netdev - RE: [PATCH net-next v7 9/9] xen-netback: Aggregate TX unmap operations

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Message-ID: <9AAE0902D5BC7E449B7C8E4E778ABCD0292550@AMSPEX01CL01.citrite.net>
Date:	Thu, 20 Mar 2014 11:14:51 +0000
From:	Paul Durrant <Paul.Durrant@...rix.com>
To:	Wei Liu <wei.liu2@...rix.com>, Zoltan Kiss <zoltan.kiss@...rix.com>
CC:	Ian Campbell <Ian.Campbell@...rix.com>,
	Wei Liu <wei.liu2@...rix.com>,
	"xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Jonathan Davies" <Jonathan.Davies@...rix.com>
Subject: RE: [PATCH net-next v7 9/9] xen-netback: Aggregate TX unmap
 operations

> -----Original Message-----
> From: Wei Liu [mailto:wei.liu2@...rix.com]
> Sent: 20 March 2014 10:49
> To: Zoltan Kiss
> Cc: Ian Campbell; Wei Liu; xen-devel@...ts.xenproject.org;
> netdev@...r.kernel.org; linux-kernel@...r.kernel.org; Jonathan Davies;
> Paul Durrant
> Subject: Re: [PATCH net-next v7 9/9] xen-netback: Aggregate TX unmap
> operations
> 
> On Wed, Mar 19, 2014 at 09:16:05PM +0000, Zoltan Kiss wrote:
> > Hi,
> >
> > I'm thinking about revoking this patch: it's value is pretty small,
> > but it causes performance regression on Win7 guests. And probably it
> > is not the best solution for this problem. It might be the delay it
> > takes the dealloc thread to be scheduled is enough.
> > What do you think?
> >
> 
> Can you elaborate? What makes Win7 so special? What's performance
> impact to other guests?
> 

It won't be Win7 specifically I expect. It will likely by any version of Windows, or any other OS that limits the TXs-in-flight so aggressively. Basically you need to TX-complete reasonably frequently otherwise your throughput drops off a lot. IIRC at Solarflare we found every ~500us to be just about frequent enough for hitting 10G.

  Paul
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