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Re: Using swap though there's plenty of mem free



Matthias Scheler <tron%zhadum.org.uk@localhost> wrote:

> What would be the sense in keeping allocated pages in memory even if
> they haven't accessed for hours? Using the memory e.g. as disk
> buffers will reduce the total amount of disk I/O which is what
> a modern VM system tries to achieve.

That may be true for a desktop system, but NEVER for a server system.
If lighttpd is swapped because it wasn't used for some time, that's
bad. If the python scripts I use via fcgi are swapped out (there are 4
threads of them, so it's very likely one will get swapped out after
some time because not all are always used - and which one is used is
randomly), it's even worse. So the probability is high that the user has
to wait about 10 seconds till he gets a reply from for example hg
(remember, first lighttpd needs to beloaded from swap, then one of the
python threads). Is this desired behaviour for a server? Nope, it's
really not, definitely not. This really kills the whole idea of having 4
threads so that no new thread needs to be spawned, which would take
too long for the user to wait. Same for spamd which runs with 5 threads
here. Kills really the point of having multiple threads to speed up
spawning. It kills the whole idea of pre-spawning.

> Please use Google to look for "Using swap is bad for performance"
> and you will find some interesting comments on the topic.

On a desktop maybe, but not on a server. And as far as I know, NetBSD
is used more for servers than for desktops, though it's a very nice
system for a desktop as well.

-- 
Jonathan

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