netdev - Re: Low performance Intel 10GE NIC (3.2.10) on 2.6.38 Kernel

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Message-ID: <1302194227.3357.52.camel@edumazet-laptop>
Date:	Thu, 07 Apr 2011 18:37:07 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alexander Duyck <alexander.h.duyck@...el.com>
Cc:	Wei Gu <wei.gu@...csson.com>, netdev <netdev@...r.kernel.org>,
	"Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>
Subject: Re: Low performance Intel 10GE NIC (3.2.10) on 2.6.38 Kernel

Le jeudi 07 avril 2011 à 09:20 -0700, Alexander Duyck a écrit :

> The intel_iommu is meant to be a security feature.  Primarily it is used 
> in virtualzation where it allows KVM or Xen to direct assign a device 
> without having to worry about the guest getting access to the hosts 
> physical memory by submitting invalid DMA requests.
> 

I see

> If virtualzation isn't in use I would recommend turning it off as it can 
> have a negative impact especially on small packet performance due to the 
> extra locking overhead that is required for DMA map and unmap calls.

Sure, but then, if this thing is ON, we should copy small packets in
freshly allocated skbs and reuse old one in driver rx handler, to avoid
the expensive dma calls ?

[ The thing called copybreak ]

I understand tx path cost cannot be avoided, so it would not help Wei
use case.

Thanks !


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