Subject: Re: Performance regression with virtio_net

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 27 Jul 2017 23:38:52 +0300

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?

> 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?
Also, could you please look at mergeable_rx_buffer_size in sysfs with
and without the change?

-- 
MST