Re: [PATCH] vma limited swapin readahead

From: David Gould (
Date: Thu Feb 01 2001 - 13:59:33 EST

On Thu, Feb 01, 2001 at 11:26:01AM +0000, Stephen C. Tweedie wrote:
> On Wed, Jan 31, 2001 at 04:24:24PM -0800, David Gould wrote:
> >
> > I am skeptical of the argument that we can win by replacing "the least
> > desirable" pages with pages were even less desireable and that we have
> > no recent indication of any need for. It seems possible under heavy swap
> > to discard quite a portion of the useful pages in favor of junk that just
> > happenned to have a lucky disk address.
> When readin clustering was added to 2.2 for swap and paging,
> performance for a lot of VM-intensive tasks more than doubled. Disk
> seeks are _expensive_. If you read in 15 neighbouring pages on swapin
> and on average only one of them turns out to be useful, you have still
> halved the number of swapin IOs required. The performance advantages
> are so enormous that easily compensate for the cost of holding the
> other, unneeded pages in memory for a while.
> Also remember that the readahead pages won't actually get mapped into
> memory, so they can be recycled easily. So, under swapping you tend
> to find that the extra readin pages are going to be replacing old,
> unneeded readahead pages to some extent, rather than swapping out
> useful pages.

Ok. I am convinced. I would have even thought of this myself eventually...



David Gould                                       
SuSE, Inc.,  580 2cd St. #210,  Oakland, CA 94607          510.628.3380
You left them alone in a room with a penguin?!  Mr Gates, your men are
already dead.
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