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

[Jason Wang <jasowang@redhat.com>]

On 2019/6/10 下午11:55, Michael S. Tsirkin wrote:
> 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.


Btw, I wonder maybe it's time to make TODO wiki up to date.

Thanks