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Date:	Thu, 24 May 2012 19:32:09 +0200
From:	Jesper Dangaard Brouer <>
To:	Hans Schillstrom <>
Cc:	Eric Dumazet <>,
	David Miller <>,
	Martin Topholm <>, netdev <>
Subject: Re: [RFC PATCH] tcp: Fast/early SYN handling to mitigate SYN floods

On Thu, 2012-05-24 at 15:20 +0200, Hans Schillstrom wrote:
> Hi Jesper
> We are also working with this issue right now,
> > [RFC PATCH] tcp: Fast/early SYN handling to mitigate SYN floods
> > 
> > TCP SYN handling is on the slow path via tcp_v4_rcv(), and is
> > performed while holding spinlock bh_lock_sock().
> > 
> > Real-life and testlab experiments show, that the kernel choks
> > when reaching 130Kpps SYN floods (powerful Nehalem 16 cores).
> > Measuring with perf reveals, that its caused by
> > bh_lock_sock_nested() call in tcp_v4_rcv().
> I can confirm this too, and it doesn't scale with more cores
> > 
> > With this patch, the machine can handle 750Kpps (max of the SYN
> > flood generator) with cycles to spare.
> This looks great.

Yes, its definitely shows that there is huge performance gain hidden
here! But we still have to handle locking (which will affect perf).

> I'm also working with a solution that not trash conntack
> i.e. have conntrack working during a heavy SYN attack

Sounds interesting, but that's a separate problem.  In this case I have
disabled conntracking (I even disabled flow-control and drop the syn-ack
responses on the generator).

Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Sr. Network Kernel Developer at Red Hat
  Author of

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