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Date: Mon, 30 Mar 2015 19:06:45 -0400 From: Milosz Tanski <milosz@...in.com> To: Andrew Morton <akpm@...ux-foundation.org> Cc: Jeremy Allison <jra@...ba.org>, Christoph Hellwig <hch@...radead.org>, LKML <linux-kernel@...r.kernel.org>, "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>, "linux-aio@...ck.org" <linux-aio@...ck.org>, Mel Gorman <mgorman@...e.de>, Volker Lendecke <Volker.Lendecke@...net.de>, Tejun Heo <tj@...nel.org>, Jeff Moyer <jmoyer@...hat.com>, "Theodore Ts'o" <tytso@....edu>, Al Viro <viro@...iv.linux.org.uk>, Linux API <linux-api@...r.kernel.org>, Michael Kerrisk <mtk.manpages@...il.com>, linux-arch@...r.kernel.org, Dave Chinner <david@...morbit.com> Subject: Re: [PATCH v7 0/5] vfs: Non-blockling buffered fs read (page cache only) On Mon, Mar 30, 2015 at 6:57 PM, Andrew Morton <akpm@...ux-foundation.org> wrote: > On Mon, 30 Mar 2015 18:49:06 -0400 Milosz Tanski <milosz@...in.com> wrote: > >> > A fincore+pread solution that blocks is simply unsafe >> > to use for us. We'll have to stay with the threadpool :-(. >> >> We're getting data from a network filesystem Ceph in our case, but it >> could be pNFS. In many cases those filesystems have some kind >> hierarchy and it's not uncommon for us to se requests that take 20 to >> 25 milliseconds to complete. In this case the miss becomes very >> expensive. And it's not just that one requests experiences the slow >> down all the request being serviced by that (single) epoll thread >> experience head-of-line blocking because of one stalled request. >> >> 10K request a second is a common load for many web services / video >> servers servings chunks of data. If we experience one miss a second, >> that 25 million stall will impact 250 other requests (all of them will >> have a 25ms latency tacked on). > > I'd expect a fincore() which doesn't do SetPageReferenced() to be > orders of magnitude better than this. A fincore() which does use > SetPageReferenced() will be in the "basically never happens" region - > it would take massive and artificial memory stress to trigger. I'm just responding to the upper bound you put out in an email a few back of 0.0001% miss. And, people run web caches (like Apache Traffic Server) at much higher rates than that. -- Milosz Tanski CTO 16 East 34th Street, 15th floor New York, NY 10016 p: 646-253-9055 e: milosz@...in.com -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.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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