Re: RFC: issues concerning the next NAPI interface

Jan-Bernd Themann
2007-08-27T16:14:37Z

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. (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.