Messages in this thread | | | Date | Tue, 21 Apr 2009 11:54:29 +0200 | From | Johannes Weiner <> | Subject | Re: [patch 3/3][rfc] vmscan: batched swap slot allocation |
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On Tue, Apr 21, 2009 at 06:23:31PM +0900, KAMEZAWA Hiroyuki wrote: > On Tue, 21 Apr 2009 10:52:31 +0200 > Johannes Weiner <hannes@cmpxchg.org> wrote: > > > > Keeping multiple pages locked while they stay on private list ? > > > > Yeah, it's a bit suboptimal but I don't see a way around it. > > > Hmm, seems to increase stale swap cache dramatically under memcg ;)
Hmpf, not good.
> > > BTW, isn't it better to add "allocate multiple swap space at once" function > > > like > > > - void get_swap_pages(nr, swp_entry_array[]) > > > ? "nr" will not be bigger than SWAP_CLUSTER_MAX. > > > > It will sometimes be, see __zone_reclaim(). > > > Hm ? If I read the code correctly, __zone_reclaim() just call shrink_zone() and > "nr" to shrink_page_list() is SWAP_CLUSTER_MAX, at most.
shrink_zone() and shrink_inactive_list() use whatever is set in sc->swap_cluster_max and for __zone_reclaim() this is:
.swap_cluster_max = max_t(unsigned long, nr_pages, SWAP_CLUSTER_MAX)
SWAP_CLUSTER_MAX is 32 (2^5), so if you have an order 6 allocation doing reclaim, you end up with sc->swap_cluster_max == 64 already. Not common, but it happens.
> > I had such a function once. The interesting part is: how and when do > > you call it? If you drop the page lock in between, you need to redo > > the checks for unevictability and whether the page has become mapped > > etc. > > > > You also need to have the pages in swap cache as soon as possible or > > optimistic swap-in will 'steal' your swap slots. See add_to_swap() > > when the cache radix tree says -EEXIST. > > > > If I was you, modify "offset" calculation of > get_swap_pages() > -> scan_swap_map() > to allow that a cpu tends to find countinous swap page cluster. > Too difficult ?
This goes in the direction of extent-based allocations. I tried that once by providing every reclaimer with a cookie that is passed in for swap allocations and used to find per-reclaimer offsets.
Something went wrong, I can not quite remember. Will have another look at this.
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