Date: Wed, 22 Mar 2006 09:23:10 -0800 From: "Jin Guojun [VFFS]" <g_jin@lbl.gov> To: Arne Woerner <arne_woerner@yahoo.com> Cc: freebsd-performance@freebsd.org, Gary Thorpe <gthorpe@myrealbox.com>, oxy@field.hu Subject: Re: packet drop with intel gigabit / marwell gigabit Message-ID: <442187FE.3060300@lbl.gov> In-Reply-To: <20060322071023.70808.qmail@web30305.mail.mud.yahoo.com> References: <20060322071023.70808.qmail@web30305.mail.mud.yahoo.com>
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Arne Woerner wrote:
--- "Jin Guojun [VFFS]" [1]<g_jin@lbl.gov> wrote:
In you example:
Now your 1.6 GB/s reduced to 16MB/s or even worse just based
on this factor.
What did we show by this <<dd if=/dev/zero ...>> test? I thought
that 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=4m count=100
tests memory bandwidth if your cache is less than 2 MB
Now you may give me the real memory bandwidth on your system :-)
I would expect something around 500.
Notice that your memory copy speed will be one half of it.
/dev/null device really does nothing beside throwing away data.
That is, it can be counted as a cost for system call.
-Jin
References
1. mailto:g_jin@lbl.gov
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