Re: virtio optimization idea

[Tetsuya Mukawa <mukawa@igel.co.jp>]

On 2015/09/05 1:50, Xie, Huawei wrote:
> There is some format issue with the ascii chart of the tx ring. Update
> that chart.
> Sorry for the trouble.

Hi XIe,

Thanks for sharing a way to optimize virtio.
I have a few questions.

>
> On 9/4/2015 4:25 PM, Xie, Huawei wrote:
>> Hi:
>>
>> Recently I have done one virtio optimization proof of concept. The
>> optimization includes two parts:
>> 1) avail ring set with fixed descriptors
>> 2) RX vectorization
>> With the optimizations, we could have several times of performance boost
>> for purely vhost-virtio throughput.

When you check performance, have you optimized only virtio-net driver?
If so, can we optimize vhost backend(librte_vhost) also using your
optimization way?

>>
>> Here i will only cover the first part, which is the prerequisite for the
>> second part.
>> Let us first take RX for example. Currently when we fill the avail ring
>> with guest mbuf, we need
>> a) allocate one descriptor(for non sg mbuf) from free descriptors
>> b) set the idx of the desc into the entry of avail ring
>> c) set the addr/len field of the descriptor to point to guest blank mbuf
>> data area
>>
>> Those operation takes time, and especially step b results in modifed (M)
>> state of the cache line for the avail ring in the virtio processing
>> core. When vhost processes the avail ring, the cache line transfer from
>> virtio processing core to vhost processing core takes pretty much CPU
>> cycles.
>> To solve this problem, this is the arrangement of RX ring for DPDK
>> pmd(for non-mergable case).
>>    
>>                     avail                      
>>                     idx                        
>>                     +                          
>>                     |                          
>> +----+----+---+-------------+------+           
>> | 0  | 1  | 2 | ... |  254  | 255  |  avail ring
>> +-+--+-+--+-+-+---------+---+--+---+           
>>   |    |    |       |   |      |               
>>   |    |    |       |   |      |               
>>   v    v    v       |   v      v               
>> +-+--+-+--+-+-+---------+---+--+---+           
>> | 0  | 1  | 2 | ... |  254  | 255  |  desc ring
>> +----+----+---+-------------+------+           
>>                     |                          
>>                     |                          
>> +----+----+---+-------------+------+           
>> | 0  | 1  | 2 |     |  254  | 255  |  used ring
>> +----+----+---+-------------+------+           
>>                     |                          
>>                     +    
>> Avail ring is initialized with fixed descriptor and is never changed,
>> i.e, the index value of the nth avail ring entry is always n, which
>> means virtio PMD is actually refilling desc ring only, without having to
>> change avail ring.

For example, avail ring is like below.
struct vring_avail {
        uint16_t flags;
        uint16_t idx;
        uint16_t ring[QUEUE_SIZE];
};

My understanding is that virtio-net driver still needs to change
avail_ring.idx, but don't need to change avail_ring.ring[].
Is this correct?

Tetsuya

>> When vhost fetches avail ring, if not evicted, it is always in its first
>> level cache.
>>
>> When RX receives packets from used ring, we use the used->idx as the
>> desc idx. This requires that vhost processes and returns descs from
>> avail ring to used ring in order, which is true for both current dpdk
>> vhost and kernel vhost implementation. In my understanding, there is no
>> necessity for vhost net to process descriptors OOO. One case could be
>> zero copy, for example, if one descriptor doesn't meet zero copy
>> requirment, we could directly return it to used ring, earlier than the
>> descriptors in front of it.
>> To enforce this, i want to use a reserved bit to indicate in order
>> processing of descriptors.
>>
>> For tx ring, the arrangement is like below. Each transmitted mbuf needs
>> a desc for virtio_net_hdr, so actually we have only 128 free slots.
>>                                                                                       
>>
>>                            
>>                             ++                                                           
>>                             ||                                                           
>>                             ||                                                           
>>    +-----+-----+-----+--------------+------+------+------+                               
>>    |  0  |  1  | ... |  127 || 128  | 129  | ...  | 255  |   avail ring                  
>>    +--+--+--+--+-----+---+------+---+--+---+------+--+---+                               
>>       |     |            |  ||  |      |             |                                   
>>       v     v            v  ||  v      v             v                                   
>>    +--+--+--+--+-----+---+------+---+--+---+------+--+---+                               
>>    | 127 | 128 | ... |  255 || 127  | 128  | ...  | 255  |   desc ring for virtio_net_hdr
>>    +--+--+--+--+-----+---+------+---+--+---+------+--+---+                               
>>       |     |            |  ||  |      |             |                                   
>>       v     v            v  ||  v      v             v                                   
>>    +--+--+--+--+-----+---+------+---+--+---+------+--+---+                               
>>    |  0  |  1  | ... |  127 ||  0   |  1   | ...  | 127  |   desc ring for tx dat                        
>>
>>
>>                      
>> /huawei
>>