Date: Fri, 27 Mar 2009 15:45:43 +0300 From: Dmitriy <_pppp@mail.ru> To: Luigi Rizzo <rizzo@iet.unipi.it> Cc: freebsd-net@freebsd.org, Andrew Brampton <brampton+freebsd-net@gmail.com> Subject: Re[2]: Interrupts + Polling mode (similar to Linux's NAPI) Message-ID: <E1LnBRP-0004FN-00._pppp-mail-ru@f86.mail.ru> In-Reply-To: <20090327111654.GA95104@onelab2.iet.unipi.it> References: <20090327111654.GA95104@onelab2.iet.unipi.it>
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-----Original Message----- From: Luigi Rizzo <rizzo@iet.unipi.it> To: Andrew Brampton <brampton+freebsd-net@gmail.com> Date: Fri, 27 Mar 2009 12:16:54 +0100 Subject: Re: Interrupts + Polling mode (similar to Linux's NAPI) > On Fri, Mar 27, 2009 at 11:05:00AM +0000, Andrew Brampton wrote: > > 2009/3/27 Luigi Rizzo <rizzo@iet.unipi.it>: > > > The load of polling is pretty low (within 1% or so) even with > > > polling. The advantage of having interrupts is faster response > > > to incoming traffic, not CPU load. > > > > oh, I was under the impression that polling spun in a tight loop, thus > > using 100% of the processor. After a quick test I see this is not the > > case. I assume it will get to 100% CPU load if I saturate my network. > > Well the motivation for the original polling code in FreeBSD was > to keep the CPU usage under strict control -- you could set the > max CPU fraction that you wanted to dedicate to packet handling, > and you were guaranteed not to exceed that fraction. Well, polling(4) usually reduces the CPU load. But this is not essential for modern CPUs, except some software-only NICs (namely, Realtek 8139). It provides an average of 0.5ms delay for a packet delivery which is not suitable for many usage patterns, though. > > > There is nothing difficult in having both active, except figuring > > > out a good logic for when to disable polling on an interface > > > that has been quiet for a while. > > > > Looking at Linux's logic, it appears to poll until there are no more > > packets, and thus re-enables interrupts. > > the complete definition should be "no more packets for X seconds". > Enabling and disabling interrupts is slightly expensive so you > don't want to do it too often. I'd rather say "no more than N packets for the recent T seconds". > > cheers > luigi > _______________________________________________ > freebsd-net@freebsd.org mailing list > http://lists.freebsd.org/mailman/listinfo/freebsd-net > To unsubscribe, send any mail to "freebsd-net-unsubscribe@freebsd.org" >
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