Date: Thu, 26 Oct 2000 00:11:06 -0700 (PDT) From: Matt Dillon <dillon@earth.backplane.com> To: Anton Berezin <tobez@tobez.org> Cc: Alfred Perlstein <bright@wintelcom.net>, ps@FreeBSD.ORG, hackers@FreeBSD.ORG Subject: Re: vm_pageout_scan badness Message-ID: <200010260711.e9Q7B6q32884@earth.backplane.com> References: <20001024112708.E28123@fw.wintelcom.net> <200010242010.e9OKAJK19739@earth.backplane.com> <20001025123154.A56298@heechee.tobez.org>
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:On Tue, Oct 24, 2000 at 01:10:19PM -0700, Matt Dillon wrote:
:> Ouch. The original VM code assumed that pages would not often be
:> ripped out from under the pageadaemon, so it felt free to restart
:> whenever. I think you are absolutely correct in regards to the
:> clustering code causing nearby-page ripouts.
:>
:> I don't have much time available, but let me take a crack at the
:> problem tonight.
:
:While you are at it, would you care and have a look at PR19672. It
:seems to be at least remotely relevant. ;-)
Hmmm. Blech. contigmalloc is aweful. I'm not even sure if what
it is doing is legal! If it can't find contiguous space it
tries to flush the entire inactive and active queues. Every
single page! not to mention the insane restarting. The
algorithm is O(N^2) on an idle machine, and even worse on
machines that might be doing something.
There is no easy fix. contigmalloc would have to be completely
rewritten. We could use the placemarker idea to make the loop
'retry' the page that blocked rather then restart at the beginning,
but the fact that contigmalloc tries to flush the entire page
queue means that it could very trivially get stuck on dead devices
(e.g. like a dead NFS mount). Also, if we don't restart, there is
less of a chance that contigmalloc can find sufficient free space.
When it frees pages it does so half-hazzardly, and when it flushes
pages out it makes no attempt to free them so an active process
may reuse the page instantly. Bleh.
I'm afraid I don't have the time to rewrite contigmalloc myself, but
my brain is available to answer questions if someone else wants to
have a go at it.
-Matt
:Cheers,
:%Anton.
:--
:<img align="lawful"> and <img align="chaotic"> would be a nice addition
:to HTML specification.
:
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