Messages in this thread | | | Date | Sat, 1 Jun 2002 07:17:59 +0100 (BST) | From | Hugh Dickins <> | Subject | Re: [patch 5/18] mark swapout pages PageWriteback() |
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On Fri, 31 May 2002, Linus Torvalds wrote: > On Fri, 31 May 2002, Andrea Arcangeli wrote: > > > > In short the same way MAP_ANON must pageout correctly, also MAP_SHARED > > must swapout correctly with very vm intensive conditions.
I strongly agree with Andrea on that. Swap fault without page lock was a step forward, reinstating it in the light of observed degradation in one workload was right decision for that particular tail of release, but it's a shame to have stayed that way.
> That is true, but it is ignoring the fact that there _are_ real technical > differences between swap cache mappings and regular shared mappings. > > One major difference is the approach to the last user: a last use of a > shared mapping still needs to write out dirty state, while the last use of > a swap page is better off noticing that it should just optimize away the > write, and we can just turn the page back into a dirty anonymous page.
Poor example: isn't last use of swap page just like last use of shared mapping of an _unlinked_ file?
Offhand, I think most of the differences between swap and filesystem just come from our wish not to waste any time on that filesystem: we _expect_ the object will be unlinked before it needs to hit disk. Plus, it's important that going to disk doesn't need more memory.
Perhaps those are just attributes of a particular filesystem. Just as anon pages graduated to being put on LRU a few months ago, maybe they could graduate to being hashed pages of tmpfs objects? Probably yes, but without wasting time and memory - more doubtful.
Hugh
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