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From: Robert Watson <rwatson@FreeBSD.org>
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To: Poul-Henning Kamp <phk@phk.freebsd.dk>
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Cc: David Xu <davidxu@freebsd.org>, Marian Hettwer <MH@kernel32.de>,
	current@freebsd.org
Subject: Re: MySQL Performance 6.0rc1 
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On Thu, 27 Oct 2005, Poul-Henning Kamp wrote:

> In message <43607DD5.3020708@freebsd.org>, David Xu writes:
>
>> Check gettimeofday syscall, it follows every I/O syscall, I think
>> our gettimeofday is tooooooo expensive, if we can directly get time from
>> memory, the performance will be improved further.
>
> Why would anybody take a timestamp at all I/O syscalls ?
>
> "I wonder why my car can only go 30 km/h with the trunk full of 
> concrete" ?
>
> In a data base application I could possibly understand a timestamp after 
> every write.
>
> But after _all_ I/O syscalls ?  That's just plain stupid...

This is actually a problem that came up with netperf as well -- it turned 
out we built it by default with -DHISTOGRAM, which meant that each syscall 
was followed by a time check.

However, the issue of expensive time keeping will presumably keep coming 
up.  It would be interesting to know which time counter is in use on the 
system is question.

Robert N M Watson