On Mon, Mar 30, 2015 at 6:57 PM, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > On Mon, 30 Mar 2015 18:49:06 -0400 Milosz Tanski <milosz@xxxxxxxxx> 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@xxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
