Re: [virtio-dev] Re: [PATCH net-next v2 0/5] virtio: support packed ring

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Sep 10, 2018 at 11:00:53AM +0800, Tiwei Bie wrote:
> On Fri, Sep 07, 2018 at 09:00:49AM -0400, Michael S. Tsirkin wrote:
> > On Fri, Sep 07, 2018 at 09:22:25AM +0800, Tiwei Bie wrote:
> > > On Mon, Aug 27, 2018 at 05:00:40PM +0300, Michael S. Tsirkin wrote:
> > > > Are there still plans to test the performance with vost pmd?
> > > > vhost doesn't seem to show a performance gain ...
> > > > 
> > > 
> > > I tried some performance tests with vhost PMD. In guest, the
> > > XDP program will return XDP_DROP directly. And in host, testpmd
> > > will do txonly fwd.
> > > 
> > > When burst size is 1 and packet size is 64 in testpmd and
> > > testpmd needs to iterate 5 Tx queues (but only the first two
> > > queues are enabled) to prepare and inject packets, I got ~12%
> > > performance boost (5.7Mpps -> 6.4Mpps). And if the vhost PMD
> > > is faster (e.g. just need to iterate the first two queues to
> > > prepare and inject packets), then I got similar performance
> > > for both rings (~9.9Mpps) (packed ring's performance can be
> > > lower, because it's more complicated in driver.)
> > > 
> > > I think packed ring makes vhost PMD faster, but it doesn't make
> > > the driver faster. In packed ring, the ring is simplified, and
> > > the handling of the ring in vhost (device) is also simplified,
> > > but things are not simplified in driver, e.g. although there is
> > > no desc table in the virtqueue anymore, driver still needs to
> > > maintain a private desc state table (which is still managed as
> > > a list in this patch set) to support the out-of-order desc
> > > processing in vhost (device).
> > > 
> > > I think this patch set is mainly to make the driver have a full
> > > functional support for the packed ring, which makes it possible
> > > to leverage the packed ring feature in vhost (device). But I'm
> > > not sure whether there is any other better idea, I'd like to
> > > hear your thoughts. Thanks!
> > 
> > Just this: Jens seems to report a nice gain with virtio and
> > vhost pmd across the board. Try to compare virtio and
> > virtio pmd to see what does pmd do better?
> 
> The virtio PMD (drivers/net/virtio) in DPDK doesn't need to share
> the virtio ring operation code with other drivers and is highly
> optimized for network. E.g. in Rx, the Rx burst function won't
> chain descs.
> So the ID management for the Rx ring can be quite
> simple and straightforward, we just need to initialize these IDs
> when initializing the ring and don't need to change these IDs
> in data path anymore (the mergable Rx code in that patch set
> assumes the descs will be written back in order, which should be
> fixed. I.e., the ID in the desc should be used to index vq->descx[]).
> The Tx code in that patch set also assumes the descs will be
> written back by device in order, which should be fixed.
> 
> But in kernel virtio driver, the virtio_ring.c is very generic.
> The enqueue (virtqueue_add()) and dequeue (virtqueue_get_buf_ctx())
> functions need to support all the virtio devices and should be
> able to handle all the possible cases that may happen. So although
> the packed ring can be very efficient in some cases, currently
> the room to optimize the performance in kernel's virtio_ring.c
> isn't that much. If we want to take the fully advantage of the
> packed ring's efficiency, we need some further e.g. API changes
> in virtio_ring.c, which shouldn't be part of this patch set. So
> I still think this patch set is mainly to make the kernel virtio
> driver to have a full functional support of the packed ring, and
> we can't expect impressive performance boost with it.

So what are the cases that make things complex?
Maybe we should drop support for them completely.


> > 
> > 
> > > 
> > > > 
> > > > On Wed, Jul 11, 2018 at 10:27:06AM +0800, Tiwei Bie wrote:
> > > > > Hello everyone,
> > > > > 
> > > > > This patch set implements packed ring support in virtio driver.
> > > > > 
> > > > > Some functional tests have been done with Jason's
> > > > > packed ring implementation in vhost:
> > > > > 
> > > > > https://lkml.org/lkml/2018/7/3/33
> > > > > 
> > > > > Both of ping and netperf worked as expected.
> > > > > 
> > > > > v1 -> v2:
> > > > > - Use READ_ONCE() to read event off_wrap and flags together (Jason);
> > > > > - Add comments related to ccw (Jason);
> > > > > 
> > > > > RFC (v6) -> v1:
> > > > > - Avoid extra virtio_wmb() in virtqueue_enable_cb_delayed_packed()
> > > > >   when event idx is off (Jason);
> > > > > - Fix bufs calculation in virtqueue_enable_cb_delayed_packed() (Jason);
> > > > > - Test the state of the desc at used_idx instead of last_used_idx
> > > > >   in virtqueue_enable_cb_delayed_packed() (Jason);
> > > > > - Save wrap counter (as part of queue state) in the return value
> > > > >   of virtqueue_enable_cb_prepare_packed();
> > > > > - Refine the packed ring definitions in uapi;
> > > > > - Rebase on the net-next tree;
> > > > > 
> > > > > RFC v5 -> RFC v6:
> > > > > - Avoid tracking addr/len/flags when DMA API isn't used (MST/Jason);
> > > > > - Define wrap counter as bool (Jason);
> > > > > - Use ALIGN() in vring_init_packed() (Jason);
> > > > > - Avoid using pointer to track `next` in detach_buf_packed() (Jason);
> > > > > - Add comments for barriers (Jason);
> > > > > - Don't enable RING_PACKED on ccw for now (noticed by Jason);
> > > > > - Refine the memory barrier in virtqueue_poll();
> > > > > - Add a missing memory barrier in virtqueue_enable_cb_delayed_packed();
> > > > > - Remove the hacks in virtqueue_enable_cb_prepare_packed();
> > > > > 
> > > > > RFC v4 -> RFC v5:
> > > > > - Save DMA addr, etc in desc state (Jason);
> > > > > - Track used wrap counter;
> > > > > 
> > > > > RFC v3 -> RFC v4:
> > > > > - Make ID allocation support out-of-order (Jason);
> > > > > - Various fixes for EVENT_IDX support;
> > > > > 
> > > > > RFC v2 -> RFC v3:
> > > > > - Split into small patches (Jason);
> > > > > - Add helper virtqueue_use_indirect() (Jason);
> > > > > - Just set id for the last descriptor of a list (Jason);
> > > > > - Calculate the prev in virtqueue_add_packed() (Jason);
> > > > > - Fix/improve desc suppression code (Jason/MST);
> > > > > - Refine the code layout for XXX_split/packed and wrappers (MST);
> > > > > - Fix the comments and API in uapi (MST);
> > > > > - Remove the BUG_ON() for indirect (Jason);
> > > > > - Some other refinements and bug fixes;
> > > > > 
> > > > > RFC v1 -> RFC v2:
> > > > > - Add indirect descriptor support - compile test only;
> > > > > - Add event suppression supprt - compile test only;
> > > > > - Move vring_packed_init() out of uapi (Jason, MST);
> > > > > - Merge two loops into one in virtqueue_add_packed() (Jason);
> > > > > - Split vring_unmap_one() for packed ring and split ring (Jason);
> > > > > - Avoid using '%' operator (Jason);
> > > > > - Rename free_head -> next_avail_idx (Jason);
> > > > > - Add comments for virtio_wmb() in virtqueue_add_packed() (Jason);
> > > > > - Some other refinements and bug fixes;
> > > > > 
> > > > > Thanks!
> > > > > 
> > > > > Tiwei Bie (5):
> > > > >   virtio: add packed ring definitions
> > > > >   virtio_ring: support creating packed ring
> > > > >   virtio_ring: add packed ring support
> > > > >   virtio_ring: add event idx support in packed ring
> > > > >   virtio_ring: enable packed ring
> > > > > 
> > > > >  drivers/s390/virtio/virtio_ccw.c   |   14 +
> > > > >  drivers/virtio/virtio_ring.c       | 1365 ++++++++++++++++++++++------
> > > > >  include/linux/virtio_ring.h        |    8 +-
> > > > >  include/uapi/linux/virtio_config.h |    3 +
> > > > >  include/uapi/linux/virtio_ring.h   |   43 +
> > > > >  5 files changed, 1157 insertions(+), 276 deletions(-)
> > > > > 
> > > > > -- 
> > > > > 2.18.0
> > > > 
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