malloc_trim(0) Releases Fastbins of Thread Arenas?

malloc_trim(0) states that it can only free memory from the top of the main arena heap, so what is going on here?

It can be called "outdated" or "incorrect" documentation. Glibc have no documentation of malloc_trim function; and Linux uses man pages from man-pages project. The man page of malloc_trim http://man7.org/linux/man-pages/man3/malloc_trim.3.html was written in 2012 by maintainer of man-pages as new. Probably he used some comments from glibc malloc/malloc.c source code http://code.metager.de/source/xref/gnu/glibc/malloc/malloc.c#675

676  malloc_trim(size_t pad);677678  If possible, gives memory back to the system (via negative679  arguments to sbrk) if there is unused memory at the `high' end of680  the malloc pool. You can call this after freeing large blocks of681  memory to potentially reduce the system-level memory requirements682  of a program. However, it cannot guarantee to reduce memory. Under683  some allocation patterns, some large free blocks of memory will be684  locked between two used chunks, so they cannot be given back to685  the system.686687  The `pad' argument to malloc_trim represents the amount of free688  trailing space to leave untrimmed. If this argument is zero,689  only the minimum amount of memory to maintain internal data690  structures will be left (one page or less). Non-zero arguments691  can be supplied to maintain enough trailing space to service692  future expected allocations without having to re-obtain memory693  from the system.694695  Malloc_trim returns 1 if it actually released any memory, else 0.696  On systems that do not support "negative sbrks", it will always697  return 0.

Actual implementation in glibc is __malloc_trim and it has code for iterating over arenas:

http://code.metager.de/source/xref/gnu/glibc/malloc/malloc.c#4552

4552 int4553 __malloc_trim (size_t s)4560  mstate ar_ptr = &main_arena;4561  do4562    {4563      (void) mutex_lock (&ar_ptr->mutex);4564      result |= mtrim (ar_ptr, s);4565      (void) mutex_unlock (&ar_ptr->mutex);45664567      ar_ptr = ar_ptr->next;4568    }4569  while (ar_ptr != &main_arena);

Every arena is trimmed using mtrim() (mTRIm()) function, which calls malloc_consolidate() to convert all free segments from fastbins (they are not coalesced at free as they are fast) to normal free chunks (which are coalesced with adjacent chunks)

4498  /* Ensure initialization/consolidation */4499  malloc_consolidate (av);4111  malloc_consolidate is a specialized version of free() that tears4112  down chunks held in fastbins. 1581   Fastbins1591    Chunks in fastbins keep their inuse bit set, so they cannot1592    be consolidated with other free chunks. malloc_consolidate1593    releases all chunks in fastbins and consolidates them with1594    other free chunks.

The problem is, when the worker thread is recreated, it creates a new arena/heap instead of reusing the previous one, such that the fastbins from previous arenas/heaps are never reused.

This is strange. By design, maximum number of arenas is limited in glibc malloc by cpu_core_count * 8 (for 64-bit platform); cpu_core_count * 2 (for 32-bit platform) or by environment variable MALLOC_ARENA_MAX / mallopt parameter M_ARENA_MAX.

You can limit count of arenas for your application; call malloc_trim() periodically or call to malloc() with "large" size (it will call malloc_consolidate) and then free() for it from your threads just before destroying:

3319 _int_malloc (mstate av, size_t bytes)3368  if ((unsigned long) (nb) <= (unsigned long) (get_max_fast ())) // fastbin allocation path3405  if (in_smallbin_range (nb)) // smallbin path; malloc_consolidate may be called3437     If this is a large request, consolidate fastbins before continuing.3438     While it might look excessive to kill all fastbins before3439     even seeing if there is space available, this avoids3440     fragmentation problems normally associated with fastbins.3441     Also, in practice, programs tend to have runs of either small or3442     large requests, but less often mixtures, so consolidation is not3443     invoked all that often in most programs. And the programs that3444     it is called frequently in otherwise tend to fragment.3445   */34463447  else3448    {3449      idx = largebin_index (nb);3450      if (have_fastchunks (av))3451        malloc_consolidate (av);3452    }

PS: there is comment in man page of malloc_trim https://github.com/mkerrisk/man-pages/commit/a15b0e60b297e29c825b7417582a33e6ca26bf65:

+.SH NOTES+This function only releases memory in the main arena.+.\" malloc/malloc.c::mTRIm():+.\" return result | (av == &main_arena ? sYSTRIm (pad, av) : 0);

Yes, there is check for main_arena, but it is at very end of malloc_trim implementation mTRIm() and it is just for calling sbrk() with negative offset. Since 2007 (glibc 2.9 and newer) there is another method to return memory back to the OS: madvise(MADV_DONTNEED) which is used in all arenas (and is not documented by author of glibc patch or author of man page). Consolidate is called for every arena. There is also code for trimming (munmapping) top chunk of mmap-ed heaps (heap_trim/shrink_heap called from slow path free()), but it is not called from malloc_trim.