Re: sched_yield() makes OpenLDAP slow

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Date:	Fri, 19 Aug 2005 16:34:57 +1000
To: Howard Chu <hyc@symas.com>

Hi Howard,

Thanks for joining the discussion. One request, if I may,
can you retain the CC list on posts please?

Howard Chu wrote:
>
>> AFAIKS, sched_yield should only really be used by realtime
>> applications that know exactly what they're doing.
>
>
> pthread_yield() was deleted from the POSIX threads drafts years ago.
> sched_yield() is the officially supported API, and OpenLDAP is using it
> for the documented purpose. Anyone who says "applications shouldn't be
> using sched_yield()" doesn't know what they're talking about.
>

Linux's SCHED_OTHER policy offers static priorities in the range [0..0].
I think anything else would be a bug, because from my reading of the
standards, a process with a higher static priority shall always preempt
a process with a lower priority.

And SCHED_OTHER simply doesn't work that way.

So sched_yield() from a SCHED_OTHER task is free to basically do anything
at all. Is that the kind of behaviour you had in mind?

>> It's really more a feature than a bug that it breaks so easily
>> because they should be really using futexes instead, which have much
>> better behaviour than any sched_yield ever could (they will directly
>> wake up another process waiting for the lock and avoid the thundering
>> herd for contended locks)
>
>
> You assume that spinlocks are the only reason a developer may want to
> yield the processor. This assumption is unfounded. Case in point - the
> primary backend in OpenLDAP uses a transactional database with
> page-level locking of its data structures to provide high levels of
> concurrency. It is the nature of such a system to encounter deadlocks
> over the normal course of operations. When a deadlock is detected, some
> thread must be chosen (by one of a variety of algorithms) to abort its
> transaction, in order to allow other operations to proceed to
> completion. In this situation, the chosen thread must get control of the
> CPU long enough to clean itself up, and then it must yield the CPU in
> order to allow any other competing threads to complete their
> transaction. The thread with the aborted transaction relinquishes all of
> its locks and then waits to get another shot at the CPU to try
> everything over again. Again, this is all fundamental to the nature of

You didn't explain why you can't use a mutex to do this. From
your brief description, it seems like a mutex might just do
the job nicely.

> transactional programming. If the 2.6 kernel makes this programming
> model unreasonably slow, then quite simply this kernel is not viable as
> a database platform.
>

Actually it should still be fast. It may yield excessive CPU to
other tasks (including those that are reniced). You didn't rely
on sched_yield providing some semantics about not doing such a
thing, did you?

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• Previous message: Jens Axboe: "Re: HDAPS, Need to park the head for real"
• In reply to: Howard Chu: "re: sched_yield() makes OpenLDAP slow"
• Next in thread: Chris Wedgwood: "Re: sched_yield() makes OpenLDAP slow"
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