[nycbug-talk] NIC Device Polling in FreeBSD

Pete Wright pete at nomadlogic.org
Mon Oct 19 17:22:57 EDT 2009

On Mon, Oct 19, 2009 at 08:10:52AM -0700, Mark Saad wrote:
> George 
>   Thanks for the info, can you elaborate more on what a good or bad work load
> would be. Is there any rule of thumb like a SQL server is a bad place to use polling,
> and a high traffic nfs server is a good place , or is this more complicated. 
> Also is there any harm in deploying a kernel with Polling enabled but 
> the nic not configured to use it? 

Hi Mark - 

this page may help provide some background though:


""Device polling" (polling for brevity) refers to a technique for
handling devices which does not rely on the latter to generate
interrupts when they need attention, but rather lets the CPU poll
devices to service their needs. "

What this imples though is that you have to make sure your devices
supports polling for it be effective.  In this case you have to make
sure that your network interface supports hardware polling.  If it does
then I believe it would be worth turning on.  Esp for high-throughput
situations.  As I understand it, if you enable polling on an interface
that is not supported then you could potentially introduce a small
amount of network latency, depending upon your polling interval you have


> ----- Original Message ----
> > From: George Neville-Neil <gnn at neville-neil.com>
> > To: Mark Saad <mark.saad at ymail.com>
> > Cc: nycbug <talk at lists.nycbug.org>
> > Sent: Sat, October 17, 2009 3:51:36 PM
> > Subject: Re: [nycbug-talk] NIC Device Polling in FreeBSD
> > 
> > 
> > On Oct 15, 2009, at 17:44 , Mark Saad wrote:
> > 
> > > Hello Talk
> > >  Does anyone know off hand why "options DEVICE_POLLING" is still not in the 
> > GENERIC FreeBSD kernel ?
> > > I understand that it does not work with every card, but is there any issue 
> > with a kernel having polling enabled and the device not using it ?
> > 
> > I believe this is at least in part due to not wanting to surprise users.  Only 
> > certain workloads
> > do well with polling enabled, and so it's considered a tuning nob for experts 
> > not something
> > to be automagically turned on.
> > 
> > Best,
> > George
>  --
> Mark Saad
> mark.saad at ymail.com
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