[ILUG] Memory Usage at 98%

[Gareth Eason]

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Colin O'Keeffe wrote:
| HI
|
| Looking at phpsysinfo, my memory usuage ( have 1 gig of ram ) is at
| 98% and holding there constantly
[munch...]

	Depending on what memory usage figure you're looking at, it could be
exactly right. Linux uses a lazy memory recovery algorithm - it assumes
RAM usage is cheap (in computing terms) compared to hard disk access, so
it tries to cache as much hard disk data as possible in RAM. If you use
`free` you should see a number of figures. `vmstat` gives more. `top`
shows the same useful memory information in a slightly different format
again.

	Basically (vastly simplified - Mel Gorman would be able to comment on
the specifics of this) the kernel, or rather the memory management
subsystem, will try and keep a small amount of memory free so that new
requests for memory can be satisfied quickly. About 2% of memory sounds
right to me. Everything else will be used for active processes, inactive
processes not yet paged to disk (paging to disk is expensive, so Linux
will try not to do it unless it needs to, even though the process is
inactive. Remember - the kernel doesn't know how soon you are going to
make the process active again - it might be in the next second, or it
might not be for a week (or months, in my case :-)  )) and disk / I/O
cache memory. On my 1.5GB machine I generally reach a quiescent state
where about 9MB is free, and the rest is split about 2:1 between disk
cache (~900MB) and process memory (~600MB) (the machine does a lot of
disk I/O for both itself and other machines on my HAN, which explains
the rather high disk-cache percentage.)

	Hope this helps and explains things somewhat. You can get much more
information from `man vmstat` and `man free`

	If you're worried something is wrong, check what the memory use is
classed as, and if it's not cache it might be worth your while looking
into things a little bit more extensively. Remember that MySQL and
Apache are both demons for caching stuff for fast response (it's much
quicker to get stuff from RAM than from disk) too, so if you have some
large data-sets in MySQL or are getting lots of hits for stuff in
Apache, it's possible they are sucking up RAM to try and optimise their
reply-rates too.

	Best regards,
	-->Gar
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