Re: [PATCH net-next] tcp: reduce memory needs of out of order queue

[Eric Dumazet <eric.dumazet@gmail.com>]

Le vendredi 14 octobre 2011 à 19:18 -0400, David Miller a écrit :
> From: Rick Jones <rick.jones2@hp.com>
> Date: Fri, 14 Oct 2011 15:12:04 -0700
> 
> > From just a very quick look it looks like tcp_v[46]_rcv is called,
> > finds that the socket is owned by the user, attempts to add to the
> > backlog, but the path called by sk_add_backlog does not seem to make
> > any attempts to compress things, so when the quantity of data is <<
> > the truesize it starts tossing babies out with the bathwater.
> 
> This is why I don't believe the right fix is to add bandaids all
> around the TCP layer.
> 
> The wastage has to be avoided at a higher level.

We cant do that at higher level without smart hardware (like NIU) or
adding a copy.

Its a tradeoff between space and speed.

Most drivers have to allocate a large skb1 and post it to hardware to
receive a frame (Unknown length, only max length is known)

Some drivers have a copybreak feature, doing a copy of small incoming
frames into a smaller skb2 (skb2->truesize < skb1->truesize)

This strategy do save memory for small frames, not for 1500 bytes
frames.

I think the problem is in TCP layer (and maybe in other protocols) :

1) Either tune rcvbuf to allow more memory to be used, for a particular
tcp window,

   Or lower TCP window to allow less packets in flight for a given
rcvbuf.

2) TCP COLLAPSE already is trying to reduce memory costs of a tcp socket
with many packets in OFO queue. But fixing 1) would make these collapses
never happen in the first place. People wanting high TCP bandwidth
[ with say more than 500 in-flight packets per session ] can certainly
afford having enough memory.