Re: [Qemu-devel] interrupt mitigation for e1000

[Avi Kivity <avi@redhat.com>]

On 07/25/2012 12:56 PM, Luigi Rizzo wrote:

>> Indeed.  But please drop the #ifdef MITIGATIONs.
> 
> Thanks for the comments. The #ifdef block MITIGATION was only temporary to
> point out the differences and run the performance comparisons.

Ok.  In a patch, the '+' in front of a line serves that, and I usually
just check out the previous version to run a performance comparison.

> Similarly, the magic thresholds below will be replaced with
> appropriately commented #defines.
> 
> Note:
> On the real hardware interrupt mitigation is controlled by a total of four
> registers (TIDV, TADV, RIDV, RADV) which control it with a granularity
> of 1024ns , see
> 
> http://www.intel.com/content/dam/doc/manual/pci-pci-x-family-gbe-controllers-software-dev-manual.pdf
> 
> An exact emulation of the feature is hard, because the timer resolution we
> have is much coarser (in the ms range).

No, timers have ns precision in Linux.

> So i am inclined to use a different
> approach, similar to the one i have implemented, namely:
> - the first few packets (whether 1 or 4 or 5 will be decided on the host)
>   report an interrupt immediately;
> - subsequent interrupts are delayed through qemu_bh_schedule_idle()
>   (which is unpredictable but efficient; i tried qemu_bh_schedule()
>   but it completely defeats mitigation)
> - when the TX or RX rings are close to getting full, then again
>   an interrupt is delivered immediately.
> 
> This approach also has the advantage of not requiring specific support
> in the OS drivers.
>

But the disadvantage, that if a guest explicitly chooses not to use
interrupt mitigation, in order to reduce latency, then that choice is
ignored.

We should follow the hardware as closely as possibly (but no closer).

-- 
error compiling committee.c: too many arguments to function