Messages in this thread | | | Date | Mon, 30 Jun 2008 21:55:13 +0100 (BST) | From | Hugh Dickins <> | Subject | Re: [PATCH] splitlru: BDI_CAP_SWAP_BACKED |
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On Mon, 30 Jun 2008, Rik van Riel wrote: > > Tmpfs is often in the same boat as anonymous memory. > Used for shared memory segments, or for files that > are temporary and will be gone soon.
Anonymous memory, and temporary files, are often soon gone, okay. But I don't find that generalization compelling; and if they're soon gone, does it matter which lru they go on?
> If swap space runs out, tmpfs pages should not be > scanned.
That point I like. But I hope they'd go to the Unevictable on systems with no swap at all (of course, as with mlocking, that can change soon after).
> To me, this suggests they should probably continue > to live on the *_ANON LRUs. Worst case we make > tmpfs pages in files that are not mmaped (/tmp use) > start out on the inactive list, so they get evicted > first.
Tweaking in/active I'll gladly leave to you! Whatever proves best. What's worrying me is that we have always treated shmem/tmpfs pages as file pages (e.g. in /proc/meminfo as Cached not as SwapCached), up until the point that we retire them to swap; but in splitlru you're sending them down another path; then mem cgroups seem to want them as something else again.
Your SwapBacked may indeed turn out to be the only implementable distinction, but it does worry me. A more useful distinction, my gut tells me, would be separate LRUs for page_mapped() and !page_mapped(), which reflects the existing swappiness notion.
But that immediately hits the difficulty we have in switching LRU midstream, which your SwapBacked-throughout tmpfs neatly sidesteps.
I'd really like to be able to try page_mapped/!page_mapped versus swap-backed/file-backed, but it would need some LRU-switching infrastructure (which might come at a prohibitive performance cost, since it's the batching that poses the problem).
Hugh
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