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Date: Wed, 22 Mar 2006 09:23:10 -0800
From: "Jin Guojun [VFFS]" <g_jin@lbl.gov>
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To: Arne Woerner <arne_woerner@yahoo.com>
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Cc: freebsd-performance@freebsd.org, Gary Thorpe <gthorpe@myrealbox.com>,
	oxy@field.hu
Subject: Re: packet drop with intel gigabit / marwell gigabit
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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