Messages in this thread | | | Subject | Re: Linux v2.4.19-rc5 | From | Steven Cole <> | Date | 06 Aug 2002 08:07:17 -0600 |
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On Mon, 2002-08-05 at 22:30, Andrew Morton wrote: [snipped] > > IO in 2.5 is much more CPU efficient that in 2.4, and straight-line > bandwidth is better as well. > > The scheduling of that IO has a few problems, so in wildly seeky loads > like dbench the kernel still falls over its own feet a bit. The > two main culprits here are the lock_buffer() in block_write_full_page() > against the blockdev mapping, and the writeback of dirty pages from the > tail of the LRU in page reclaim. > > And no, the eventual dbench numbers will not be a measure of the success > of the tuning which will happen on the run in to 2.6. Dbench throughput > may well be lower, because we probably should be starting writeback > at lower dirty thresholds. > > If you want good dbench numbers: > > echo 70 > /proc/sys/vm/dirty_background_ratio > echo 75 > /proc/sys/vm/dirty_async_ratio > echo 80 > /proc/sys/vm/dirty_sync_ratio > echo 30000 > /proc/sys/vm/dirty_expire_centisecs
That last one looks like the biggest cheat. Rather than optimizing for dbench, is there a set of pessimizing numbers which would optimally turn dbench into a semi-useful tool for measuring meaningful IO performance? Or is dbench really only useful for stress testing?
Thanks for the explanations.
Steven
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