netdev - Re: [PATCH] net: allow configuration of the size of page in __netdev_alloc

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Date:	Wed, 24 Oct 2012 18:43:20 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Ian Campbell <Ian.Campbell@...rix.com>
Cc:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Eric Dumazet <edumazet@...gle.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>
Subject: Re: [PATCH] net: allow configuration of the size of page in
 __netdev_alloc_frag

On Wed, 2012-10-24 at 17:22 +0100, Ian Campbell wrote:
> On Wed, 2012-10-24 at 16:21 +0100, Eric Dumazet wrote:

> > If you really have such problems, why locally generated TCP traffic
> > doesnt also have it ?
> 
> I think it does. The reason I noticed the original problem was that ssh
> to the machine was virtually (no pun intended) unusable.
> 
> > Your patch doesnt touch sk_page_frag_refill(), does it ?
> 
> That's right. It doesn't. When is (sk->sk_allocation & __GFP_WAIT) true?
> Is it possible I'm just not hitting that case?
> 

I hope not. GFP_KERNEL has __GFP_WAIT.

> Is it possible that this only affects certain traffic patterns (I only
> really tried ssh/scp and ping)? Or perhaps its just that the swiotlb is
> only broken in one corner case and not the other.

Could you try a netperf -t TCP_STREAM ?

Because ssh use small packets, and small TCP packets dont use frags but
skb->head.

You mentioned a 70% drop of performance, but what test have you used
exactly ?



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