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Date: Sun, 8 Dec 2002 14:23:30 +0100 (CET)
From: Ingo Molnar <mingo@elte.hu>
Reply-To: Ingo Molnar <mingo@elte.hu>
To: Andrew Morton <akpm@digeo.com>
Cc: "Martin J. Bligh" <mbligh@aracnet.com>, Christoph Hellwig <hch@sgi.com>,
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Subject: Re: [PATCH] set_cpus_allowed() for 2.4
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On Mon, 2 Dec 2002, Andrew Morton wrote:

> I have observed two problems with the new scheduler, both serious IMO:
> 
> 1) Changed sched_yield() semantics.  [...]

we noticed this OpenOffice/StarOffice problem in July, while beta-testing
RH 8.0. In July Andrea already had another yield implementation in his
tree, which was addressing an unrelated yield()-related regression. I'd
like to note here that StarOffice/OpenOffice sucked just as much under
Andrea's yield() variant as the original (and 2.5) O(1) scheduler variant
did.

So i talked to Andrea, and we agreed in a rough solution that worked
sufficiently well for OpenOffice and the other regression as well. I
implemented it and tested it for OpenOffice. You can see (an i suspect
later incarnation) of that implementation in Andrea's current tree. My
position back then was that we should not try to move the arguably broken
2.4 yield() implementation to 2.5.

So this is the history of O(1) yield().

fortunately, things have changed since July, since due to NPTL threading
the architectural need for user-space yield() has decreased significantly
(NPTL uses futexes, no yielding anywhere), so the only worry is behavioral
compatibility with LinuxThreads (and other yield() users). I'll forward
port the new (well, old) yield() semantics to 2.5 as well, which will be
quite similar to the yield() implementation in Andrea's tree.

there's another (this time unique) bit implemented by Andrea, a variant of
giving newly forked children priority in a more subtle way - i'm testing
this change currently, to see whether it has any positive effect on
compilation workloads.

does this clarify things?

	Ingo

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