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

[Paul Durrant]

> -----Original Message-----
> From: Wei Liu [mailto:wei.liu2@xxxxxxxxxx]
> Sent: 20 March 2014 10:49
> To: Zoltan Kiss
> Cc: Ian Campbell; Wei Liu; xen-devel@xxxxxxxxxxxxxxxxxxxx;
> netdev@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; 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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