netdev - Re: [RFC PATCH v8 00/16] Provide a zero-copy method on KVM virtio-net.

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Date:	Fri, 30 Jul 2010 08:46:11 -0700
From:	Shirley Ma <mashirle@...ibm.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	xiaohui.xin@...el.com, netdev@...r.kernel.org, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, mst@...hat.com, mingo@...e.hu,
	davem@...emloft.net, herbert@...dor.hengli.com.au,
	jdike@...ux.intel.com
Subject: Re: [RFC PATCH v8 00/16] Provide a zero-copy method on KVM
 virtio-net.

Hello Avi,

On Fri, 2010-07-30 at 08:02 +0300, Avi Kivity wrote:
> get_user_pages() is indeed slow.  But what about
> get_user_pages_fast()?
> 
> Note that when the page is first touched, get_user_pages_fast() falls 
> back to get_user_pages(), so the latency needs to be measured after 
> quite a bit of warm-up. 

Yes, I used get_user_pages_fast, however if falled back to
get_user_pages() when the apps doesn't allocate buffer on the same page.
If I run a single ping, the RTT is extremely high, but when running
multiple pings, the RTT time reduce significantly, but still it is not
as fast as copy from my initial test. I am thinking that we might need
to pre-pin memory pool.

Shirley

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