mysql scaling questions

[Gergely CZUCZY]

On Tue, Jan 01, 2008 at 03:19:29PM +0100, Kris Kennaway wrote:
> Vlad GALU wrote:
> >On 1/1/08, Kris Kennaway <kris at freebsd.org> wrote:
> >>Gergely CZUCZY wrote:
> >>>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.
> >>FreeBSD does on amd64.  It still doesn't make syscalls free, so the
> >>architectural principle of "cache data close to where it is needed"
> >>continues to apply.
> >>
> >>Anyway, it remains to be understood whether linux really does have
> >>faster syscalls, i.e. to understand exactly what unixbench is reporting
> >>when it emits pretty numbers.  For example, how is it determining
> >>"syscall overhead"?  Often this is done by calling a syscall that the
> >>microbenchmark assumes is doing almost no work in the kernel.  This is
> >>often chosen to be getpid() which may well be NULL on Linux, but
> >>actually does do work on FreeBSD unless you remove COMPAT_43BSD from
> >>your kernel.  Also I believe glibc caches getpid() in libc (again that
> >>pesky architectural principle) so you need to be careful you are
> >>actually doing the syscalls you think you are.
> >>
> >   BTW, now with COMPAT_43 gone out of GENERIC, is it necesary to keep
> >COMPAT_43TTY, even when Linux emulation is not needed?
> 
> I think a lot of old software (e.g. in ports) still uses it although someone is working on converting them to less archaic APIs.
Is there some wiki pages or any writings on this issue? I'm not so
familiar with this COMPAT_43 obsolated stuff, and I'd like to
know what's going on, what's the problem, and so on...

Sincerely,

Gergely Czuczy
mailto: gergely.czuczy at harmless.hu

-- 
Weenies test. Geniuses solve problems that arise.
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