Messages in this thread | | | Date | Sat, 25 Aug 2001 23:02:25 -0300 (BRST) | From | Rik van Riel <> | Subject | Re: [resent PATCH] Re: very slow parallel read performance |
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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 -- IA64: a worthy successor to i860.
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