Re: [PATCH net-next 2/2] net: core: increase the default size of GRO_NORMAL skb lists to flush

[Eric Dumazet]

On Sat, Oct 12, 2019 at 2:22 AM Alexander Lobakin <alobakin@xxxxxxxx> wrote:

>
> I've generated an another solution. Considering that gro_normal_batch
> is very individual for every single case, maybe it would be better to
> make it per-NAPI (or per-netdevice) variable rather than a global
> across the kernel?
> I think most of all network-capable configurations and systems has more
> than one network device nowadays, and they might need different values
> for achieving their bests.
>
> One possible variant is:
>
> #define THIS_DRIVER_GRO_NORMAL_BATCH    16
>
> /* ... */
>
> netif_napi_add(dev, napi, this_driver_rx_poll, NAPI_POLL_WEIGHT); /*
> napi->gro_normal_batch will be set to the systcl value during NAPI
> context initialization */
> napi_set_gro_normal_batch(napi, THIS_DRIVER_GRO_NORMAL_BATCH); /* new
> static inline helper, napi->gro_normal_batch will be set to the
> driver-speficic value of 16 */
>
> The second possible variant is to make gro_normal_batch sysctl
> per-netdevice to tune it from userspace.
> Or we can combine them into one to make it available for tweaking from
> both driver and userspace, just like it's now with XPS CPUs setting.
>
> If you'll find any of this reasonable and worth implementing, I'll come
> with it in v2 after a proper testing.

Most likely the optimal tuning is also a function of the host cpu caches.

Building a too big list can also lead to premature cache evictions.

Tuning the value on your test machines does not mean the value will be good
for other systems.

Adding yet another per device value should only be done if you demonstrate
a significant performance increase compared to the conservative value
Edward chose.

Also the behavior can be quite different depending on the protocols,
make sure you test handling of TCP pure ACK packets.

Accumulating 64 (in case the device uses standard NAPI_POLL_WEIGHT)
of them before entering upper stacks seems not a good choice, since 64 skbs
will need to be kept in the GRO system, compared to only 8 with Edward value.