On Tue, May 11, 2010 at 10:16:13PM -0400, Martin Langhoff wrote: > What you are saying is that this is a different issue from what dsd > tracked down -- entirely possible that these are 2 orthogonal issues.
Yes, entirely possible. > Does the frequency and/or std deviation change if you tweak the > vm.dirty_* sysctl values? Yes, significantly. What I've observed is most likely entirely due to vm.dirty_* behaviour. The default on os123 is vm.dirty_background_bytes=0 vm.dirty_background_ratio=10 vm.dirty_bytes=0 vm.dirty_expire_centisecs=3000 vm.dirty_ratio=20 vm.dirty_writeback_centisecs=500 Changing this to vm.dirty_background_ratio=90 vm.dirty_expire_centisecs=60000 vm.dirty_ratio=90 vm.dirty_writeback_centisecs=0 causes the dd to finish in 7 seconds instead of 92 seconds, the sync to take much longer, and the write latencies are much smaller. So the effect I was seeing was documented ... "process which is generating disk writes will itself start writing out dirty data.", per vm.txt in Documentation/ -- James Cameron http://quozl.linux.org.au/ _______________________________________________ Devel mailing list [email protected] http://lists.laptop.org/listinfo/devel
