Re: [PATCH] swapin readahead and fixes

[Rik van Riel <H.H.vanRiel@phys.uu.nl>]

On Mon, 7 Dec 1998, Billy Harvey wrote:
> Rik van Riel wrote:
> > 
> > On Mon, 7 Dec 1998, Billy Harvey wrote:
> > 
> > > Has anyone ever looked at the following concept?  In addition to a
> > > swap-in read-ahead, have a swap-out write-ahead.  The idea is to use
> > > all the avaialble swap space as a mirror of memory.
> > 
> > We do something a bit like this in 2.1.130+. Writing out all
> > pages to swap will use far too much I/O bandwidth though, so
> > we will never do that...
> 
> That's my point though about not taking I/O time away from other
> tasks.  Only mirror pages to swap if there's nothing else blocked
> for I/O - put any free time to work, and mirror pages if swap memory
> allows in anticipation that it may be swapped out later. 

Write-ahead only makes sense when we can cluster the extra
I/O with the operation we were already going to do.

> I suppose a least-recently-used approach on the pages would have the
> highest payback. 

LRU would be a very bad strategy since it wastes too much CPU
and it prevents us from writing the blocks to disk in such a
way that it makes swapin readahead efficient.

Remember that disk seek time is about 10 times as expensive
as transfer time. This means that we've got to optimize our
I/O patterns mainly for seek time -- transferring a few
blocks extra in one big I/O sweep isn't really costing us
anything. And once we do that, expensive schemes like LRU
really don't matter any more, do they?

regards,

Rik -- the flu hits, the flu hits, the flu hits -- MORE
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