Messages in this thread | | | Date | Wed, 2 Mar 2011 07:57:51 +0900 | Subject | Re: [PATCH 2/2] mm: compaction: Minimise the time IRQs are disabled while isolating pages for migration | From | Minchan Kim <> |
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On Wed, Mar 2, 2011 at 7:34 AM, Andrew Morton <akpm@linux-foundation.org> wrote: > On Wed, 2 Mar 2011 07:22:33 +0900 > Minchan Kim <minchan.kim@gmail.com> wrote: > >> On Wed, Mar 2, 2011 at 1:19 AM, Andrea Arcangeli <aarcange@redhat.com> wrote: >> > On Wed, Mar 02, 2011 at 12:35:58AM +0900, Minchan Kim wrote: >> >> On Tue, Mar 01, 2011 at 01:49:25PM +0900, KAMEZAWA Hiroyuki wrote: >> >> > On Tue, 1 Mar 2011 13:11:46 +0900 >> >> > Minchan Kim <minchan.kim@gmail.com> wrote: >> >> > >> >> ... >> >> > pages freed from irq shouldn't be PageLRU. >> >> Hmm.. >> As looking code, it seems to be no problem and I didn't see the any >> comment about such rule. It should have been written down in >> __page_cache_release. >> Just out of curiosity. >> What kinds of problem happen if we release lru page in irq context? > > put_page() from irq context has been permissible for ten years. I > expect there are a number of sites which do this (via subtle code > paths, often). It might get messy. > >> > >> > deferring freeing to workqueue doesn't look ok. firewall loads runs >> > only from irq and this will cause some more work and a delay in the >> > freeing. I doubt it's worhwhile especially for the lru_lock. >> > >> >> As you said, if it is for decreasing lock contention in SMP to deliver >> overall better performance, maybe we need to check again how much it >> helps. >> If it doesn't help much, could we remove irq_save/restore of lru_lock? >> Do you know any benchmark to prove it had a benefit at that time or >> any thread discussing about that in lkml? > > > : commit b10a82b195d63575958872de5721008b0e9bef2d > : Author: akpm <akpm> > : Date: Thu Aug 15 18:21:05 2002 +0000 > : > : [PATCH] make pagemap_lru_lock irq-safe > : > : It is expensive for a CPU to take an interrupt while holding the page > : LRU lock, because other CPUs will pile up on the lock while the > : interrupt runs. > : > : Disabling interrupts while holding the lock reduces contention by an > : additional 30% on 4-way. This is when the only source of interrupts is > : disk completion. The improvement will be higher with more CPUs and it > : will be higher if there is networking happening. > : > : The maximum hold time of this lock is 17 microseconds on 500 MHx PIII, > : which is well inside the kernel's maximum interrupt latency (which was > : 100 usecs when I last looked, a year ago). > : > : This optimisation is not needed on uniprocessor, but the patch disables > : IRQs while holding pagemap_lru_lock anyway, so it becomes an irq-safe > : spinlock, and pages can be moved from the LRU in interrupt context. > : > : pagemap_lru_lock has been renamed to _pagemap_lru_lock to pick up any > : missed uses, and to reliably break any out-of-tree patches which may be > : using the old semantics. > : > : BKrev: 3d5bf1110yfdAAur4xqJfiLBDJ2Cqw > > > Ancient stuff, and not a lot of detail. But I did measure it. I > measured everything ;) And, as mentioned, I'd expect that the > contention problems would worsen on higher CPU machines and higher > interrupt frequencies.
Thanks for giving the important information.
> > I expect we could eliminate the irqsave requirement from > rotate_reclaimable_page() simply by switching to a trylock. Some pages > will end up at the wrong end of the LRU but the effects may be > negligible. Or perhaps they may not - disk seeks are costly. > >
Releasing 14 pages should not have much cost about interrupt latency and It's a general concept we have been used. If it really has a problem, I think it would be better to reduce PAGEVEC_SIZE rather than fixing the rotate_reclaimable_page.
-- Kind regards, Minchan Kim -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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