RE: Speeding up swap
Helge Hafting (helge.hafting@idb.hist.no)
Tue, 26 Oct 1999 11:29:05 +0200
> > I am looking for a mechanism that enables a user process to
> > advise the kernel on a per-fd basis not to keep more than say
> > two pages of a certain file any buffer cache based on the
> > promise that the user process will read that file only in a
> > sequentially-forward or even non-seeking manner. I exspect
> > performance improvements from such a mechanism because I
> > observed that unrelated user processes get paged out when a very
> > large file is passed thru a streaming application.
>
> If you allow it to be used by non-privileged processes, you create the
> potential for an untrusted process to cause ridiculous amounts of physical
> I/O.
They can do that already by abusing fsync for every byte written or some
such.
This ridiculous reading won't be suc a big deal anyway, the abusive
process will
be blocked for IO a lot of the time letting others run, and the others
will find
their data in the now un-polluted cache.
Of course there are ways to prevent abuse too. The minimum limit could
be
8 pages instead of 2 perhaps. And the kernel don't *need* to honor such
a request,
it is a 'hint', not a 'rule'. The kernel can ditch the hint if it
proves
ineffective. A backwards seek outside the limit could
raise the limit so that sort of seek may be cached the next time, for
example.
Helge Hafting
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