Re: [PATCH v2] tcp: splice as many packets as possible at once

Evgeniy Polyakov
2009-02-03T12:12:46Z

On Tue, Feb 03, 2009 at 05:05:14AM -0800, david@lang.hm (david@lang.hm) wrote:
> >Maybe just do not allow jumbo frames when memory is fragmented enough
> >and fallback to the smaller MTU in this case? With LRO/GRO stuff there
> >should be not that much of the overhead compared to multiple-page
> >copies.
>
>
> 1. define 'fragmented enough'

When allocator can not provide requested amount of data.

> 2. the packet size was already negotiated on your existing connections,
> how are you going to change all those on the fly?

I.e. MTU can not be changed on-flight? Magic world.

> 3. what do you do when a remote system sends you a large packet? drop it
> on the floor?

We already do just that when jumbo frame can not be allocated :)

> having some pool of large buffers to receive into (and copy out of those
> buffers as quickly as possible) would cause a performance hit when things
> get bad, but isn't that better than dropping packets?

It is a solution, but I think it will behave noticebly worse than
with decresed MTU.

> as for the number of buffers to use. make a reasonable guess. if you only
> have a small number of packets around, use the buffers directly, as you
> use more of them start copying, as useage climbs attempt to allocate more.
> if you can't allocate more (and you have all of your existing ones in use)
> you will have to drop the packet, but at that point are you really in any
> worse shape than if you didn't have some mechanism to copy out of the
> large buffers?

That's the main point: how to deal with broken hardware? I think (but
have no strong numbers though) that having 6 packets with 1500 MTU
combined into GRO/LRO frame will be processed way faster than copying 9k
MTU into 3 pages and process single skb.

--
Evgeniy Polyakov