Messages in this thread | | | Date | Wed, 29 Sep 2004 20:46:28 -0700 | From | Andrew Morton <> | Subject | Re: OSDL aio-stress results on latest kernels show buffered random read issue |
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Judith Lebzelter <judith@osdl.org> wrote: > > Hello; > > I am running aio-stress on the most recent kernels and have > found that on linux-2.6.8, 2.6.9-rc2 and 2.6.9-rc2-mm4 the > performance of buffered random reads is poor compared to the > buffered random writes: > > 2.6.8 2.6.9-rc2 2.6.9-rc2-mm4 > -------------------------------------------- > random write 35.66 MB/s 34.80 MB/s 29.89 MB/s > random read 7.69 MB/s 7.50 MB/s 7.68 MB/s > > ** 2CPU hosts with striped Megaraid. 1G RAM. 4G File. > > > This shows up on our 4CPU host as well. (striped AACRAID.4G > RAM. 8G File): > 2.6.9-rc2 2.6.9-rc2-mm4 2.6.9-rc2-mm1 > ------------------------------------------- > random write 31.36 MB/s 18.92 MB/s 18.97 MB/s > random read 11.13 MB/s 9.74 MB/s 11.05 MB/s > > > There seems to be an issue with the reads. Usually, reads > should be at least as fast as writes of the same type. > > Also, there seems to be a substantial drop-off in the performance > of AIO buffered-random writes in the mm kernels. (14% on 2CPU, > 40% on 4CPU) >
Well one would expect writes to be much faster than reads because writes usually do not involve performing physical I/O, and when pagecache writeback finally happens it has vastly more data to work with and hence can schedule I/O more efficiently.
Unless you are using O_SYNC or fsync(), in which case ignore the above.
The regression within random write performance is unexpected. Can you please provide a URL to the current version of the test tool, and a description of how you are invoking it? What sort of I/O system, what filesystem, etc.
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