Re: [PATCH] vma limited swapin readahead

[David Gould]

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...

Thanks

-dg
 
-- 
David Gould                                                 [e-mail]
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You left them alone in a room with a penguin?!  Mr Gates, your men are
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