There is some format issue with the ascii chart of the tx ring. Update that chart. Sorry for the trouble.
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. > > 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 > > +--+--+--+--+-----+---+------+---+--+---+------+--+---+ > > | | | || | | | > > 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 >
