mi_collect(false) does not always complete scheduled purges #1246

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1. mianyuntong commented on Mar 19, 2026

   

   mianyuntong

   on Mar 19, 2026

   Last edited by mianyuntong

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   I just found this problem on 3.1.5 yesterday, and I modified 'mi_arena_schedule_purge' to test, it seems right to me now.

   static void mi_arena_schedule_purge(mi_arena_t* arena, size_t slice_index, size_t slice_count) {
     const long delay = mi_arena_purge_delay();
     if (arena->memid.is_pinned || delay < 0 || _mi_preloading()) return;  // is purging allowed at all?
   
     mi_assert_internal(mi_bbitmap_is_clearN(arena->slices_free, slice_index, slice_count));
     if (delay == 0) {
       // purge directly
       mi_arena_purge(arena, slice_index, slice_count);
     }
     else {
       // schedule purge
       const mi_msecs_t expire = _mi_clock_now() + delay;
       mi_msecs_t expire0 = 0;
       if (mi_atomic_casi64_strong_acq_rel(&arena->purge_expire, &expire0, expire)) {
         // expiration was not yet set
         // maybe set the global arenas expire as well (if it wasn't set already)
         mi_assert_internal(expire0==0);
         mi_atomic_casi64_strong_acq_rel(&arena->subproc->purge_expire, &expire0, expire);
       }
       else {
         // already an expiration was set
         // MY MODIFICATION:
         // Arena already had an expiration set (expire0 holds its current value).
         // Still update subproc->purge_expire so try_purge can run: when subproc is 0
         // we must set it; when subproc is set, set to the earlier of the two.
         if (expire0 != 0) {
           mi_msecs_t subproc_exp = mi_atomic_loadi64_acquire(&arena->subproc->purge_expire);
           mi_msecs_t new_exp = (subproc_exp == 0) ? expire0
               : ((expire0 < subproc_exp) ? expire0 : subproc_exp);
           if (new_exp != subproc_exp) {
               mi_msecs_t expected = subproc_exp;
               mi_atomic_casi64_strong_acq_rel(&arena->subproc->purge_expire, &expected, new_exp);
           }
         }
       }
       mi_bitmap_setN(arena->slices_purge, slice_index, slice_count, NULL);
     }
   }
   

   👀React with 👀1Reacted by Daan
2. Daan (daanx)

   added 2 commits that reference this issue on Mar 26, 2026

   avoid resetting the global purge counter if arena purge expire was no…

   ...

   4e50cec

   merge fix for resetting subproc purge counter from dev (possible fix …

   ...

   11f676c
3. daanx commented on Mar 26, 2026

   

   Daan (daanx)

   on Mar 26, 2026

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   Thanks! I added a potential fix -- a bit different what you did as I now check the return value of mi_arena_purge to distinguish if a purge didn't happen because the arena->purge_expire time has not been reached yet. Let me know if this fixes the issue for you.

   👀React with 👀1Reacted by mianyuntong
4. 4jsquared commented on Mar 31, 2026

   

   4jsquared

   on Mar 31, 2026

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   Daan (@daanx) - tested those fixes (slightly adjusted for the version I have here), and that seems to fix the issue - thanks!
5. Daan (daanx)

   closed this as completedon Apr 15, 2026