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 queues.
Robert N M Watson
Computer Laboratory
University of Cambridge
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