Re: [PATCH 0/5] forcedeth: several proposed updates for testing

Jeff Garzik
2007-10-07T11:34:56Z

Ingo Molnar wrote:
> * Jeff Garzik <jeff@garzik.org> wrote:
>
>> * I feel TX NAPI is a useful tool, because it provides an independent TX
>> process control point and system load feedback point.
>> Thus I felt this was slightly superior to tasklets.
>
> /me agrees violently
>
> btw., when i played with this tunable under -rt:
>
> enum {
> NV_OPTIMIZATION_MODE_THROUGHPUT,
> NV_OPTIMIZATION_MODE_CPU
> };
> static int optimization_mode = NV_OPTIMIZATION_MODE_THROUGHPUT;
>
> the MODE_CPU one gave (much) _higher_ bandwidth. The queueing model in
> forcedeth seemed to be not that robust and i think a single queueing
> model should be adopted instead of this tunable. (which i think just hid
> some bug/dependency) But i never got to the bottom of it so it's just
> the impression i got.

That's interesting. It will be informative to narrow down the variables
affected by this. My changes stirred the pot quite a bit :)

* 'throughput' mode enables MSI-X, and separate interrupt vectors for RX
and TX. so, NVIDIA's MSI-X implementation, our generic MSI-X support,
or "Known bugs" (see top of file) may be a factor here.

* 'throughput' mode also changes the NIC's timer interrupt frequency

* do you recall if you were running in NAPI mode? It defaulted to off
in Kconfig, but I turned it on unconditionally.

* I think TX NAPI has the potential to make the optimization_mode
irrelevant (along with the other changes, most notably the interrupt
handling change)

* and overall, yes, if we can have a single queueing model /
optimization mode I am strongly in favor of that.

Testing welcome ;-) Though these patches are raw and "hot off the
presses", so unrelated bugs are practically a certainty. And I am
worrying about the "Known bugs" note at the top. My gut feeling is that
this was, in part, misunderstanding on the part of reverse-engineers,
since corrected when NVIDIA started contributing to the driver.

Jeff