Messages in this thread | | | Date | Wed, 15 Apr 2009 11:33:16 +0200 | From | Brice Goglin <> | Subject | Re: [PATCH] migration: only migrate_prep() once per move_pages() |
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KAMEZAWA Hiroyuki wrote: > On Wed, 15 Apr 2009 09:32:10 +0200 > Brice Goglin <Brice.Goglin@ens-lyon.org> wrote: > > >> migrate_prep() is fairly expensive (72us on 16-core barcelona 1.9GHz). >> Commit 3140a2273009c01c27d316f35ab76a37e105fdd8 improved move_pages() >> throughput by breaking it into chunks, but it also made migrate_prep() >> be called once per chunk (every 128pages or so) instead of once per >> move_pages(). >> >> This patch reverts to calling migrate_prep() only once per chunk >> as we did before 2.6.29. >> It is also a followup to commit 0aedadf91a70a11c4a3e7c7d99b21e5528af8d5d >> mm: move migrate_prep out from under mmap_sem >> >> This improves migration throughput on the above machine from 600MB/s >> to 750MB/s. >> >> Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr> >> >> > Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> > > I think this patch is good. page migration is best-effort syscall ;) >
My next feeling now is about improving migrate_prep() itself. It makes the move_pages() startup overhead very high.
But lru_add_drain_all() touches some code that I am far from understanding :/ Can we imagine using IPI instead of a deferred work_struct for this kind of things? Or maybe, for each processor, check whether drain_cpu_pagevecs() would have something to do before actually scheduling the local work_struct? It's racy, but migrate_prep() doesn't guarantee anyway that pages won't be moved out of the LRU before the actual migration, so...
Also I don't see why the cost of lru_add_drain_all() seems to increase linearly with the number of cores in the machine. There may be some lock contention, but it should scale better when there's pretty-much nothing in the CPU lists...
> BTW, current users of sys_move_pages() does retry when it gets -EBUSY ? >
I'd say they ignore it since it doesn't happen often :)
Brice
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