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:

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On Thu, Jul 27, 2017 at 11:38:52PM +0300, Michael S. Tsirkin wrote:

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On Thu, Jul 27, 2017 at 12:09:42PM -0500, Seth Forshee wrote:

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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.

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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.

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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