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

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Date:   Fri, 22 Sep 2017 12:03:21 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Matthew Rosato <mjrosato@...ux.vnet.ibm.com>,
        netdev@...r.kernel.org
Cc:     davem@...emloft.net, mst@...hat.com
Subject: Re: Regression in throughput between kvm guests over virtual bridge



On 2017年09月21日 03:38, Matthew Rosato wrote:
>> 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.

This probably means some other cases of the wakeups were missed. Could 
you please record the callers of __wake_up_sync_key()?

>
>> 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)
>

  I still can't see a reason that can lead more wakeups, will take more 
time to look at this issue and keep you posted.

Thanks