Date: Wed, 22 Mar 2006 09:53:36 -0800 (PST) From: Arne Woerner <arne_woerner@yahoo.com> To: "Jin Guojun \[VFFS\]" <g_jin@lbl.gov> Cc: freebsd-performance@freebsd.org, Gary Thorpe <gthorpe@myrealbox.com>, oxy@field.hu Subject: Re: packet drop with intel gigabit / marwell gigabit Message-ID: <20060322175336.59008.qmail@web30309.mail.mud.yahoo.com> In-Reply-To: <442187FE.3060300@lbl.gov>
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--- "Jin Guojun [VFFS]" <g_jin@lbl.gov> wrote: >Arne Woerner wrote: >> What did we show by this <<dd if=/dev/zero ...>> test? I thoughtthat >> would prove the memory bandwidth is about 8Gbit/sec(1GByte/sec; >> 2 * <dd's bytes/sec >>number>/2^30). >> > It depends on how you use /dev/zero. > dd of=/dev/null if=/dev/zero bs=4k count=100k > tests cache speed % dd of=/dev/null if=/dev/zero bs=4k count=100k 102400+0 records in 102400+0 records out 419430400 bytes transferred in 0.204511 secs (2050894814 bytes/sec) about 32Gbit/sec? > dd of=/dev/null if=/dev/zero bs=4m count=100 > tests memory bandwidth if your cache is less than 2 MB > % dd of=/dev/null if=/dev/zero bs=4m count=100 100+0 records in 100+0 records out 419430400 bytes transferred in 2.587341 secs (162108677 bytes/sec) about 2.4Gbit/sec? I had an mpeg encoder in the background, when i did those benchmarks... :-) > Now you may give me the real memory bandwidth on your > system :-) > I would expect something around 500. > Hmm... 500Mbit/sec? even if i divide 2.4Gbit by 4, i still get 600Mbit/sec on a quite busy (50%) system... Oh... The docu TV series episode is over now and I re-ran the tests: % dd of=/dev/null if=/dev/zero bs=4m count=100 100+0 records in 100+0 records out 419430400 bytes transferred in 1.513610 secs (277106012 bytes/sec) % dd of=/dev/null if=/dev/zero bs=4k count=500k 512000+0 records in 512000+0 records out 2097152000 bytes transferred in 0.945430 secs (2218199591 bytes/sec) > Notice that your memory copy speed will be one half of it. > Why "half"? dd causes two copies but counts each byte just once... Maybe "dd" in combination with /dev/zero is not the right way to measure memory bandwidth? -Arne __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com
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