From mboxrd@z Thu Jan  1 00:00:00 1970
From: Asias He <asias.hejun@gmail.com>
Subject: Re: [PATCH 1/2] kvm tools: Respect ISR status in virtio header
Date: Sat, 07 May 2011 22:24:16 +0800
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References: <1304735660-10844-1-git-send-email-asias.hejun@gmail.com> <20110507093027.GD27657@elte.hu> <4DC529E8.8030104@gmail.com> <20110507140030.GA2859@elte.hu>
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Cc: "Michael S. Tsirkin" <mst@redhat.com>,
	Rusty Russell <rusty@rustcorp.com.au>,
	Mark McLoughlin <markmc@redhat.com>,
	Anthony Liguori <aliguori@us.ibm.com>,
	Pekka Enberg <penberg@kernel.org>,
	Cyrill Gorcunov <gorcunov@gmail.com>,
	Sasha Levin <levinsasha928@gmail.com>,
	Prasad Joshi <prasadjoshi124@gmail.com>, kvm@vger.kernel.org
To: Ingo Molnar <mingo@elte.hu>
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On 05/07/2011 10:00 PM, Ingo Molnar wrote:
> 
> * Asias He <asias.hejun@gmail.com> wrote:
> 
>> BTW. The ping latency was caused by the movement of irq injection outside the 
>> loop. Suppose we have 5 available buffers and only 1 buffer from tap device. 
>> We will sleep on read without giving the buffer from tap to guest. The 
>> latency will be huge in this case.
>>
>>   while(virt_queue__available(vq)) {
>> 	...
>> 	read(tap_fd)
>> 	...
>>    }
>>    trigger_irq()
> 
> But ... in one of the mails one of you claimed that even when moving the irq 
> notification inside the loop (which we all agreed was necessary to avoid 
> latencies!) the latencies would still occur during stress-tests.

What I got in stress-tests(ping -f) with irq notification inside the
loop is only network hangs.

> 
> So something is still not understood here and could hit us anytime with any of 
> the virtio drivers in the future and such bugs are not always so nice to debug 
> like the latency problem here ...

Yes, Especially when they are under tremendous stress.

> 
> Thanks,
> 
> 	Ingo
> 


-- 
Best Regards,
Asias He