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From: Don Bowman <don@sandvine.com>
To: 'Terry Lambert' <tlambert2@mindspring.com>,
	"Michael O. Boev" <mike@tric.tomsk.gov.ru>
Date: Thu, 9 Oct 2003 11:04:14 -0400 
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cc: freebsd-current@FreeBSD.ORG
Subject: RE: Why is em nic generating interrupts?
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From: Terry Lambert [mailto:tlambert2@mindspring.com]
> "Michael O. Boev" wrote:
> > I've got a [uniprocessor 5.1-RELEASE] router machine with 
> fxp and em nics.
> > I've built my kernel with the following included:
> > 
> > options         DEVICE_POLLING
> > options         HZ=2500
> > 
> > and enabled polling in /etc/sysctl.conf.
> [ ... ]
> > What's happening? Is polling working in my case?
> > If yes, why is vmstat showing interrupts? I see clearly,
> > that fxp's counter doesn't increase, and em's is constantly growing.
> > 
> > Is there anyone who knows for sure that em's polling works?
> 
> You may want to ask Luigi; polling is his code.
> 
> However, I believe the issue is that polling doesn't start
> until you take an interrupt, and it stops as soon as there is
> no more data to process, and waits for the next interrupt.
> 
> If you were to jack your load way up, you would probably see
> an increase in interrupts, then them dropping off dramatically.
> 
> If all else fails, read the source code... 8-).

FWIW, this works for me with 4.7. As terry says, you
do see a couple of initial interrupts, as below, and
then they stop.

$ vmstat -i
interrupt                   total       rate
em0 irq16                       4          0
em1 irq17                       4          0
ahc0 irq18                   5653          0
ahc1 irq19                     15          0
em2 irq20                       4          0
mux irq21                       3          0
sio0 irq4                    1583          0
sio1 irq3                       1          0
clk irq0                 14545633       2501
rtc irq8                   744224        128
Total                    15297124       2631