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

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Message-Id: <c117c0c5-7fc0-013b-6f54-2e722d115101@linux.vnet.ibm.com>
Date:   Wed, 25 Oct 2017 16:21:26 -0400
From:   Matthew Rosato <mjrosato@...ux.vnet.ibm.com>
To:     Jason Wang <jasowang@...hat.com>, Wei Xu <wexu@...hat.com>,
        mst@...hat.com
Cc:     netdev@...r.kernel.org, davem@...emloft.net
Subject: Re: Regression in throughput between kvm guests over virtual bridge

On 10/22/2017 10:06 PM, Jason Wang wrote:
> 
> 
> On 2017年10月19日 04:17, Matthew Rosato wrote:
>>> 2. It might be useful to short the traffic path as a reference, What
>>> I am running
>>> is briefly like:
>>>      pktgen(host kernel) -> tap(x) -> guest(DPDK testpmd)
>>>
>>> The bridge driver(br_forward(), etc) might impact performance due to
>>> my personal
>>> experience, so eventually I settled down with this simplified testbed
>>> which fully
>>> isolates the traffic from both userspace and host kernel stack(1 and
>>> 50 instances,
>>> bridge driver, etc), therefore reduces potential interferences.
>>>
>>> The down side of this is that it needs DPDK support in guest, has
>>> this ever be
>>> run on s390x guest? An alternative approach is to directly run XDP
>>> drop on
>>> virtio-net nic in guest, while this requires compiling XDP inside
>>> guest which needs
>>> a newer distro(Fedora 25+ in my case or Ubuntu 16.10, not sure).
>>>
>> I made an attempt at DPDK, but it has not been run on s390x as far as
>> I'm aware and didn't seem trivial to get working.
>>
>> So instead I took your alternate suggestion & did:
>> pktgen(host) -> tap(x) -> guest(xdp_drop)
>>
>> When running this setup, I am not able to reproduce the regression.  As
>> mentioned previously, I am also unable to reproduce when running one end
>> of the uperf connection from the host - I have only ever been able to
>> reproduce when both ends of the uperf connection are running within a
>> guest.
>>
> 
> Thanks for the test. Looking at the code, the only obvious difference
> when BATCH is 1 is that one spinlock which was previously called by
> tun_peek_len() was avoided since we can do it locally. I wonder whether
> or not this speeds up handle_rx() a little more then leads more wakeups
> during some rates/sizes of TCP stream. To prove this, maybe you can try:
> 
> - enable busy polling, using poll-us=1000, and to see if we can still
> get the regression

Enabled poll-us=1000 for both guests - drastically reduces throughput,
but can still see the regression between host 4.12->4.13 running the
uperf workload


> - measure the pps pktgen(vm1) -> tap1 -> bridge -> tap2 -> vm2
> 

I'm getting apparent stalls when I run pktgen from the guest in this
manner...  (pktgen thread continues spinning after the first 5000
packets make it to vm2, but no further packets get sent).  Not sure why yet.