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Date: Tue, 01 Jan 2008 05:04:56 +0100
From: Kris Kennaway <kris@FreeBSD.org>
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To: Ivan Voras <ivoras@freebsd.org>
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Cc: freebsd-performance@freebsd.org
Subject: Re: mysql scaling questions
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Ivan Voras wrote:
> Kris Kennaway wrote:
>> Gergely CZUCZY wrote:
> 
>>>> It looks like myisam is doing huge numbers of concurrent reads of the
>>>> same file which is running into exclusive locking in the kernel
>>>> (vnode interlock and lockbuilder mtxpool).  Does it not do any
>>>> caching of the data in userspace but relies on querying into the
>>>> kernel every time? innodb doesn't have this behaviour.
>>> Sorry, but was this a rethorical kind of question, or was this
>>> addressed to me? :)
>>> If the later, then how do I find this out?
>> It's a general question.  It looks like myisam either has a design
>> deficiency in this regard or it has poor defaults.  If it can be made to
>> improve caching of the data in userland then performance should improve.
> 
> Isn't this common for software developed for Linux? I mean assuming
> syscalls are cheap; for example: gettimeofday(2), settitle(2), etc. I
> don't think the applications should be blamed for relying on performance
> optimizations not present in FreeBSD. Saying applications must do their
> own caching instead of relying on the kernel and need to avoid
> concurrent accesses to the same file seems like a doctrine from the dark
> ages.

Why?  Even if Linux magically has faster syscalls somehow, they are 
still not zero cost so avoiding huge numbers of unnecessary trips into 
the kernel is in no sense a "doctrine from the dark ages".  Besides, if 
my hypothesis about the problem is correct then mysql itself does this 
with the alternate innodb backend anyway.

Kris