Re: [PATCH net-next] tuntap: introduce tx skb ring

[Jason Wang <jasowang@redhat.com>]

On 2016年05月18日 17:55, Michael S. Tsirkin wrote:
> On Wed, May 18, 2016 at 11:21:29AM +0200, Jesper Dangaard Brouer wrote:
>> On Wed, 18 May 2016 11:21:59 +0300
>> "Michael S. Tsirkin" <mst@redhat.com> wrote:
>>
>>> On Wed, May 18, 2016 at 10:16:31AM +0200, Jesper Dangaard Brouer wrote:
>>>> On Tue, 17 May 2016 09:38:37 +0800 Jason Wang <jasowang@redhat.com> wrote:
>>>>    
>>>>>>> And if tx_queue_length is not power of 2,
>>>>>>> we probably need modulus to calculate the capacity.
>>>>>> Is that really that important for speed?
>>>>> Not sure, I can test.
>>>> In my experience, yes, adding a modulus does affect performance.
>>> How about simple
>>> 	if (unlikely(++idx > size))
>>> 		idx = 0;
>> So, you are exchanging an AND-operation with a mask, for a
>> branch-operation.  If the branch predictor is good enough in the CPU
>> and code-"size" use-case, then I could be just as fast.
>>
>> I've actually played with a lot of different approaches:
>>   https://github.com/netoptimizer/prototype-kernel/blob/master/kernel/include/linux/alf_queue_helpers.h
>>
>> I cannot remember the exact results. I do remember micro benchmarking
>> showed good results with the advanced "unroll" approach, but IPv4
>> forwarding, where I know I-cache is getting evicted, showed best
>> results with the more simpler implementations.
> This is all assuming you can somehow batch operations.
> We can do this for transmit sometimes (when linux
> is the source of the packets) but not always.
>
>>>>> Right, this sounds a good solution.
>>>> Good idea.
>>> I'm not that sure - it's clearly wasting memory.
>> Rounding up to power of two.  In this case I don't think the memory
>> wast is too high.  As we are talking about max 16 bytes elements.
> It almost doubles it.
> E.g. queue size of 10000 (rather common) will become 16K, wasting 6K.

It depends on the user, e.g default tx_queue_len is around 1000 for real 
cards. If we really care about the wasting, we can add a threshold and 
fall back to normal linked list during resizing.

>
>> I am concerned about memory in another way. We need to keep these
>> arrays/rings small, due to data cache usage.  A 4096 ring queue is bad
>> because e.g. 16*4096=65536 bytes, and typical L1 cache is 32K-64K. As
>> this is a circular buffer, we walk over this memory all the time, thus
>> evicting the L1 cache.
> Depends on the usage I guess.
> Entries pointed to are much bigger, and you are
> going to access them - is this really an issue?
> If yes this shouldn't be that hard to fix ...
>
>> -- 
>> Best regards,
>>    Jesper Dangaard Brouer
>>    MSc.CS, Principal Kernel Engineer at Red Hat
>>    Author of http://www.iptv-analyzer.org
>>    LinkedIn: http://www.linkedin.com/in/brouer