From: Eric Dumazet <eduma...@google.com>
Date: Tue, 12 Jan 2021 13:32:56 +0100
> On Tue, Jan 12, 2021 at 11:56 AM Alexander Lobakin <aloba...@pm.me> wrote:
>>
>
>>
>> Ah, I should've mentioned that I use UDP GRO Fraglists, so these
>> numbers are for GRO.
>>
>
> Right, this suggests UDP GRO fraglist is a pathological case of GRO,
> not saving memory.
>
> Real GRO (TCP in most cases) will consume one skb, and have page
> fragments for each segment.
>
> Having skbs linked together is not cache friendly.
OK, so I rebased test setup a bit to clarify the things out.
I disabled fraglists and GRO/GSO fraglists support advertisement
in driver to exclude any "pathological" cases and switched it
from napi_get_frags() + napi_gro_frags() to napi_alloc_skb() +
napi_gro_receive() to disable local skb reusing (napi_reuse_skb()).
I also enabled GSO UDP L4 ("classic" one: one skbuff_head + frags)
for forwarding, not only local traffic, and disabled NF flow offload
to increase CPU loading and drop performance below link speed so I
could see the changes.
So, the traffic flows looked like:
- TCP GRO (one head + frags) -> NAT -> hardware TSO;
- UDP GRO (one head + frags) -> NAT -> driver-side GSO.
Baseline 5.11-rc3:
- 865 Mbps TCP, 866 Mbps UDP.
This patch (both separate caches and Edward's unified cache):
- 899 Mbps TCP, 893 Mbps UDP.
So that's cleary *not* only "pathological" UDP GRO Fraglists
"problem" as TCP also got ~35 Mbps from this, as well as
non-fraglisted UDP.
Regarding latencies: I remember there were talks about latencies when
Edward introduced batched GRO (using linked lists to pass skbs from
GRO layer to core stack instead of passing one by one), so I think
it's a perennial question when it comes to batching/caching.
Thanks for the feedback, will post v2 soon.
The question about if this caching is reasonable isn't closed anyway,
but I don't see significant "cons" for now.
> So I would try first to make this case better, instead of trying to
> work around the real issue.
Al
