Re: [dpdk-stable] [PATCH 1/2] net/virtio: fix performance regression due to TSO enabling

[Yuanhan Liu <yuanhan.liu@linux.intel.com>]

On Wed, Jan 11, 2017 at 03:51:22PM +0100, Thomas Monjalon wrote:
> 2017-01-11 12:27, Yuanhan Liu:
> > The fact that virtio net header is initiated to zero in PMD driver
> > init stage means that these costly writes are unnecessary and could
> > be avoided:
> > 
> >     if (hdr->csum_start != 0)
> >         hdr->csum_start = 0;
> > 
> > And that's what the macro ASSIGN_UNLESS_EQUAL does. With this, the
> > performance drop introduced by TSO enabling is recovered: it could
> > be up to 20% in micro benchmarking.
> 
> This patch is adding a condition to assignments.
> We need a benchmark on other architectures like ARM. Please anyone?

I think the cost of condition should be way lower than the cost from the
penalty introduced by the cache issue, that I don't see it would perform
bad on other platforms.

But, of course, testing is always welcome!

	--yliu
> 
> 
> [...]
> > +/* avoid write operation when necessary, to lessen cache issues */
> > +#define ASSIGN_UNLESS_EQUAL(var, val) do {	\
> > +	if ((var) != (val))			\
> > +		(var) = (val);			\
> > +} while (0)