[dpdk-dev] virtio optimization idea

[Stephen Hemminger]

On Fri, 4 Sep 2015 08:25:05 +0000
"Xie, Huawei" <huawei.xie at intel.com> 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.
> 
> 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.
> 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
> with fixed descriptor                
>   
> +--+--+--+--+-----+---+------+---+--+---+------+--+---+                              
> 
>       |     |            |  ||  |      |            
> |                                  
>       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       
>   
> +-----+-----+-----+--------------+------+------+------+                        
> 

Does this still work with Linux (or BSD) guest/host.
If you are assuming both virtio/vhost are DPDK this is never going
to be usable.

On a related note, have you looked at getting virtio to support the
new standard (not legacy) mode?