Re: [PATCH] vhost: Add polling mode

On Tue, Aug 12, 2014 at 01:57:05PM +0300, Razya Ladelsky wrote:

"Michael S. Tsirkin" [off-list ref] wrote on 12/08/2014 12:18:50 PM:

quoted

From: "Michael S. Tsirkin" <mst@redhat.com>
To: David Miller <davem@davemloft.net>
Cc: Razya Ladelsky/Haifa/IBM@IBMIL, kvm@vger.kernel.org, Alex 
Glikson/Haifa/IBM@IBMIL, Eran Raichstein/Haifa/IBM@IBMIL, Yossi 
Kuperman1/Haifa/IBM@IBMIL, Joel Nider/Haifa/IBM@IBMIL, 
abel.gordon@gmail.com, linux-kernel@vger.kernel.org, 
netdev@vger.kernel.org, virtualization@lists.linux-foundation.org
Date: 12/08/2014 12:18 PM
Subject: Re: [PATCH] vhost: Add polling mode

On Mon, Aug 11, 2014 at 12:46:21PM -0700, David Miller wrote:

quoted

From: "Michael S. Tsirkin" <mst@redhat.com>
Date: Sun, 10 Aug 2014 21:45:59 +0200

quoted

On Sun, Aug 10, 2014 at 11:30:35AM +0300, Razya Ladelsky wrote:

 ...

quoted

And, did your tests actually produce 100% load on both host CPUs?

 ...

Michael, please do not quote an entire patch just to ask a one line
question.

I truly, truly, wish it was simpler in modern email clients to delete
the unrelated quoted material because I bet when people do this they
are simply being lazy.

Thank you.

Lazy - mea culpa, though I'm using mutt so it isn't even hard.

The question still stands: the test results are only valid
if CPU was at 100% in all configurations.
This is the reason I generally prefer it when people report
throughput divided by CPU (power would be good too but it still
isn't easy for people to get that number).

Hi Michael,

Sorry for the delay, had some problems with my mailbox, and I realized 
just now that 
my reply wasn't sent.
The vm indeed ALWAYS utilized 100% cpu, whether polling was enabled or 
not.
The vhost thread utilized less than 100% (of the other cpu) when polling 
was disabled.
Enabling polling increased its utilization to 100% (in which case both 
cpus were 100% utilized). 

Hmm this means the testing wasn't successful then, as you said:

	The idea was to get it 100% loaded, so we can see that the polling is
	getting it to produce higher throughput.

in fact here you are producing more throughput but spending more power
to produce it, which can have any number of explanations besides polling
improving the efficiency. For example, increasing system load might
disable host power management.

quoted

-- 
MST