linux-kernel - Re: [PATCH net-next 2/2] net: exit busy loop when another process is runnable

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Message-ID: <54080BF5.1070504@linux.intel.com>
Date:	Thu, 04 Sep 2014 09:51:33 +0300
From:	Eliezer Tamir <eliezer.tamir@...ux.intel.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	Jason Wang <jasowang@...hat.com>, Ingo Molnar <mingo@...nel.org>,
	Mike Galbraith <umgwanakikbuti@...il.com>, davem@...emloft.net,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH net-next 2/2] net: exit busy loop when another process
 is runnable

On 03/09/2014 10:51, Michael S. Tsirkin wrote:
> On Wed, Sep 03, 2014 at 09:49:10AM +0300, Eliezer Tamir wrote:
>> On 02/09/2014 11:31, Michael S. Tsirkin wrote:
>>> On Tue, Sep 02, 2014 at 09:15:18AM +0300, Eliezer Tamir wrote:

>> Busy polling is not a general purpose feature, it's not something you
>> can casually turn on and will "just work". Most applications should not
>> be using busy polling. Currently it is used by multiserver applications
>> that you spend days tuning to specific platforms.
>>
>> What the user wants is to lower both avg and maximum latencies, at the
>> expense of everything else including power efficiency and sometimes
>> even throughput. The only exception is making the system crash ;)
>>
>> While letting other things take precedence over busy polling might not
>> hurt the avg latency much, it will kill your maximum latency.
> 
> If scheduler happens to run both server and client on the
> same CPU, polling will hurt maximum latency even more.
> So I guess different users want different things.
> 
> How about applications giving us a hint what they prefer?
> For example, a new flag that says "I don't have anything useful to do so
> let's do busy polling but my server is on the local system, so please
> only poll if CPU is otherwise idle".

I'm sorry for being ambiguous, when I said multi-server application, I
meant an app that runs on more than one server machine.

The loopback test is as far as I know not interesting.

Of course if busypoll becomes interesting for virtualization over
loopback, I have no problem with that, provided that there is a way to
get the old behavior and that it is well documented.

-Eliezer
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