Re: sched_yield() makes OpenLDAP slow

[Florian Weimer <fw@deneb.enyo.de>]

* Howard Chu:

>>> Has anybody contacted the Sleepycat people with a description of
>>> the problem yet?

>> Berkeley DB does not call sched_yield, but OpenLDAP does in some
>> wrapper code around the Berkeley DB backend.

> That's not the complete story. BerkeleyDB provides a 
> db_env_set_func_yield() hook to tell it what yield function it should 
> use when its internal locking routines need such a function. If you 
> don't set a specific hook, it just uses sleep(). The OpenLDAP backend 
> will invoke this hook during some (not necessarily all) init sequences, 
> to tell it to use the thread yield function that we selected in autoconf.

And this helps to increase performance substantially?

> Note that (on systems that support inter-process mutexes) a BerkeleyDB 
> database environment may be used by multiple processes concurrently.

Yes, I know this, and I haven't experienced that much trouble with
deadlocks.  Maybe the way you structure and access the database
environment can be optimized for deadlock avoidance?

> As such, the yield function that is provided must work both for
> threads within a single process (PTHREAD_SCOPE_PROCESS) as well as
> between processes (PTHREAD_SCOPE_SYSTEM).

If I understand you correctly, what you really need is a syscall along
the lines "don't run me again until all threads T that share property
X have run, where the Ts aren't necessarily in the same process".  The
kernel is psychic, it can't really know which processes to schedule to
satisfy such a requirement.  I don't even think "has joined the
Berkeley DB environment" is the desired property, but something like
"is part of this cycle in the wait-for graph" or something similar.

I would have to check the Berkeley DB internals in order to tell what
is feasible to implement.  This code shouldn't be on the fast path, so
some kernel-based synchronization is probably sufficient.