Messages in this thread |  | | | Date | Mon, 28 Jul 2008 08:30:34 -0700 | | From | "Ray Lee" <> | | Subject | Re: PERF: performance tests with the split LRU VM in -mm |
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On Mon, Jul 28, 2008 at 7:57 AM, Rik van Riel <riel@redhat.com> wrote: > On Thu, 24 Jul 2008 22:25:10 -0400 > Rik van Riel <riel@redhat.com> wrote: > >> TEST 1: dd if=/dev/sda of=/dev/null bs=1M >> >> kernel speed swap used >> >> 2.6.26 111MB/s 500kB >> -mm 110MB/s 59MB (ouch, system noticably slower) >> noforce 111MB/s 128kB >> stream 108MB/s 0 (slight regression, not sure why yet) >> >> This patch shows that the split LRU VM in -mm has a problem >> with large streaming IOs: the working set gets pushed out of >> memory, which makes doing anything else during the big streaming >> IO kind of painful. >> >> However, either of the two patches posted fixes that problem, >> though at a slight performance penalty for the "stream" patch. > > OK, the throughput number with this test turns out not to mean > nearly as much as I thought. > > Switching off CPU frequency scaling, pinning the CPUs at the > highest speed, resulted in a throughput of only 102MB/s. > > My suspicion is that faster running code on the CPU results > in IOs being sent down to the device faster, resulting in > smaller IOs and lower throughput.
Or the IOs are getting sent in a different order, and so coalescing/merging isn't occurring as often. Getting some instrumentation (something as simple as a histogram) on the IO sizes could be useful.
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