Re: [PATCH 5/5] Stop dropping so many RX packets in tap (v3)

[Avi Kivity <avi@qumranet.com>]

Anthony Liguori wrote:
>> How about the other way round: when the vlan consumer detects it can 
>> no longer receive packets, it tells that to the vlan.  When all vlan 
>> consumers can no longer receive, tell the producer to stop 
>> producing.  For the tap producer, this is simply removing its fd from 
>> the read poll list.  When a vlan consumer becomes ready to receive 
>> again, it tells the vlan, which tells the producers, which then 
>> install their fds back again.
>
> Yeah, that's a nice idea.   I'll think about it.  I don't know if it's 
> really worth doing as an intermediate step though.  What I'd really 
> like to do is have a vlan interface where consumers published all of 
> their receive buffers.  Then there's no need for notifications of 
> receive-ability.

That's definitely better, and is also more multiqueue nic / vringfd 
friendly.

I still think interrupt-on-halfway-mark is needed much more urgently.  
It deals with concurrency much better:

rx:
  host interrupts guest on halfway mark
  guest starts processing packets
  host keeps delivering

tx:
  guest kicks host on halfway mark
  host starts processing packets
  second vcpu on guest keeps on queueing

It's also much better with multiqueue NICs, where there's no socket 
buffer to hold the packets while we're out of descriptors.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.


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