git: hammer2 - Try to reduce no-activity stalls during complex flushes

[Matthew Dillon]

commit caf661fcf8eadefbd5a83af42a3fcb41ac93c805
Author: Matthew Dillon <dillon at apollo.backplane.com>
Date:   Thu Oct 19 23:00:44 2023 -0700

    hammer2 - Try to reduce no-activity stalls during complex flushes
    
    * Hammer2 keeps track of directory dependencies to maintain
      meta-data consistency at flush boundaries.  This can cause
      issues when heavy simultaneous front-end activity blows out
      dirty buffer limits and stalls in 'h2memw'.
    
      These front-end stalls are not supposed to be holding vnodes,
      but there do appear to be cases where the backend flusher is
      not able to immediately acquire some vnode locks during the flush.
      This causes the backend flush to skip that vnode but also
      introduce some static delays (rather than becoming cpu-bound).
      The backend flush ultimately restarts the flush and tries again.
    
      Situations can develop where the backend also stalls in a
      sequence of 'h2syndel' tsleep delays, resulting in zero
      cpu activity (frontend is stalled in 'h2memw'), and zero
      disk activity (backend is also stalled) for a short period of
      time.
    
    * This problem does not lead to permanent deadlocks, however.
      H2 is always able to recover.
    
    * Rearrange a 'h2syndel' tsleep() call in the backend flusher.
      Instead of tsleep on a per-failed-to-lock-vnode basis, we
      now finish flushing the remaining vnodes, then try to wakeup
      processes blocked in 'h2memw' on the frontend, and THEN sleep
      for a few ticks before restarting.
    
      This is an attempt to close the gap causing these periods of
      no-activity.

Summary of changes:
 sys/vfs/hammer2/hammer2_vfsops.c | 33 +++++++++++++++++++++++----------
 1 file changed, 23 insertions(+), 10 deletions(-)

http://gitweb.dragonflybsd.org/dragonfly.git/commitdiff/caf661fcf8eadefbd5a83af42a3fcb41ac93c805


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