Re: jitter bursts

[Michael Olbrich <m.olbrich@pengutronix.de>]

On Fri, Aug 31, 2007 at 09:25:08PM +0530, Girish kathalagiri wrote:
> We also had similar problems when where we face these kind of periodic
> jitter. In our case we used Intel 82551ER Fast Ethernet Controller(
> e100 driver ). The problem with ours was the  with the microcode (in
> e100.c) of the adapter which had default algorithm where the hardware
> was holding of the interrupts until X packets or Y usecs have expired.
> 
> This generated a periodic jitter that was 400-500 us more than the usual one.
> This was solved by making the hardware generate interrupt on every frame.
> In e100.c this can be done by changing the values of INTDELAY,
> BUNDDLEMAX and BUNDDLESMALL (CPU saver parameters)

Unfortunately not but it did get me on the right track. The problem is
caused by the e100_watchdog function. As far as I can tell e100_watchdog
is called from a timer queue and therefore runs with the priority of
that thread. And while I set higher priorities for softirq-net-rx and
the application I missed the thread for the hardware interrupt of the
network interface. So whenever a packet arived while e100_watchdog was
running it got delayed. The periodic pattern is a result of this and
speed difference of sender and receiver clocks.
The jitter is now <50us but still with some periodic patterns. So I'll
need to investigate further.

michael

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