Re: [resent PATCH] Re: very slow parallel read performance

Rik van Riel
2001-08-26T02:03:39Z

On Sun, 26 Aug 2001, Marc A. Lehmann wrote:
> On Sat, Aug 25, 2001 at 10:33:36PM +0100, Alan Cox <alan@lxorguk.ukuu.org.uk> wrote:
> > > exactly this is a point: my disk can do 5mb/s with almost random seeks,
> > > and linux indeed reads 5mb/s from it. but the userpsace process doing
> > > read() only ever sees 2mb/s because the kernel throes away all the nice
> > > pages.
> >
> > Which means the VM in the relevant kernel is probably crap or your working
> > set exceeds ram.
>
> The relevant kernel is linux (all 2.4 versions I tested), and no,
> working set exceeding ram should never result in such excessive
> thrashing. So yes, the VM in the kernel is crap (in this particular
> case, namely high-volume fileserving) ;)

This is because the readahead windows are too large so the
kernel ends up evicting data before its needed and has to
re-read the data.

Also see http://linux-mm.org/wiki/moin.cgi/StreamingIo
in the Linux-MM wiki.

This problem should be relatively easy to fix for 2.4.

regards,

Rik
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