Re: [Xen-devel] [PATCH RFC] tcp: Allow sk_wmem_alloc to exceed sysctl_tcp_limit_output_bytes

[George Dunlap <George.Dunlap@eu.citrix.com>]

On Mon, Apr 13, 2015 at 4:03 PM, Malcolm Crossley
<malcolm.crossley@citrix.com> wrote:
>>
>> But the main concern here is it basically breaks back pressure.
>>
>> And you do not want this, unless there is no other choice.
>>
>
> virtio_net already use's skb_orphan() in it's transmit path. It seems
> only fair that other virtual network drivers behave in the same way.
>
> There are no easy solutions to decrease the transmit latency for
> netback/netfront. We map the guest memory through to the backend to
> avoid memory copies. The frontend memory can only be freed once the
> network driver has completed transmitting the packet in the backend.
>
> Modern network drivers can be quite slow at freeing the skb's once
> transmitted (the packet is already on the wire as far as they are
> concerned) and this delay is compounded by needing the signal the
> completion of the transmit back to the frontend (by IPI in worst case).
>
> From a networking point of view, the backend is a switch. Is it OK to
> consider the packet to have been transmitted from the guest point of
> view once the backend is aware of the packet?
>
> This would help justify the skb_orphan() in the frontend.

This sounds sensible to me, particularly if virtio_net is already doing it.

 -George