mysql scaling questions

[Gergely CZUCZY]

On Tue, Jan 01, 2008 at 05:04:56AM +0100, Kris Kennaway wrote:
> 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.
There's this SYSCALL CPU extension with the SYSENTER/SYSEXIT features. IIRC
Linux takes advantage of this, while FreeBSD doesn't. I might be wrong here,
of course.

Sincerely,

Gergely Czuczy
mailto: gergely.czuczy at harmless.hu

-- 
Weenies test. Geniuses solve problems that arise.
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