Subject: Is this a scheduler issue?
To: None <>
From: Jeff Rizzo <>
List: tech-kern
Date: 07/13/2006 21:41:14
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I'm currently working on a project in which I'm using ICMP to measure
RTT across a switch fabric over long periods of time; currently sending
about 50 packets per minute between pairs of hosts.  (In the production
deployment, there will be one probe per switch, to look for inter-switch
issues)  I'd really like to use NetBSD for this, as I've got a lovely
build/distribution setup for this that will greatly ease the deployment
of new or replacement systems.   However, the userland portion of the
ping setup periodically shows high RTT spikes - 5-6 times a day, the max
RTT jumps from ~0.5ms to as high as 25 ms. (That is, 5 or 6 packets per
day exhibit this)

Two (identical hardware - soekris net4801) Linux boxes pinging each
other don't exhibit this behavior; neither does a Linux box pinging a
NetBSD host.   nice -20 helps a little, but not much - for whatever
reason, a few times a day (irregular intervals, and I've employed static
ARP to remove that variable) I get these spikes, and I would very much
like not to.  :)

Does anyone have suggestions for something I can do to improve the
situation?  I understand that NetBSD doesn't have any realtime
capability - but I was hoping to get "close enough", as these boxes
aren't doing anything else.  I've already got my build framework set up,
and switching to another OS (especially if that's Linux) will be a
_major_ hassle...  I'm willing to render the system sub-optimal for
"other" work.

This is 3.0_STABLE;  though I have verified that a -current (as of
yesterday) kernel exhibits the same behaviour...


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