Re: pthread_mutex_unlock (was Re: sched_yield() makes OpenLDAP slow)

[linux-os (Dick Johnson)]

On Thu, 26 Jan 2006, Howard Chu wrote:

> Nick Piggin wrote:
>> No, a spec is something that is written unambiguously, and generally
>> the wording leads me to believe they attempted to make it so (it
>> definitely isn't perfect - your mutex unlock example is one that could
>> be interpreted either way). If they failed to say something that should
>> be there then the spec needs to be corrected -- however in this case
>> I don't think you've shown what's missing.
>
> What is missing: sched_yield is a core threads function but it's defined
> using language that only has meaning in the presence of an optional
> feature (Process Scheduling.) Since the function must exist even in the
> absence of these options, the definition must be changed to use language
> that has meaning even in the absence of these options.
>
>> And actually your reading things into the spec that "they failed to say"
>> is wrong I believe (in the above sched_yield example).
>>
>>> interpretation would come from saying "hey, this spec is only defined
>>> for realtime behavior, WTF is it supposed to do for the default
>>> non-realtime case?" and getting a clear definition in the spec.
>>
>> However they do not omit to say that. They quite explicitly say that
>> SCHED_OTHER is considered a single priority class in relation to its
>> interactions with other realtime classes, and is otherwise free to
>> be implemented in any way.
>>
>> I can't see how you still have a problem with that...
>>
> I may be missing the obvious, but I couldn't find this explicit
> statement in the SUS docs. Also, it would not address the core
> complaint, that sched_yield's definition has no meaning when the Process
> Scheduling option doesn't exist.
>
> The current Open Group response to my objection reads:
> >>>
>
> Add to APPLICATION USAGE
> Since there may not be more than one thread runnable in a process
> a call to sched_yield() might not relinquish the processor at all.
> In a single threaded application this will always be case.
>
> <<<
> The interesting point one can draw from this response is that
> sched_yield is only intended to yield to other runnable threads within a
> single process. This response is also problematic, because restricting
> it to threads within a process makes it useless for Process Scheduling.
> E.g., the Process Scheduling language would imply that a single-threaded
> app could yield the processor to some other process. As such, I think
> this response is also flawed, and the definition still needs more work.
>
> --
>  -- Howard Chu
>  Chief Architect, Symas Corp.  http://www.symas.com
>  Director, Highland Sun        http://highlandsun.com/hyc
>  OpenLDAP Core Team            http://www.openldap.org/project/
>

To fix the current problem, you can substitute usleep(0); It will
give the CPU to somebody if it's computable, then give it back to
you. It seems to work in every case that sched_yield() has
mucked up (perhaps 20 to 30 here).

Cheers,
Dick Johnson
Penguin : Linux version 2.6.13.4 on an i686 machine (5589.66 BogoMips).
Warning : 98.36% of all statistics are fiction.
.

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