[NMLUG] Linux rocks

[niosop at gmail.com (Matthew Bowie)]

Just thought I'd share some stuff that happened today.

I just finished moving a LAMP(php/perl) stack over from a Pentium 4 3.4GHz/1GB
RAM machine to a Pentium D 3.4/2GB RAM server.  There was about 1.5GB of
data in the MySQL database, and the client had a long running query that
would aggregate much of it and display it on a page.  It would normally take
about 20-30 seconds to run.

After moving over the scripts and database, the client tested the script,
and as expected, it ran only slightly faster than it did on the other
machine.  Then the client wanted to run two instances of the script and see
how the dual core system handled it.  He started one, but before he could
start the second, the first one was complete.  He was pretty concerned,
thinking that something much have been broken by the move, and that a cached
copy was being served.  How could a 30 second query be served in less than a
second?

A quick look at "free -m" showed that although the OS and LAMP stack was
only taking 95MB of RAM, over 1.5GB was being used as cache.  The majority
of the normal 30 second query time was spent reading the data off disk.
After the first query, Linux cached all the database files in RAM, so the
next time the queries didn't have to pull anything off disk.  That combined
with MySQL's query cache made a big difference.  The client was pretty
impressed.

The moral?
Linux's aggressive file system caching is a good thing (usually).
Linux doesn't let unused RAM sit around doing nothing, and sometimes it
shows in a big way.  So, if you can afford it, go for more RAM.


-- 
Matthew Bowie
Programmer/IT Consultant
niosop at gmail.com
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