netdev - Re: Performance regression with virtio

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Date:   Fri, 28 Jul 2017 08:12:56 -0500
From:   Seth Forshee <seth.forshee@...onical.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Jason Wang <jasowang@...hat.com>,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org
Subject: Re: Performance regression with virtio_net

On Fri, Jul 28, 2017 at 12:30:54AM +0300, Michael S. Tsirkin wrote:
> On Thu, Jul 27, 2017 at 04:14:30PM -0500, Seth Forshee wrote:
> > On Thu, Jul 27, 2017 at 11:38:52PM +0300, Michael S. Tsirkin wrote:
> > > On Thu, Jul 27, 2017 at 12:09:42PM -0500, Seth Forshee wrote:
> > > > I'm seeing a performance regression with virtio_net that looks to have
> > > > started in 4.12-rc1. I only see it in one context though, downloading
> > > > snap packages from the Ubuntu snap store. For example:
> > > > 
> > > >  https://api.snapcraft.io/api/v1/snaps/download/b8X2psL1ryVrPt5WEmpYiqfr5emixTd7_1797.snap
> > > > 
> > > > which redirects to Internap's CDN. Normally this downloads in a few
> > > > seconds at ~10 MB/s, but with 4.12 and 4.13 it takes minutes with a rate
> > > > of ~150 KB/s. Everything else I've tried downloads as normal speeds.
> > > 
> > > So just wget that URL should be enough?
> > 
> > Yes. Note that sometimes it starts out faster then slows down.
> > > > I bisected this to 680557cf79f8 "virtio_net: rework mergeable buffer
> > > > handling". If I revert this on top of 4.13-rc2 (along with other changes
> > > > needed to successfully revert it) speeds return to normal.
> > > > 
> > > > Thanks,
> > > > Seth
> > > 
> > > 
> > > Interesting. A more likely suspect would be
> > > e377fcc8486d40867c6c217077ad0fa40977e060 - could you please try
> > > reverting that one instead?
> > 
> > I tried it, and I still get slow download speeds. I did test at
> > 680557cf79f82623e2c4fd42733077d60a843513 during the bisect so I'm
> > reasonably confident that this is the one where things went bad.
> > > Also, could you please look at mergeable_rx_buffer_size in sysfs with
> > > and without the change?
> > 
> > In all cases (stock 4.13-rc2, 680557cf79f8 reverted, and e377fcc8486d
> > reverted) mergeable_rx_buffer_size was 1536.
> > 
> > Thanks,
> > Seth
> 
> Do you see any error counters incrementing after it slows down?

I see rx_dropped and rx_length_errors increasing in lockstep once it
slows down.

Thanks,
Seth