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

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Date:	Fri, 14 Oct 2011 18:00:52 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Rick Jones <rick.jones2@...com>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH net-next] tcp: reduce memory needs of out of order queue

Le vendredi 14 octobre 2011 à 08:50 -0700, Rick Jones a écrit :

> Is the wireless problem strictly a wireless problem?  Many of the 
> drivers where Eric has been fixing the truesize accounting have been 
> wired devices no?

Yes, but the goal of such fixes it to make bugs happen too with said
wired devices ;)

About WIFI, I get these TCP Collapses on two different machines, one
using drivers/net/wireless/rt2x00 driver

Extract from drivers/net/wireless/rt2x00/rt2x00queue.h

/**
 * DOC: Entry frame size
 * 
 * Ralink PCI devices demand the Frame size to be a multiple of 128 bytes,
 * for USB devices this restriction does not apply, but the value of
 * 2432 makes sense since it is big enough to contain the maximum fragment
 * size according to the ieee802.11 specs. 
 * The aggregation size depends on support from the driver, but should
 * be something around 3840 bytes.
 */
#define DATA_FRAME_SIZE         2432
#define MGMT_FRAME_SIZE         256
#define AGGREGATION_SIZE        3840

You understand why we endup using skb->truesize > 4096 buffers 

I liked doing the copybreak only if needed, I found the OFO case was
 most of the time responsible of the Collapses.

Now we also could do the copybreak for frames queued into regular
 receive_queue, if current wmem_alloc is above 25% of rcvbuf space...

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