linux-kernel - Re: [patch, v2.6.22-rc6] sys

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Date:	Tue, 26 Jun 2007 10:35:29 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Roman Zippel <zippel@...ux-m68k.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Jesper Juhl <jesper.juhl@...il.com>,
	linux-kernel@...r.kernel.org, John Stultz <johnstul@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [patch, v2.6.22-rc6] sys_time() speedup

On Tue, 26 Jun 2007 19:08:27 +0200 (CEST) Roman Zippel <zippel@...ux-m68k.org> wrote:

> > This current ... interesting piece of Roman about a _single_ trivial 
> > unlikely() branch in do_gettimeofday() borders on the ridiculous. My 
> > patch might be wrong for various reasons, but that single
> > 'if (unlikely())' statement is not one of those reasons =B-)
> 
> That's even more nonsense, that wasn't what my mail was about and Andrew 
> understood me correctly, so you could have too.

umm, yeah.  Ingo went a bit over the top there, IMO.

It boils down to: is sys_time() called at more or less than 1/2000th the
frequency of gettimeofday(), across the expected lifetime of 2.6.23 and
later?  Ingo has a couple of (surprising) examples where the sys_time()
call frequency _is_ high, but whether that will remain true across 2.6.23
and later is an open question.

How does mysql call sys_time() at all, if time(2) uses the vsyscall page??

Will contemporary-to-2.6.23-and-later mysqls do this?

All this isn't super-trivial silliness, either.  gettimeofday() is, for
many workloads, the kernel's most time-critical codepath bar none, I
believe.

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