Re: [PATCH 6/6] vhost_net: remove the max pending check

On Fri, Aug 23, 2013 at 04:55:49PM +0800, Jason Wang wrote:

On 08/20/2013 10:48 AM, Jason Wang wrote:

quoted

On 08/16/2013 06:02 PM, Michael S. Tsirkin wrote:

quoted

quoted

On Fri, Aug 16, 2013 at 01:16:30PM +0800, Jason Wang wrote:

quoted

quoted

We used to limit the max pending DMAs to prevent guest from pinning too many
pages. But this could be removed since:

- We have the sk_wmem_alloc check in both tun/macvtap to do the same work
- This max pending check were almost useless since it was one done when there's
  no new buffers coming from guest. Guest can easily exceeds the limitation.
- We've already check upend_idx != done_idx and switch to non zerocopy then. So
  even if all vq->heads were used, we can still does the packet transmission.

We can but performance will suffer.

The check were in fact only done when no new buffers submitted from
guest. So if guest keep sending, the check won't be done.

If we really want to do this, we should do it unconditionally. Anyway, I
will do test to see the result.

There's a bug in PATCH 5/6, the check:

nvq->upend_idx != nvq->done_idx

makes the zerocopy always been disabled since we initialize both
upend_idx and done_idx to zero. So I change it to:

(nvq->upend_idx + 1) % UIO_MAXIOV != nvq->done_idx.

But what I would really like to try is limit ubuf_info to VHOST_MAX_PEND.
I think this has a chance to improve performance since
we'll be using less cache.
Of course this means we must fix the code to really never submit
more than VHOST_MAX_PEND requests.

Want to try?

With this change on top, I didn't see performance difference w/ and w/o
this patch.

Did you try small message sizes btw (like 1K)? Or just netperf
default of 64K?

-- 
MST