributed between the areans it can't be reclaimed.
The application uses 64 arenas (maximum is 8 per core? we have 8 core test
system) and our threads  depending on the task need sometimes a lot of memory
(several 100MB). Since the thread if it has bad luck gets uses only one arena
it can't use the free memory of the others.

Our solution was to try to limit the number of arenas in the malloc
implementation. For that the M_ARENA_MAX / MALLOC_ARENA_MAX parameters
(environment or mallopt() are available. According to the documentation they
define the maximum number of arenas used.
In our tests we configured for only 1 arena. But the application tends to use
around 10 arenas which is more than the specified maximum. It is of course less
than the 64 arenas if the parameters not sed and works also better in terms of
free memory fragmentation.
But the limit is exceed which seems to be a bug since the documentation also in
Ulrich Dreppers blog defines that as a hard limit.

We tested e.g. on RHEL 6 with latest updates.

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