Messages in this thread | | | Date | Tue, 8 Dec 1998 04:00:10 +0100 (CET) | From | Rik van Riel <> | Subject | Re: [PATCH] swapin readahead and fixes |
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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 +-------------------------------------------------------------------+ | Linux memory management tour guide. H.H.vanRiel@phys.uu.nl | | Scouting Vries cubscout leader. http://www.phys.uu.nl/~riel/ | +-------------------------------------------------------------------+
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