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To: Jiawei Ye <leafy7382@gmail.com>
From: "Poul-Henning Kamp" <phk@phk.freebsd.dk>
In-Reply-To: Your message of "Fri, 28 Oct 2005 17:48:19 +0800."
	<c21e92e20510280248g6b47a7c6t79cb07e53a7db118@mail.gmail.com> 
Date: Fri, 28 Oct 2005 12:14:48 +0200
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Cc: Mark Kirkwood <markir@paradise.net.nz>,
	Pawel Jakub Dawidek <pjd@freebsd.org>, David Xu <davidxu@freebsd.org>,
	"Yuriy N. Shkandybin" <jura@networks.ru>, current@freebsd.org
Subject: Re: Timers and timing, was: MySQL Performance 6.0rc1 
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In message <c21e92e20510280248g6b47a7c6t79cb07e53a7db118@mail.gmail.com>, Jiawe
i Ye writes:

Very typical numbers...

>kern.timecounter.hardware: ACPI-fast -> TSC
>            getpid(): 0.94267
>      gettimeofday(): 1.25332

So timekeeping "as such" takes no more than 310nsec and that
includes a TSC read of unknown duration.

I consider the 310 nsec acceptable, but if this is a really
big problem for people, somebody is welcome to start looking
at doing inline assembler code to speed it up

>kern.timecounter.hardware: TSC -> ACPI-fast
>            getpid(): 0.94788
>      gettimeofday(): 2.29598

ACPI-fast does an inl() which seems to take a microsecond longer
than rdtsc().

One microsecond is disgusting considering that the hardware lives
in the chipset and was meant to improve timekeeping performance.

>kern.timecounter.hardware: ACPI-fast -> i8254
>            getpid(): 0.96921
>      gettimeofday(): 5.01292

And i8254 does a outb() and several 8 bit inb on a simulated ISA
bus and therefore takes 3.75 usec longer than rdtsc().

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.