netdev - Re: Regression in throughput between kvm guests over virtual bridge

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Date:   Wed, 20 Sep 2017 15:38:25 -0400
From:   Matthew Rosato <mjrosato@...ux.vnet.ibm.com>
To:     Jason Wang <jasowang@...hat.com>, netdev@...r.kernel.org
Cc:     davem@...emloft.net, mst@...hat.com
Subject: Re: Regression in throughput between kvm guests over virtual bridge


> Seems to make some progress on wakeup mitigation. Previous patch tries
> to reduce the unnecessary traversal of waitqueue during rx. Attached
> patch goes even further which disables rx polling during processing tx.
> Please try it to see if it has any difference.

Unfortunately, this patch doesn't seem to have made a difference.  I
tried runs with both this patch and the previous patch applied, as well
as only this patch applied for comparison (numbers from vhost thread of
sending VM):

4.12    4.13     patch1   patch2   patch1+2
2.00%   +3.69%   +2.55%   +2.81%   +2.69%   [...] __wake_up_sync_key

In each case, the regression in throughput was still present.

> And two questions:
> - Is the issue existed if you do uperf between 2VMs (instead of 4VMs)

Verified that the second set of guests are not actually required, I can
see the regression with only 2 VMs.

> - Can enable batching in the tap of sending VM improve the performance
> (ethtool -C $tap rx-frames 64)

I tried this, but it did not help (actually seemed to make things a
little worse)