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Date:      Wed, 5 Feb 2003 10:20:50 +0100
From:      "Jens Wiggers" <>
To:        <hackers@FreeBSD.ORG>, <freebsd-net@FreeBSD.ORG>
Subject:   Correlation between CPU load and network usage
Message-ID:  <000001c2ccf7$dfcdcc70$1d01a8c0@iis2>

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I'm using two FreeBSD machines in an small switched network. One of them
blows out tcp packets as fast as possible for five seconds. The other
machine just receives the packets and does nothing else.

What I see is that for short packets the senders CPU load is just 100
percent, but for larger packets the senders CPU load drops down rapidly.

>From the applications point of view the data to send gets into the
socket buffer and via tcp_output, ip_output and ether_output into the
interface buffer. The rest of work is done by the interrupt handler of
the network controller.

Therefore the application returns from the socket send syscall and calls
it again. So where is sparetime for the CPU in this cycle?

Any help would be appreciated.


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