[Gllug] performance difference between a swap partition vs a swap file.>>>>>

John Hearns hearnsj at googlemail.com
Wed Dec 2 18:11:46 UTC 2009

2009/12/2 Richard Jones <rich at annexia.org>:
> > What you'd really want in this case is some way to lock the relevant
> parts you need into physical RAM.  The kernel is already "locked" in
> RAM, so you'd have to mlock pages from getty too.  And login, and
> bash, and any diagnostic programs.  Never seen this done in real life
> though :-)  People who want such highly available systems tend to
> configure them without swap and without memory overcommit in the first
> place.

As someone with a clue once explained to me (old-time SGI employee and
trainer on the Altix Evaluation and Performance Tuning course)
the problem is that the Linux cache has no floor - agreeing with what
you are saying. In Irix it was possible to set a floor to the cache,
so you would still have some space
for applications like that. In the Linux world, when a system has run
out of memory it is common to say "I can't get a login prompt because
the system is too busy reading/writing to disk", in fact what has
happened is that the executables you need have been swapped out - and
you haven't a snowball's chance of getting them back.
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