Re: RFC: issues concerning the next NAPI interface

James Chapman
2007-08-27T17:06:14Z

Jan-Bernd Themann wrote:
> On Monday 27 August 2007 17:51, James Chapman wrote:
>
>> In the second half of my previous reply (which seems to have been
>> deleted), I suggest a way to avoid this problem without using hardware
>> interrupt mitigation / coalescing. Original text is quoted below.
>>
>> >> I've seen the same and I'm suggesting that the NAPI driver keeps
>> >> itself in polled mode for N polls or M jiffies after it sees
>> >> workdone=0. This has always worked for me in packet forwarding
>> >> scenarios to maximize packets/sec and minimize latency.
>>
>> To implement this, there's no need for timers, hrtimers or generic NAPI
>> support that others have suggested. A driver's poll() would set an
>> internal flag and record the current jiffies value when finding
>> workdone=0 rather than doing an immediate napi_complete(). Early in
>> poll() it would test this flag and if set, do a low-cost test to see if
>> it had any work to do. If no work, it would check the saved jiffies
>> value and do the napi_complete() only if no work has been done for a
>> configurable number of jiffies. This keeps interrupts disabled longer at
>> the expense of many more calls to poll() where no work is done. So
>> critical to this scheme is modifying the driver's poll() to fastpath the
>> case of having no work to do while waiting for its local jiffy count to
>> expire.
>>
>
> The problem I see with this approach is that the time that passes between
> two jiffies might be too long for 10G ethernet adapters.

Why would staying in polled mode for 2 jiffies be too long in the 10G
case? I don't see why 10G makes any difference. Your poll() would be
called as fast as your CPU allows during those 2 jiffies (it would
actually be between 1 and 2 jiffies in practice). It is therefore
critical that the driver's poll() implementation is as efficient as
possible for the "no work" case to minimize the overhead of the extra
poll() calls. Your poll might be called thousands of times in 1-2
jiffies with nothing to do...

> (I tried to implement
> a timer based approach with usual timers and the result was a disaster).
> HW interrupts / or HP timer avoid the jiffy problem as they activate softIRQs
> as soon as you call netif_rx_schedule.

My scheme doesn't use timers to do netif_rx_schedule() because the
device stays in polled mode for 1-2 jiffies _after_ it detects it has no
more work. So the device remains scheduled, processing packets as usual.
The device deschedules itself and re-enables its interrupts only when it
has a period of 1-2 jiffies of doing no work.

BTW, I chose 2 jiffies in the example patch just to keep the patch
simple. It might be more for systems with large HZ or those that want to
be even more aggressive at staying in polled mode. I envisage it being
another parameter that can be tweaked using ethtool if people see a
benefit of this scheme.

--
James Chapman
Katalix Systems Ltd
http://www.katalix.com
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