Re: [Qemu-devel] [QUESTION] How to reduce network latency to improve netperf TCP_RR drastically?

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Jun 04, 2019 at 03:10:43PM +0800, Like Xu wrote:
> Hi Michael，
> 
> At https://www.linux-kvm.org/page/NetworkingTodo, there is an entry for
> network latency saying:
> 
> ---
> reduce networking latency:
>  allow handling short packets from softirq or VCPU context
>  Plan:
>    We are going through the scheduler 3 times
>    (could be up to 5 if softirqd is involved)
>    Consider RX: host irq -> io thread -> VCPU thread ->
>    guest irq -> guest thread.
>    This adds a lot of latency.
>    We can cut it by some 1.5x if we do a bit of work
>    either in the VCPU or softirq context.
>  Testing: netperf TCP RR - should be improved drastically
>           netperf TCP STREAM guest to host - no regression
>  Contact: MST
> ---
> 
> I am trying to make some contributions to improving netperf TCP_RR.
> Could you please share more ideas or plans or implemental details to make it
> happen?
> 
> Thanks,
> Like Xu


So some of this did happen. netif_receive_skb is now called
directly from tun_get_user.

Question is about the rx/tun_put_user path now.

If the vhost thread is idle, there's a single packet
outstanding then maybe we can forward the packet to userspace
directly from BH without waking up the thread.

For this to work we need to map some userspace memory into kernel
ahead of the time. For example, maybe it can happen when
guest adds RX buffers? Copying Jason who's looking into
memory mapping matters.

-- 
MST