Jan-Bernd Themann wrote:
Hi David
David Miller schrieb:
Interrupt mitigation only works if it helps you avoid interrupts.
This scheme potentially makes more of them happen.
The hrtimer is just another interrupt, a cpu locally triggered one,
but it has much of the same costs nonetheless.
So if you set this timer, it triggers, and no packets arrive, you are
taking more interrupts and doing more work than if you had disabled
NAPI.
In fact, for certain packet rates, your scheme would result in
twice as many interrupts than the current scheme
That depends how smart the driver switches between timer
polling and plain NAPI (depending on load situation).
This is one of several reasons why hardware is the only truly proper
place for this kind of logic. Only the hardware can see the packet
arrive, and do the interrupt deferral without any cpu intervention
whatsoever.
What I'm trying to improve with this approach is interrupt
mitigation for NICs where the hardware support for interrupt
mitigation is limited. I'm not trying to improve this for NICs
that work well with the means their HW provides. I'm aware of
the fact that this scheme has it's tradeoffs and certainly
can not be as good as a HW approach.
So I'm grateful for any ideas that do have less tradeoffs and
provide a mechanism to reduce interrupts without depending on
HW support of the NIC.
In the end I want to reduce the CPU utilization. And one way
to do that is LRO which also works only well if there are more
then just a very few packets to aggregate. So at least our
driver (eHEA) would benefit from a mix of timer based polling
and plain NAPI (depending on load situations).
Wouldn't you achieve the same result by enabling hardware interrupt
mitigation in eHEA in combination with NAPI? Presumably a 10G interface
has hardware mitigation features?
If there is no need for a generic mechanism for this kind of
network adapters, then we can just leave this to each device
driver.
I've been looking at this from a different angle. My goal is to optimize
NAPI packet forwarding rates while minimizing packet latency. Using
hardware interrupt mitigation hurts latency so I'm investigating ways to
turn it off without risking NAPI poll on/off thrashing at certain packet
rates.
Jan-Bernd, I think I've found a solution to the issue that you
highlighted with my scheme yesterday and it doesn't involve generating
other interrupts using hrtimers etc. :) Initial results are very
encouraging in my setups. Would you be willing to test it with eHEA? I
don't have a 10G setup. If results are encouraging, I'll post an RFC to
ask for review / feedback from the NAPI experts here. What do you think?
--
James Chapman
Katalix Systems Ltd
http://www.katalix.com
Catalysts for your Embedded Linux software development
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