Messages in this thread | | | Date | Mon, 30 Jun 2008 17:17:21 -0400 | From | Rik van Riel <> | Subject | Re: [PATCH] splitlru: BDI_CAP_SWAP_BACKED |
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On Mon, 30 Jun 2008 21:55:13 +0100 (BST) Hugh Dickins <hugh@veritas.com> wrote: > 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?
Temporary files are often soon gone.
Anonymous memory and shmem segments tend to stick around for longer.
> > 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).
If we have them on the *_ANON LRUs, we will automatically not scan them when swap space runs out. Just like we do not scan anonymous pages when there is no swap space left.
> > 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
This is a performance problem for database systems, where the system ends up swapping out the shared memory segment.
> (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.
Having the mem cgroups consistent with the global LRU implementation would be good, indeed. That will make balancing a bit easier.
> 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).
Agreed, we understand the page_mapped/!page_mapped distinction quite well. On the other hand, we do not understand LRU acrobatics (moving pages between lists on mmap/munmap) and the consequences of that...
I suspect we'll just have to tweak the swap-backed/file-backed code until it works right. Once Andrew comes out with a new -mm (with all the stability fixes), I will create a kernel RPM with the latest split LRU code for Fedora 9, so we can get some wider testing.
I have some performance tweaks in mind already, but not enough data yet to justify them. I will continue working on that.
-- All rights reversed.
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