netdev - Re: GSO/GRO and UDP performance

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Message-ID: <1378495955.31445.52.camel@edumazet-glaptop>
Date:	Fri, 06 Sep 2013 12:32:35 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	James Yonan <james@...nvpn.net>
Cc:	Rick Jones <rick.jones2@...com>, netdev <netdev@...r.kernel.org>
Subject: Re: GSO/GRO and UDP performance

On Fri, 2013-09-06 at 13:26 -0600, James Yonan wrote:

> Where does the 16+1 come from?  I'm getting my 43x from the ratio of max 
> legal IP packet size (64KB) / internet MTU (1500).  Are you saying that 
> GRO cannot aggregate up to 64 KB?
> 

Yes this is what I said.

Hint : MAX_SKB_FRAGS is the number of fragments per skb

Each aggregated frame consumes at least one fragment.

Hint : some drivers uses more than one fragment per datagram.

-> A fragment in skb does not necessarily contains one and exactly one
datagram

> >> I think we cannot aggregate UDP packets, because UDP lacks sequence
> >> numbers, so reorders would be a problem.
> 
> >> You really need something that is not UDP generic.
> 
> Right -- that's why I'm proposing a hook for UDP GSO/GRO providers that 
> know about specific app-layer protocols and can provide segmentation and 
> aggregation methods for them.  Such a provider would be implemented in a 
> kernel module and would know about the specific app-layer protocol, so 
> it would be able to losslessly segment and aggregate it (i.e. it could 
> use a sequence number from the app-layer protocol).

Its not a choice given by application.

As I said you'll have to make sure all the stack will understand the
meaning of datagram aggregation.



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