netdev - Re: [PATCH v2 net-next] pkt_sched: fq: Fair Queue packet scheduler

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Date:	Thu, 05 Sep 2013 11:39:36 +0800
From:	Jason Wang <jasowang@...hat.com>
To:	Daniel Borkmann <dborkman@...hat.com>
CC:	Eric Dumazet <eric.dumazet@...il.com>,
	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>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH v2 net-next] pkt_sched: fq: Fair Queue packet scheduler

On 09/04/2013 07:59 PM, Daniel Borkmann wrote:
> On 09/04/2013 01:27 PM, Eric Dumazet wrote:
>> On Wed, 2013-09-04 at 03:30 -0700, Eric Dumazet wrote:
>>> On Wed, 2013-09-04 at 14:30 +0800, Jason Wang wrote:
>>>
>>>>> And tcpdump would certainly help ;)
>>>>
>>>> See attachment.
>>>>
>>>
>>> Nothing obvious on tcpdump (only that lot of frames are missing)
>>>
>>> 1) Are you capturing part of the payload only (like tcpdump -s 128)
>>>
>>> 2) What is the setup.
>>>
>>> 3) tc -s -d qdisc
>>
>> If you use FQ in the guest, then it could be that high resolution timers
>> have high latency ?
>
> Probably they internally switch to a lower resolution clock event
> source if
> there's no hardware support available:
>
>   The [source event] management layer provides interfaces for hrtimers to
>   implement high resolution timers [...] [and it] supports these more
> advanced
>   functions only when appropriate clock event sources have been
> registered,
>   otherwise the traditional periodic tick based behaviour is retained.
> [1]
>
> [1] https://www.kernel.org/doc/ols/2006/ols2006v1-pages-333-346.pdf 

Maybe, AFAIK, kvm-clock does not provide a clock event, only a pv
clocksource were provided.
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