Re: [Xen-devel] Poor network performance between DomU with multiqueue support

["Zhangleiqiang (Trump)" <zhangleiqiang@xxxxxxxxxx>]

> On Fri, Dec 05, 2014 at 01:17:16AM +0000, Zhangleiqiang (Trump) wrote:
> [...]
> > > I think that's expected, because guest RX data path still uses
> > > grant_copy while guest TX uses grant_map to do zero-copy transmit.
> >
> > As far as I know, there are three main grant-related operations used in 
> > split
> device model: grant mapping, grant transfer and grant copy.
> > Grant transfer has not used now, and grant mapping and grant transfer both
> involve "TLB" refresh work for hypervisor, am I right?  Or only grant transfer
> has this overhead?
> 
> Transfer is not used so I can't tell. Grant unmap causes TLB flush.
> 
> I saw in an email the other day XenServer folks has some planned improvement
> to avoid TLB flush in Xen to upstream in 4.6 window. I can't speak for sure 
> it will
> get upstreamed as I don't work on that.
> 
> > Does grant copy surely has more overhead than grant mapping?
> >
> 
> At the very least the zero-copy TX path is faster than previous copying path.
> 
> But speaking of the micro operation I'm not sure.
> 
> There was once persistent map prototype netback / netfront that establishes a
> memory pool between FE and BE then use memcpy to copy data. Unfortunately
> that prototype was not done right so the result was not good.

The newest mail about persistent grant I can find is sent from 16 Nov 2012 
(http://lists.xen.org/archives/html/xen-devel/2012-11/msg00832.html). Why is it 
not done right and not merged into upstream?

And I also search for virtio support in XEN, and I find that the one who are 
familiar with it is you, too, (http://wiki.xen.org/wiki/Virtio_On_Xen), :-). I 
am wondering what is the current state for virtio on XEN?

> > >From the code, I see that in TX, netback will do gnttab_batch_copy as well
> as gnttab_map_refs:
> >
> > <code> //netback.c:xenvif_tx_action
> >     xenvif_tx_build_gops(queue, budget, &nr_cops, &nr_mops);
> >
> >     if (nr_cops == 0)
> >             return 0;
> >
> >     gnttab_batch_copy(queue->tx_copy_ops, nr_cops);
> >     if (nr_mops != 0) {
> >             ret = gnttab_map_refs(queue->tx_map_ops,
> >                                   NULL,
> >                                   queue->pages_to_map,
> >                                   nr_mops);
> >             BUG_ON(ret);
> >     }
> > </code>
> >
> 
> The copy is for the packet header. Mapping is for packet data.
> 
> We need to copy header from guest so that it doesn't change under netback's
> feet.
> 
> Wei.

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