netdev - Re: [PATCH] NET_DMA: free skbs periodically

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Message-Id: <20100318.203644.102533317.davem@davemloft.net>
Date:	Thu, 18 Mar 2010 20:36:44 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	steve@...idescorp.com
Cc:	netdev@...r.kernel.org, kuznet@....inr.ac.ru, pekkas@...core.fi,
	jmorris@...ei.org, yoshfuji@...ux-ipv6.org, kaber@...sh.net,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] NET_DMA: free skbs periodically

From: "Steven J. Magnani" <steve@...idescorp.com>
Date: Tue, 16 Mar 2010 10:22:44 -0500

> Under NET_DMA, data transfer can grind to a halt when userland issues a
> large read on a socket with a high RCVLOWAT (i.e., 512 KB for both).
> This appears to be because the NET_DMA design queues up lots of memcpy 
> operations, but doesn't issue or wait for them (and thus free the 
> associated skbs) until it is time for tcp_recvmesg() to return. 
> The socket hangs when its TCP window goes to zero before enough data is 
> available to satisfy the read.
> 
> Periodically issue asynchronous memcpy operations, and free skbs for ones 
> that have completed, to prevent sockets from going into zero-window mode.
> 
> Signed-off-by: Steven J. Magnani <steve@...idescorp.com>

Can one of the NET DMA folks review this.

It's a pretty fundamental problem with how this stuff works
it seems.
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