I have done at work modification to the polling
code to do SMP polling (previously posted to this ml).
SMP polling (dynamic group of interface binded to
CPU) does not significantly improve the throughput (lock
contention seems to be the cause here).
The main advantage of polling with modern
interface is not the PPS (which is nearly the same) but the
global efficiency of the system when using multiple
interfaces (which is the case for Firewall).
The best configuration we have found with FreeBSD
6.3 is to do polling on one CPU and keep the other CPU free
for other processing. In this configuration the whole system
is more efficient than with interrupt where all
the CPU are busy processing interrupt thread.
out of curiosity: did you try polling on 4.x? i know
it doesn't "support" SMP over there, but last
time i tried polling on 7.x (or was it 6.x? i don't
remember...)
i found it didn't gave any benefit, while
switching the system to 4.x showed a huge improvement.
yes rewriting the core polling code started at half because
the polling code on 6.x and up perform badly (in our env)
regarding performance.
today 4.x is unbeatable regarding network perf (6.2 ->
7.0 at least, i need to do more test on 7_stable and 8).
the other half of the work was to explore the SMP scaling
of the polling code to gain what we loose with fine grained
SMP kernel.
The problem with all of this "analysis" is that it assumes that SMP
coding scales intuitively; when the opposite is actually true.
What you fail to address is the basic fact that moderated interrupts
(ie holding off interrupts to a set number of ints/second) is exactly
the same as polling, as on an active system you'll get exactly X
interrupts per second at equal intervals. So all of this chatter about
polling being more efficient is simply bunk.
I agree with you with one interface. When you use ten interface it is
not the case.
The truth is that polling requires additional overhead to the system
while
interrupts do not. So if polling did better for you, its simply
because
either
1) The polling code in the driver is better
or
2) You tuned polling better than you tuned interrupt moderation.
Barney
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