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

On 05/12/14 12:42, Wei Liu wrote:
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 

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

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.

>From the code, I see that in TX, netback will do gnttab_batch_copy as well as 

<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,

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.

It is also important because if the above mentioned "TLB flush avoidance" patch goes in to Xen, it will be important to grant copy the header rather than grant map plus memcpy. The latter is the old way, it touches the page so you can't avoid TLB flush.


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