Re: [PATCH RFC] ptr_ring: add ptr_ring_unconsume

[Jason Wang <jasowang@redhat.com>]

On 2017年04月24日 20:00, Michael S. Tsirkin wrote:
> On Mon, Apr 24, 2017 at 07:54:18PM +0800, Jason Wang wrote:
>> On 2017年04月24日 07:28, Michael S. Tsirkin wrote:
>>> On Tue, Apr 18, 2017 at 11:07:42AM +0800, Jason Wang wrote:
>>>> On 2017年04月17日 07:19, Michael S. Tsirkin wrote:
>>>>> Applications that consume a batch of entries in one go
>>>>> can benefit from ability to return some of them back
>>>>> into the ring.
>>>>>
>>>>> Add an API for that - assuming there's space. If there's no space
>>>>> naturally we can't do this and have to drop entries, but this implies
>>>>> ring is full so we'd likely drop some anyway.
>>>>>
>>>>> Signed-off-by: Michael S. Tsirkin<mst@redhat.com>
>>>>> ---
>>>>>
>>>>> Jason, in my mind the biggest issue with your batching patchset is the
>>>>> backet drops on disconnect.  This API will help avoid that in the common
>>>>> case.
>>>> Ok, I will rebase the series on top of this. (Though I don't think we care
>>>> the packet loss).
>>> E.g. I care - I often start sending packets to VM before it's
>>> fully booted. Several vhost resets might follow.
>> Ok.
>>
>>>>> I would still prefer that we understand what's going on,
>>>> I try to reply in another thread, does it make sense?
>>>>
>>>>>     and I would
>>>>> like to know what's the smallest batch size that's still helpful,
>>>> Yes, I've replied in another thread, the result is:
>>>>
>>>>
>>>> no batching   1.88Mpps
>>>> RX_BATCH=1    1.93Mpps
>>>> RX_BATCH=4    2.11Mpps
>>>> RX_BATCH=16   2.14Mpps
>>>> RX_BATCH=64   2.25Mpps
>>>> RX_BATCH=256  2.18Mpps
>>> Essentially 4 is enough, other stuf looks more like noise
>>> to me. What about 2?
>> The numbers are pretty stable, so probably not noise. Retested on top of
>> batch zeroing:
>>
>> no  1.97Mpps
>> 1   2.09Mpps
>> 2   2.11Mpps
>> 4   2.16Mpps
>> 8   2.19Mpps
>> 16  2.21Mpps
>> 32  2.25Mpps
>> 64  2.30Mpps
>> 128 2.21Mpps
>> 256 2.21Mpps
>>
>> 64 performs best.
>>
>> Thanks
> OK but it might be e.g. a function of the ring size, host cache size or
> whatever. As we don't really understand the why, if we just optimize for
> your setup we risk regressions in others.  64 entries is a lot, it
> increases the queue size noticeably.  Could this be part of the effect?
> Could you try changing the queue size to see what happens?

I increase tx_queue_len to 1100, but only see less than 1% improvement 
on pps number (batch = 1) in my machine. If you care about the 
regression, we probably can leave the choice to user through e.g module 
parameter. But I'm afraid we have already had too much choices for them. 
Or I can test this with different CPU types.

Thanks

>