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

[Thomas Monjalon <thomas.monjalon@6wind.com>]

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?


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