Re: [PATCH v1 0/3] Provide a zero-copy method on KVM virtio-net.

From: Michael S. Tsirkin
Date: Sun Mar 07 2010 - 05:57:49 EST

On Sat, Mar 06, 2010 at 05:38:35PM +0800, wrote:
> The idea is simple, just to pin the guest VM user space and then
> let host NIC driver has the chance to directly DMA to it.
> The patches are based on vhost-net backend driver. We add a device
> which provides proto_ops as sendmsg/recvmsg to vhost-net to
> send/recv directly to/from the NIC driver. KVM guest who use the
> vhost-net backend may bind any ethX interface in the host side to
> get copyless data transfer thru guest virtio-net frontend.
> We provide multiple submits and asynchronous notifiicaton to
> vhost-net too.
> Our goal is to improve the bandwidth and reduce the CPU usage.
> Exact performance data will be provided later. But for simple
> test with netperf, we found bindwidth up and CPU % up too,
> but the bindwidth up ratio is much more than CPU % up ratio.
> What we have not done yet:
> packet split support
> To support GRO
> Performance tuning

Am I right to say that nic driver needs changes for these patches
to work? If so, please publish nic driver patches as well.

> what we have done in v1:
> polish the RCU usage
> deal with write logging in asynchroush mode in vhost
> add notifier block for mp device
> rename page_ctor to mp_port in netdevice.h to make it looks generic
> add mp_dev_change_flags() for mp device to change NIC state
> add CONIFG_VHOST_MPASSTHRU to limit the usage when module is not load
> a small fix for missing dev_put when fail
> using dynamic minor instead of static minor number
> a __KERNEL__ protect to mp_get_sock()
> performance:
> using netperf with GSO/TSO disabled, 10G NIC,
> disabled packet split mode, with raw socket case compared to vhost.
> bindwidth will be from 1.1Gbps to 1.7Gbps
> CPU % from 120%-140% to 140%-160%

That's pretty low for a 10Gb nic. Are you hitting some other bottleneck,
like high interrupt rate? Also, GSO support and performance tuning
for raw are incomplete. Try comparing with e.g. tap with GSO.

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