Messages in this thread | | | Date | Tue, 3 Feb 2009 15:12:19 +0300 | From | Evgeniy Polyakov <> | Subject | Re: [PATCH v2] tcp: splice as many packets as possible at once |
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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
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