cvs commit: src/sys/dev/bge if_bge.c
oleg at FreeBSD.org
Sat Dec 23 14:33:49 PST 2006
On Sat, Dec 23, 2006 at 09:36:33PM +0000, Robert Watson wrote:
> On Sat, 23 Dec 2006, John Polstra wrote:
> >>That said, dropping and regrabbing the driver lock in the rxeof routine
> >>of any driver is bad. It may be safe to do, but it incurs horrible
> >>performance penalties. It essentially allows the time-critical, high
> >>priority RX path to be constantly preempted by the lower priority
> >>if_start or if_ioctl paths. Even without this preemption and priority
> >>inversion, you're doing an excessive number of expensive lock ops in the
> >>fast path.
> >We currently make this a lot worse than it needs to be by handing off the
> >received packets one at a time, unlocking and relocking for every packet.
> >It would be better if the driver's receive interrupt handler would harvest
> >all of the incoming packets and queue them locally. Then, at the end, hand
> >off the linked list of packets to the network stack wholesale, unlocking
> >and relocking only once. (Actually, the list could probably be handed off
> >at the very end of the interrupt service routine, after the driver has
> >already dropped its lock.) We wouldn't even need a new primitive, if
> >ether_input() and the other if_input() functions were enhanced to deal
> >with a possible list of packets instead of just a single one.
> I try this experiement every few years, and generally don't measure much
> improvement. I'll try it again with 10gbps early next year once back in
> the office again. The more interesting transition is between the link
> layer and the network layer, which is high on my list of topics to look
> into in the next few weeks. In particular, reworking the ifqueue handoff.
> The tricky bit is balancing latency, overhead, and concurrency...
> FYI, there are several sets of patches floating around to modify if_em to
> hand off queues of packets to the link layer, etc. They probably need
> updating, of course, since if_em has changed quite a bit in the last year.
> In my implementaiton, I add a new input routine that accepts mbuf packet
I'm just curious, do you remember average length of mbuf queue in your
tests? While experimenting with bge(4) driver (taskqueue, interrupt moderation,
converted bge_rxeof() to above scheme), i've found it's quite easy to exhaust
available mbuf clusters under load (trying to queue hundreids of received
packets). So i had to limit rx queue to rather low length.
> Robert N M Watson
> Computer Laboratory
> University of Cambridge
=== Oleg Bulyzhin -- OBUL-RIPN -- OBUL-RIPE -- oleg at rinet.ru ===
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