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

From: Tiwei Bie <[email-masked]>
Date: Fri, 7 Sep 2018 09:22:25 +0800

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!

> 
> 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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