On Mon, 22 May 2017, Michael S. Tsirkin wrote:
> On Mon, May 22, 2017 at 10:25:19AM -0400, Mikulas Patocka wrote:
> > Hi
> >
> > I see severe network performance degradation with the kernels 4.12-rc1 and
> > 4.12-rc2 in the network virtio driver. Download rate drops down to about
> > 100kB/s.
> >
> > I bisected it and it is caused by patch
> > d85b758f72b05a774045545f24d70980e3e9aac4 ("virtio_net: fix support for
> > small rings"). When I revert this patch, the problem goes away.
> >
> > The host is Debian Jessie with kernel 4.4.62, the guest is Debian Sid with
> > kernel 4.12-rc.
> >
> > Mikulas
>
> Thanks for the report!
> I'm not sure what causes it on this host. I'll try to reproduce.
> Meanwhile, could you please locate this line of code:
> + vi->rq[i].min_buf_len = mergeable_min_buf_len(vi, vi->rq[i].vq);
>
> and add something like
> printk(KERN_ERR, "min buf = 0x%x expected 0x%x size 0x%x big %d\n",
> vi->rq[i].min_buf_len, GOOD_PACKET_LEN,
> virtqueue_get_vring_size(vi->rq[i].vq),
> (int)vi->big_packets);
>
> after it?
> Then boot and capture the output.
>
> Thanks!
It prints one line "min buf = 0x101 expected 0x5ee size 0x100 big 1".
Mikulas
