linux-kernel - Re: [RFC] CPU hard limits

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Message-ID: <20090605113217.GA20786@in.ibm.com>
Date:	Fri, 5 Jun 2009 17:02:17 +0530
From:	Srivatsa Vaddagiri <vatsa@...ibm.com>
To:	Paul Menage <menage@...gle.com>
Cc:	bharata@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	Dhaval Giani <dhaval@...ux.vnet.ibm.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	Gautham R Shenoy <ego@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Pavel Emelyanov <xemul@...nvz.org>,
	Avi Kivity <avi@...hat.com>, kvm@...r.kernel.org,
	Linux Containers <containers@...ts.linux-foundation.org>,
	Herbert Poetzl <herbert@...hfloor.at>
Subject: Re: [RFC] CPU hard limits

On Fri, Jun 05, 2009 at 01:53:15AM -0700, Paul Menage wrote:
> This claim (and the subsequent long thread it generated on how limits
> can provide guarantees) confused me a bit.
> 
> Why do we need limits to provide guarantees when we can already
> provide guarantees via shares?

I think the interval over which we need guarantee matters here. Shares
can generally provide guaranteed share of resource over longer (sometimes
minutes) intervals. For high-priority bursty workloads, the latency in 
achieving guaranteed resource usage matters. By having hard-limits, we are 
"reserving" (potentially idle) slots where the high-priority group can run and 
claim its guaranteed share almost immediately.

- vatsa
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