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

[Alexander Lobakin]

Hi Edward,

Edward Cree wrote 10.10.2019 21:16:
> On 10/10/2019 15:42, Alexander Lobakin wrote:
> > Commit 323ebb61e32b ("net: use listified RX for handling GRO_NORMAL
> > skbs") have introduced a sysctl variable gro_normal_batch for defining
> > a limit for listified Rx of GRO_NORMAL skbs. The initial value of 8 is
> > purely arbitrary and has been chosen, I believe, as a minimal safe
> > default.
> 8 was chosen by performance tests on my setup with v1 of that patch;
> Âsee https://www.spinics.net/lists/netdev/msg585001.html .
> Sorry for not including that info in the final version of the patch.
> While I didn't re-do tests on varying gro_normal_batch on the final
> Âversion, I think changing it needs more evidence than just "we tested
> Âit; it's better". In particular, increasing the batch size should be
> Âaccompanied by demonstration that latency isn't increased in e.g. a
> Âmulti-stream ping-pong test.
>
> > However, several tests show that it's rather suboptimal and doesn't
> > allow to take a full advantage of listified processing. The best and
> > the most balanced results have been achieved with a batches of 16 skbs
> > per flush.
> > So double the default value to give a yet another boost for Rx path.
>
> > It remains configurable via sysctl anyway, so may be fine-tuned for
> > each hardware.
> I see this as a reason to leave the default as it is; the combination
> Âof your tests and mine have established that the optimal size does
> Âvary (I found 16 to be 2% slower than 8 with my setup), so any
> Âtweaking of the default is likely only worthwhile if we have data
> Âover lots of different hardware combinations.

Agree, if you've got slower results on 16, we must leave the default
value, as it seems to be VERY hardware- and driver- dependent.
So, patch 2/2 is not actual any more (I supposed that it would likely
go away before sending this series).

> > Signed-off-by: Alexander Lobakin <alobakin@xxxxxxxx>
> > ---
> >  net/core/dev.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/net/core/dev.c b/net/core/dev.c
> > index a33f56b439ce..4f60444bb766 100644
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
> > @@ -4189,7 +4189,7 @@ int dev_weight_tx_bias __read_mostly = 1;  /* 
> > bias for output_queue quota */
> >  int dev_rx_weight __read_mostly = 64;
> >  int dev_tx_weight __read_mostly = 64;
> >  /* Maximum number of GRO_NORMAL skbs to batch up for list-RX */
> > -int gro_normal_batch __read_mostly = 8;
> > +int gro_normal_batch __read_mostly = 16;
> >
> >  /* Called with irq disabled */
> >  static inline void ____napi_schedule(struct softnet_data *sd,

Regards,
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