On Tue, May 31, 2011 at 3:47 PM, <m...@freebsd.org> wrote: > On Tue, May 31, 2011 at 2:48 PM, Pieter de Goeje <pie...@degoeje.nl> wrote: >> On Sunday 29 May 2011 05:01:57 m...@freebsd.org wrote: >>> On Sat, May 28, 2011 at 12:03 PM, Pieter de Goeje <pie...@degoeje.nl> wrote: >>> > To me it looks like it's not able to cache the zeroes anymore. Is this >>> > intentional? I tried to change ZERO_REGION_SIZE back to 64K but that >>> > didn't help. >>> >>> Hmm. I don't have access to my FreeBSD box over the weekend, but I'll >>> run this on my box when I get back to work. >>> >>> Meanwhile you could try setting ZERO_REGION_SIZE to PAGE_SIZE and I >>> think that will restore things to the original performance. >> >> Indeed it does. I couldn't find any authoritative docs stating wether or not >> the cache on this CPU is virtually indexed, but apparently at least some of >> it is. > > On my physical box (some Dell thing from about 2008), I ran 10 loops > of dd if=/dev/zero of=/dev/null bs=XX count=XX where bs went by powers > of 2 from 512 bytes to 2M, and count was set so that the dd always > transferred 8GB. I compared ZERO_REGION_SIZE of 64k and 2M on amd64. > > The summary of the ministat(1) output is: > > bs=512b - no difference > bs=1K - no difference > bs=2k - no difference > bs=4k - no difference > bs=8k - no difference > bs=16k - no difference > bs=32k - no difference > bs=64k - no difference > bs=128k - 2M is 0.69% faster > bs=256k - 2M is 0.98% faster > bs=512k - 2M is 0.65% faster > bs=1M - 2M is 1.02% faster > bs=2M - 2M is 2.17% slower > > I'll play again with a 4K buffer.
The data is harder to parse precisely, but in general it looks like on my box using a 4K buffer results in significantly worse performance when the dd(1) block size is larger than 4K. How much worse depends on the block size, but it goes from 6% at bs=8k to 17% at bs>=256k. Showing 4k/64k/2M ZERO_REGION_SIZE graphically in the ministat(1) output also makes it clear that the difference between 64k and 2M is nearly insignificant on my box compared to using 4k. http://people.freebsd.org/~mdf/zero-ministat.txt Cheers, matthew _______________________________________________ svn-src-all@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/svn-src-all To unsubscribe, send any mail to "svn-src-all-unsubscr...@freebsd.org"
