Re: RFC: issues concerning the next NAPI interface

Jan-Bernd Themann
2007-08-28T11:49:14Z

On Tuesday 28 August 2007 11:22, James Chapman wrote:
> > So in this scheme what runs ->poll() to process incoming packets?
> > The hrtimer?
>
> No, the regular NAPI networking core calls ->poll() as usual; no timers
> are involved. This scheme simply delays the napi_complete() from the
> driver so the device stays in the poll list longer. It means that its
> ->poll() will be called when there is no work to do for 1-2 jiffies,
> hence the optimization at the top of ->poll() to efficiently handle that
> case. The device's ->poll() is called by the NAPI core until it has
> continuously done no work for 1-2 jiffies, at which point it finally
> does the netif_rx_complete() and re-enables its interrupts.
>
I'm not sure if I understand your approach correctly.
This approach may reduce the number of interrupts, but it does so
by blocking the CPU for up to 1 jiffy (that can be quite some time
on some platforms). So no other application / tasklet / softIRQ type
can do anything in between. The CPU utilization does not drop at all,
and I thought that is one reason why we try to reduce the number of interrupts.

> If people feel that holding the device in the poll list for 1-2 jiffies
> is too long (because there are too many wasted polls), a counter could
> be used to to delay the netif_rx_complete() by N polls instead. N would
> be a value depending on CPU speed. I use the jiffy sampling method
> because it results in some natural randomization of the actual delay
> depending on when the jiffy value was sampled in relation to the jiffy tick.
>

Waiting for N polls seems to make no sense if there are no further network adapters
in that machine. It would take no time to call poll N times in a row when no
new packets arrive. There is no real delay as the net_rx_action function will
do nothing else between the poll calls.

Please correct me if I'm wrong.

Regards,
Jan-Bernd