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Re: Thread pool efficiency

Howard Chu wrote:
>Hallvard B Furuseth wrote:
>> Then I'll at least reduce pool_wrapper a bit.

Done.   Noticed a few other things underway:

- slapd does not set ltp_max_pending, we could drop it or add some
  slapd.conf option to set it.  Or make it ltp_max_tasks instead,
  then the test in _submit() can be moved inside the branch which
  mallocs a new task.  (The freelist size will enforce the limit.)

- It's slightly less work to maintain number of pending + active tasks
  than number of pending tasks, or for that matter a count which
  reduces the "paused or create new thread?" test to a single compare,
  but if _submit() grows smarter it may need to undo that.
  (That's the change I thought was best to delay for now.)

- Unless pool_purgekey() gets called from the main thread and expects
  to update other threads (I have no idea), we can kill thread_keys[]
  now and support multiple pools.  Is that useful?

  Replace thread_keys[] with: a circular list with prev/next pointers
  in the thread contexts, a pool->ltp_ctx_list which points to one of
  them, and a pool_context() call in pool_purgekey() to get at the list.

>>> (...) a single shared resource like this [ltp_mutex] is
>>> just a really bad idea.
>> True enough.  Still, slapd has a lot of mutexes.  Should perhaps check
>> if this one stands out before rearraning scheduling around it.
> Using back-null makes this fairly clear. There are no globally shared
> mutexes in the connection manager at all, so this is the only
> remaining culprit.

After grepping around a bit I'm not sure what you mean.  There are
several other singled shared mutexes which seem to be frequently used.
I'm far from sure of all of these, but here are some in slapd:

attr.c: attr_mutex, for attr_alloc() and attrs_alloc().
daemon.c: slap_daemon.sd_mutex via slapd_<set/clr>_<read/write>()
          and in slapd_daemon_task().
daemon.c: slapd_rq.rq_mutex?
operation.c: slap_op_mutex, for slap_op_time().
gmtime_mutex if !HAVE_GMTIME_R, per connection in back-monitor
at least.  slap_get_csn() does not #ifndef HAVE_GMTIME_R, should it?