netdev - Re: TCPBacklogDrops during aggressive bursts of traffic

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Date:	Wed, 23 May 2012 18:12:10 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alexander Duyck <alexander.h.duyck@...el.com>
Cc:	Kieran Mansley <kmansley@...arflare.com>,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
	Ben Hutchings <bhutchings@...arflare.com>,
	netdev@...r.kernel.org
Subject: Re: TCPBacklogDrops during aggressive bursts of traffic

On Wed, 2012-05-23 at 09:04 -0700, Alexander Duyck wrote:
> On 05/23/2012 05:09 AM, Eric Dumazet wrote:
> > On Wed, 2012-05-23 at 11:44 +0200, Eric Dumazet wrote:
> >
> >> I believe that as soon as ixgbe can use build_skb() and avoid the 1024
> >> bytes overhead per skb, it should go away.
> >
> > Here is the patch for ixgbe, for reference.
> I'm confused as to what this is trying to accomplish.
> 
> Currently the way the ixgbe driver works is that we allocate the
> skb->head using netdev_alloc_skb, which after your recent changes should
> be using a head frag.  If the buffer is less than 256 bytes we have
> pushed the entire buffer into the head frag, and if it is more we only
> pull everything up to the end of the TCP header.  In either case if we
> are merging TCP flows we should be able to drop one page or the other
> along with the sk_buff giving us a total truesize addition after merge
> of ~1K for less than 256 bytes or 2K for a full sized frame.
> 
> I'll try to take a look at this today as it is in our interest to have
> TCP performing as well as possible on ixgbe.

With current driver, a MTU=1500 frame uses :

sk_buff (256 bytes)
skb->head : 1024 bytes  (or more exaclty now : 512 + 384)
one fragment of 2048 bytes

At skb free time,  one kfree(sk_buff) and two put_page().

After this patch :

sk_buff (256 bytes)
skb->head : 2048 bytes 

At skb free time, one kfree(sk_buff) and only one put_page().

Note that my patch doesnt change the 256 bytes threshold: Small frames
wont have one fragment and their use is :

sk_buff (256 bytes)
skb->head : 512 + 384 bytes 


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