netdev - Re: [PATCH v2 net-next] tcp: avoid expensive pskb_expand

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Date:	Thu, 19 Apr 2012 20:00:36 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Rick Jones <rick.jones2@...com>
Cc:	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>,
	Neal Cardwell <ncardwell@...gle.com>,
	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Tom Herbert <therbert@...gle.com>,
	Maciej Żenczykowski <maze@...gle.com>,
	Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [PATCH v2 net-next] tcp: avoid expensive pskb_expand_head()
 calls

On Thu, 2012-04-19 at 10:48 -0700, Rick Jones wrote:
> On 04/19/2012 10:25 AM, Eric Dumazet wrote:
> > On Thu, 2012-04-19 at 10:20 -0700, Rick Jones wrote:
> >
> >> By copying them to smaller buffers? Or just by altering truesize?
> >> Wasn't the whole point of fixing all the broken truesize settings to
> >> accurately account for memory consumed?
> >
> > I checked, their truesize are OK (1024+256) for ixgbe driver.
> > They could be little smaller, but not that much. (512 + 256)
> >
> > No, its only the sk_rcvbuf is small for a tcp sender,
> > and sk_add_backlog() makes sure we dont queue more than sk_rcvbuf
> > bytes in backlog.
> 
> Sounds like a variation on the theme of wildly divergent 
> inbound/outbound bandwidth and constraining ACKs constraining throughput 
> - only with buffer sizes.
> 
> 87380 is the default SO_RCVBUF right?  That should have allowed 
> 87380/1280 or 68 ACKs to be queued.  Without ACK stretching from GRO 
> that should have covered 68 * 2896 or 196928 bytes.  Based on your 
> previous 54 usec to transmit 64 KB that would be 162+ usecs to 
> accumulate those ACKs, so I guess a question becomes if TCP can be 
> held-off processing ACKs for > 162 usecs, and if so and that cannot be 
> changed, the autotuning will have to start increasing SO_SNDBUF 
> alongside so_sndbuf even if the endpoint is not receiving.  As a 
> handwave, since TCP does not know the buffer size(s) used by the driver, 
> by 1 MSS for every 2 MSS it adds to SO_SNDBUF.  Or find some way to do 
> it "on demand" in the about to drop path.
> 
> That or bare ACKs have to be excluded from the overhead checks somehow I 
> guess, or there be more aggressive copying into smaller buffers.
> 

Nope, we need a limit or risk OOM if a malicious peer send ACK flood ;)

To be clear, if I change the tcp_rmem[1] from 87380 to big value, I no
longer have ACK drops, but still poor performance, I am still
investigating.


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