Re: [PATCH] virtio_net: free transmit skbs in a timer

[Avi Kivity <avi@qumranet.com>]

Rusty Russell wrote:
>> Sorry to barge in late, but IMO the timer should be on the host, which
>> is cheaper than on the guest (well, a 100ms timer is likely zero cost,
>> but  I still don't like it).
>>
>> the host should fire a tx completion interrupt whenever the completion
>> queue has "enough" entries, where we can define "enough" now as the
>> halfway mark or a timer expiry, whichever comes earlier.
>>
>> We can later improve "enough" to be "just enough so the timer never
>> triggers" and adjust it dynamically.  It probably doesn't matter for
>> Linux, but I don't want to punish guests that can do true async
>> networking and depend on timely completion notification.
>>     
>
> This implies that we should not be supressing notifications in the guest at 
> all (unless we're sure there are more packets to come, which currently we 
> never are: that needs new net infrastructure).
>   

We don't have to be sure, just reasonably confident.  If we see a stream 
of packets, we open the window, but set a timer in case we're wrong.  
The expectation is that the timer will only fire when tx rate drops (or 
tx stops completely).

> But that means we'd get a notification on every xmit at the moment.  
> Benchmarks anyone?
>   

Notification on every xmit will surely kill performance.  I'm trying to 
get batching to work but also good latency when the link is not saturated.

-- 
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