Re: unexpected paging during large file reads in 2.1.127
David Feuer (feuer@his.com)
Tue, 17 Nov 1998 22:40:15 -0500
Rik van Riel wrote:
>
> On Mon, 16 Nov 1998, Stephen C. Tweedie wrote:
> > On Mon, 16 Nov 1998 21:48:35 +0100 (CET), Rik van Riel
> > <H.H.vanRiel@phys.uu.nl> said:
> > > On Mon, 16 Nov 1998, Stephen C. Tweedie wrote:
> > >> The real cure is to disable page aging in the page cache completely.
> > >> Now that we have disabled it for swap, it makes absolutely no sense at
> > >> all to keep it in the page cache.
> >
> > > This is not entirely true. There is a major difference
> > > between pages in the page cache and pages that can go
> > > into swap. The latter kind will always be mapped inside
> > > the address space of a program (where it gets proper
> > > aging and stuff)
> >
> > No it doesn't, that's what I'm saying. Linus removed swap page aging in
> > the recent kernels. That throws the balance between swap and cache
> > completely out of the window: removing the page cache aging is necessary
> > to restore balance. There are many many reports of massive cache growth
> > on the latest kernels as a result of this.
>
> I meant the page aging that occurs in vmscan.c, where we
> decide on which page to unmap from a program's address
> space. There we do aging while we don't age pages from
> files that are read().
>
> > > Now we can get severe problems with readahead when we
> > > are evicting just read-in data because it isn't mapped,
> >
> > No, we don't. We don't evict just-read-in data, because we mark such
> > pages as PG_Referenced. It takes two complete shrink_mmap() passes
> > before we can evict such pages.
>
> OK, I can (and have for quite a while) agree with this.
> Kernels with this feature and enough memory will run great,
> maybe small machines (<16M) will have a bit of trouble
> keeping up readahead performance (since kswapd will have
> made it's round a bit fast) but those machines will have
> sucky performance anyway :)
>
> Rik -- slowly getting used to dvorak kbd layout...
<snip>
The Dvorak layout is obviously superior to that ancient Sholes thing
most people store on their desks :)
It might not be elegant, but wouldn't it be possible to have different
MM code depending on the amount of memory on the system? This could
either be compiled-in (yuck) or else set at boot time...... Don't know
how this would impact efficiency.
--
______________________________
/ David Feuer \
| dfeuer@binx.mbhs.edu |
| feuer@his.com |
\ david@feuer.his.com /
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