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Date:	Tue, 03 Sep 2013 22:59:36 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jason Wang <jasowang@...hat.com>
Cc:	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Yuchung Cheng <ycheng@...gle.com>,
	Neal Cardwell <ncardwell@...gle.com>,
	"Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: [PATCH v2 net-next] pkt_sched: fq: Fair Queue packet scheduler

On Wed, 2013-09-04 at 13:26 +0800, Jason Wang wrote:

> I see both degradation and jitter when using fq with virtio-net. Guest
> to guest performance drops from 8Gb/s to 3Gb/s-7Gb/s. Guest to local
> host drops from 8Gb/s to 4Gb/s-6Gb/s. Guest to external host with ixgbe
> drops from 9Gb/s to 7Gb/s
> 
> I didn't meet the issue when using sfq or disabling pacing.
> 
> So it looks like it was caused by the inaccuracy and jitter of the
> pacing estimation in a virt guest?

Well, using virtio-net means you use FQ without pacing.

Make sure you do not have reorders because of a bug in queue selection.

TCP stack has the ooo_okay thing, I do not think a VM can get it.

nstat >/dev/null ; <your test> ; nstat

And tcpdump would certainly help ;)


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