Messages in this thread | | | Subject | Re: [PATCH] mm: fadvise: avoid expensive remote LRU cache draining after FADV_DONTNEED | From | Vlastimil Babka <> | Date | Tue, 13 Dec 2016 13:32:58 +0100 |
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On 12/12/2016 04:55 PM, Johannes Weiner wrote: > On Mon, Dec 12, 2016 at 10:21:24AM +0100, Vlastimil Babka wrote: >> On 12/10/2016 06:26 PM, Johannes Weiner wrote: >>> When FADV_DONTNEED cannot drop all pages in the range, it observes >>> that some pages might still be on per-cpu LRU caches after recent >>> instantiation and so initiates remote calls to all CPUs to flush their >>> local caches. However, in most cases, the fadvise happens from the >>> same context that instantiated the pages, and any pre-LRU pages in the >>> specified range are most likely sitting on the local CPU's LRU cache, >>> and so in many cases this results in unnecessary remote calls, which, >>> in a loaded system, can hold up the fadvise() call significantly. >> >> Got any numbers for this part? > > I didn't record it in the extreme case we observed, unfortunately. We > had a slow-to-respond system and noticed it spending seconds in > lru_add_drain_all() after fadvise calls, and this patch came out of > thinking about the code and how we commonly call FADV_DONTNEED. > > FWIW, I wrote a silly directory tree walker/searcher that recurses > through /usr to read and FADV_DONTNEED each file it finds. On a 2 > socket 40 ht machine, over 1% is spent in lru_add_drain_all(). With > the patch, that cost is gone; the local drain cost shows at 0.09%.
Thanks, worth adding to changelog :)
Vlastimil
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