netdev - Re: [PATCH V3 0/8] macvtap/vhost TX zero copy support

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Date:	Thu, 21 Apr 2011 10:29:04 -0400
From:	Jon Mason <jdmason@...zu.us>
To:	Shirley Ma <mashirle@...ibm.com>
Cc:	David Miller <davem@...emloft.net>, mst@...hat.com,
	Eric Dumazet <eric.dumazet@...il.com>,
	Avi Kivity <avi@...hat.com>, Arnd Bergmann <arnd@...db.de>,
	netdev@...r.kernel.org, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH V3 0/8] macvtap/vhost TX zero copy support

On Wed, Apr 20, 2011 at 3:36 PM, Shirley Ma <mashirle@...ibm.com> wrote:
> This patchset add supports for TX zero-copy between guest and host
> kernel through vhost. It significantly reduces CPU utilization on the
> local host on which the guest is located (It reduced 30-50% CPU usage
> for vhost thread for single stream test). The patchset is based on
> previous submission and comments from the community regarding when/how
> to handle guest kernel buffers to be released. This is the simplest
> approach I can think of after comparing with several other solutions.
>
> This patchset includes:
>
> 1/8: Add a new sock zero-copy flag, SOCK_ZEROCOPY;
>
> 2/8: Add a new device flag, NETIF_F_ZEROCOPY for lower level device
> support zero-copy;
>
> 3/8: Add a new struct skb_ubuf_info in skb_share_info for userspace
> buffers release callback when lower device DMA has done for that skb;
>
> 4/8: Add vhost zero-copy callback in vhost when skb last refcnt is gone;
> add vhost_zerocopy_add_used_and_signal to notify guest to release TX skb
> buffers.
>
> 5/8: Add macvtap zero-copy in lower device when sending packet is
> greater than 128 bytes.
>
> 6/8: Add Chelsio 10Gb NIC to zero copy feature flag
>
> 7/8: Add Intel 10Gb NIC zero copy feature flag
>
> 8/8: Add Emulex 10Gb NIC zero copy feature flag

Why are only these 3 drivers getting support?  As far as I can tell,
the only requirement is HIGHDMA.  If this is the case, is there really
a need for an additional flag to support this?  If you can key off of
HIGHDMA, all devices that support this would get the benefit.



> The patchset is built against most recent linux 2.6.git. It has passed
> netperf/netserver multiple streams stress test on above NICs.
>
> The single stream test results from 2.6.37 kernel on Chelsio:
>
> 64K message size: copy_from_user dropped from 40% to 5%; vhost thread
> cpu utilization dropped from 76% to 28%
>
> I am collecting more test results against 2.6.39-rc3 kernel and will
> provide the test matrix later.
>
> Thanks
> Shirley
>
>
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