[ale] mysql and high load levels

[djinn at djinnspace.com]

This showed up on the mysql list...and it caught my attention given what you guys
are saying about 2.4 and its wierd memory handling....
This post (below) describes my situation almost exactly...he's using more
'spensive hardware than I am, but scale it down a bit and it's *exactly* what I'm
seeing.  I'm also seeing 98% RAM usage and even swapping on my web
server....which has 1GB of RAM and should NEVER need to swap.  Ever.  Even with
lots of PHP usage...the old server with 256MB ram never swapped.

So...given this and ya'll's advice...should I drop back to 2.2.19 and forget 2.4
altogether, or should I disable swap, compile himem into my 2.4.7 kernel, and see
if it fixes it?

jenn

<originally posted on lists.mysql.com by  Jeremy Zawodny >
-------------------------------------------------
We had a couple of Linux 2.2.19 servers with 2 CPUs (P3-850) and 1GB
of RAM.  They were solid and they flew.  The never swapped.  They had
well tuned my.cnf files.  But we needed large files and ReiserFS, so
we upgraded to 2.4.9.

After the upgrade, IO performance on the master server began to suck
at backup/snapshot time.  In fact, Linux claimed that it was running
out of RAM.  So we added RAM (a 2nd gigabyte).  The problem didn't go
away.

So I poked around and watched it really closely for a couple days.
What I found is that the server would run pretty well until I
initiated some other type of intensive IO in addition to MySQL.  For
example, I'd copy a 3GB data file from the main RAID-5 data volume to
/tmp.  As it copied, Linux decided to swap out most of mysqld (which
is in the neighborhood of 400MB) to use it for a disk buffer.  This
not only killed MySQL performance, because it had to swap those pages
back into physical memory as soon as they were needed, it also made
the copy take much longer than it should have.

I could go on, but the pattern was clear.  Linux tried to use as much
memory as it could for disk buffering--even if it meant "stealing" RAM
from mysqld.

The solution?  I disabled swap and it's been humming along just fine
ever since.  After all, with 2GB of RAM and a good understanding of
what our RAM requirements really are, the system should never swap.
-----------------------------------------------------

Joseph Andrew Knapka wrote:

> Jeff Hubbs wrote:
> >
> > Jenn -
> >
> > If it were me - and please understand that I have very, very little
> > experience with MySQL - but I'd try going back to a 2.2 kernel first.
>
> Seconded (including the disclaimer :-). There seem to be a number of
> ways the 2.4 kernels can snafu WRT memory management. When your load
> goes up, what process(es) are using all that CPU? I'll place a small
> wager on kswapd.
>
> Cheers,
>
> --
> # Joe Knapka
> # "You know how many remote castles there are along the
> #  gorges? You can't MOVE for remote castles!" - Lu Tze re. Uberwald
> # Linux MM docs:
> http://home.earthlink.net/~jknapka/linux-mm/vmoutline.html
> --
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