Re: sched_yield() makes OpenLDAP slow

[Nick Piggin]

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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