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%. > > Try to avoid the remote call by flushing the local LRU cache before > > even attempting to invalidate anything. It's a cheap operation, and > > the local LRU cache is the most likely to hold any pre-LRU pages in > > the specified fadvise range. > > Anyway it looks like things can't be worse after this patch, so... > > > Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx> > > Acked-by: Vlastimil Babka <vbabka@xxxxxxx> Thanks! -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>