From mboxrd@z Thu Jan  1 00:00:00 1970
From: Ingo Molnar <mingo@elte.hu>
Subject: Re: [PATCH 1/2] kvm tools: Respect ISR status in virtio header
Date: Sat, 7 May 2011 16:00:30 +0200
Message-ID: <20110507140030.GA2859@elte.hu>
References: <1304735660-10844-1-git-send-email-asias.hejun@gmail.com>
 <20110507093027.GD27657@elte.hu>
 <4DC529E8.8030104@gmail.com>
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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: Asias He <asias.hejun@gmail.com>
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* 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.

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 ...

Thanks,

	Ingo