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

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Message-ID: <4A283D72.6070603@google.com>
Date:	Thu, 04 Jun 2009 14:32:34 -0700
From:	Mike Waychison <mikew@...gle.com>
To:	Avi Kivity <avi@...hat.com>
CC:	bharata@...ux.vnet.ibm.com,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Pavel Emelyanov <xemul@...nvz.org>,
	Dhaval Giani <dhaval@...ux.vnet.ibm.com>, kvm@...r.kernel.org,
	Gautham R Shenoy <ego@...ibm.com>,
	Linux Containers <containers@...ts.linux-foundation.org>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>
Subject: Re: [RFC] CPU hard limits

Avi Kivity wrote:
> Bharata B Rao wrote:
>> 2. Need for hard limiting CPU resource
>> --------------------------------------
>> - Pay-per-use: In enterprise systems that cater to multiple clients/customers
>>   where a customer demands a certain share of CPU resources and pays only
>>   that, CPU hard limits will be useful to hard limit the customer's job
>>   to consume only the specified amount of CPU resource.
>> - In container based virtualization environments running multiple containers,
>>   hard limits will be useful to ensure a container doesn't exceed its
>>   CPU entitlement.
>> - Hard limits can be used to provide guarantees.
>>   
> How can hard limits provide guarantees?

Hard limits are useful and desirable in situations where we would like 
to maintain deterministic behavior.

Placing a hard cap on the cpu usage of a given task group (and 
configuring such that this cpu time is not overcommited) on a system 
allows us to create a hard guarantee that throughput for that task group 
will not fluctuate as other workloads are added and removed on the system.

Cache use and bus bandwidth in a multi-workload environment can still 
cause a performance deviation, but these are second order compared to 
the cpu scheduling guarantees themselves.

Mike Waychison
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