netdev - Re: [PATCH 2/6] vhost_net: use vhost_add_used_and_signal_n() in vhost_zerocopy_signal

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Date:	Fri, 23 Aug 2013 16:50:38 +0800
From:	Jason Wang <jasowang@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/6] vhost_net: use vhost_add_used_and_signal_n() in vhost_zerocopy_signal_used()

On 08/20/2013 10:33 AM, Jason Wang wrote:
> On 08/16/2013 05:54 PM, Michael S. Tsirkin wrote:
>> On Fri, Aug 16, 2013 at 01:16:26PM +0800, Jason Wang wrote:
>>>> Switch to use vhost_add_used_and_signal_n() to avoid multiple calls to
>>>> vhost_add_used_and_signal(). With the patch we will call at most 2 times
>>>> (consider done_idx warp around) compared to N times w/o this patch.
>>>>
>>>> Signed-off-by: Jason Wang <jasowang@...hat.com>
>> So? Does this help performance then?
>>
> Looks like it can especially when guest does support event index. When
> guest enable tx interrupt, this can saves us some unnecessary signal to
> guest. I will do some test.

Have done some test. I can see 2% - 3% increasing in both aggregate
transaction rate and per cpu transaction rate in TCP_RR and UDP_RR test.

I'm using ixgbe. W/o this patch, I can see more than 100 calls of
vhost_add_used_signal() in one vhost_zerocopy_signaled_used(). This is
because ixgbe (and other modern ethernet driver) tends to free old tx
skbs in a loop during tx interrupt, and vhost tend to batch the adding
used and signal in vhost_zerocopy_callback(). Switching to use
vhost_add_use_and_signal_n() means saving 100 times of used idx updating
and memory barriers.
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