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Subject: Re: pthread_mutex_unlock (was Re: sched_yield() makes OpenLDAP slow)
Date: Thu, 26 Jan 2006 12:34:12 -0500
Message-ID: <Pine.LNX.4.61.0601261231460.9298@chaos.analogic.com>
Thread-Topic: pthread_mutex_unlock (was Re: sched_yield() makes OpenLDAP slow)
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References: <20060124225919.GC12566@suse.de>  <20060124232142.GB6174@inferi.kami.home> <20060125090240.GA12651@suse.de>  <20060125121125.GH5465@suse.de> <43D78262.2050809@symas.com>  <43D7BA0F.5010907@nortel.com>  <43D7C2F0.5020108@symas.com> <1138223212.3087.16.camel@mindpipe> <43D7F863.3080207@symas.com> <43D88E55.7010506@yahoo.com.au> <43D8DB90.7070601@symas.com> <43D8E298.3020402@yahoo.com.au> <43D8E96B.3070606@symas.com> <43D8EFF7.3070203@yahoo.com.au> <43D8FC76.2050906@symas.com>
From: "linux-os \(Dick Johnson\)" <linux-os@analogic.com>
To: "Howard Chu" <hyc@symas.com>
Cc: "Nick Piggin" <nickpiggin@yahoo.com.au>,
       "Lee Revell" <rlrevell@joe-job.com>,
       "Christopher Friesen" <cfriesen@nortel.com>,
       "Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
       <hancockr@shaw.ca>
Reply-To: "linux-os \(Dick Johnson\)" <linux-os@analogic.com>
Sender: linux-kernel-owner@vger.kernel.org
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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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