Stephen Hemminger wrote:
On Fri, 24 Aug 2007 17:47:15 +0200
Jan-Bernd Themann <[email protected]> wrote:
Hi,
On Friday 24 August 2007 17:37, [email protected] wrote:
On Fri, Aug 24, 2007 at 03:59:16PM +0200, Jan-Bernd Themann wrote:
.......
3) On modern systems the incoming packets are processed very fast. Especially
on SMP systems when we use multiple queues we process only a few packets
per napi poll cycle. So NAPI does not work very well here and the interrupt
rate is still high. What we need would be some sort of timer polling mode
which will schedule a device after a certain amount of time for high load
situations. With high precision timers this could work well. Current
usual timers are too slow. A finer granularity would be needed to keep the
latency down (and queue length moderate).
We found the same on ia64-sn systems with tg3 a couple of years
ago. Using simple interrupt coalescing ("don't interrupt until
you've received N packets or M usecs have elapsed") worked
reasonably well in practice. If your h/w supports that (and I'd
guess it does, since it's such a simple thing), you might try
it.
I don't see how this should work. Our latest machines are fast enough that they
simply empty the queue during the first poll iteration (in most cases).
Even if you wait until X packets have been received, it does not help for
the next poll cycle. The average number of packets we process per poll queue
is low. So a timer would be preferable that periodically polls the
queue, without the need of generating a HW interrupt. This would allow us
to wait until a reasonable amount of packets have been received in the meantime
to keep the poll overhead low. This would also be useful in combination
with LRO.
You need hardware support for deferred interrupts. Most devices have it (e1000, sky2, tg3)
and it interacts well with NAPI. It is not a generic thing you want done by the stack,
you want the hardware to hold off interrupts until X packets or Y usecs have expired.
Does hardware interrupt mitigation really interact well with NAPI? In my
experience, holding off interrupts for X packets or Y usecs does more
harm than good; such hardware features are useful only when the OS has
no NAPI-like mechanism.
When tuning NAPI drivers for packets/sec performance (which is a good
indicator of driver performance), I make sure that the driver stays in
NAPI polled mode while it has any rx or tx work to do. If the CPU is
fast enough that all work is always completed on each poll, I have the
driver stay in polled mode until dev->poll() is called N times with no
work being done. This keeps interrupts disabled for reasonable traffic
levels, while minimizing packet processing latency. No need for hardware
interrupt mitigation.
The parameters for controlling it are already in ethtool, the issue is finding a good
default set of values for a wide range of applications and architectures. Maybe some
heuristic based on processor speed would be a good starting point. The dynamic irq
moderation stuff is not widely used because it is too hard to get right.
I agree. It would be nice to find a way for the typical user to derive
best values for these knobs for his/her particular system. Perhaps a
tool using pktgen and network device phy internal loopback could be
developed?
--
James Chapman
Katalix Systems Ltd
http://www.katalix.com
Catalysts for your Embedded Linux software development
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