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From John Lilley <john.lil...@redpoint.net>
Subject RE: Containers and CPU
Date Tue, 02 Jul 2013 19:28:45 GMT
Sandy,
Thanks, I think I understand.  So it only makes a difference if cgroups is on AND the AM requests
multiple cores?  E.g. if each task wants 4 cores the RM would only allow two containers per
8-core node?
John


From: Sandy Ryza [mailto:sandy.ryza@cloudera.com]
Sent: Tuesday, July 02, 2013 1:26 PM
To: user@hadoop.apache.org
Subject: Re: Containers and CPU

Use of cgroups for controlling CPU is off by default, but can be turned on as a nodemanager
configuration with yarn.nodemanager.linux-container-executor.resources-handler.class.  So
it is site-wide.  If you want tasks to purely fight it out in the OS thread scheduler, simply
don't change from the default.

Even with cgroups on, all tasks will have access to all CPU cores.  We don't do any pinning
of tasks to cores.  If a task is requested with a single vcore and placed on an otherwise
empty machine with 8 cores, it will have access to all 8 cores.  If 3 other tasks that requested
a single vcore are later placed on the same node, and all tasks are using as much CPU as they
can get their hands on, then each of the tasks will get 2 cores of CPU-time.

On Tue, Jul 2, 2013 at 12:12 PM, John Lilley <john.lilley@redpoint.net<mailto:john.lilley@redpoint.net>>
wrote:
Sandy,
Sorry, I don't completely follow.
When you say "with cgroups on", is that an attribute of the AM, the Scheduler, or the Site/RM?
 In other words is it site-wide or something that my application can control?
With cgroups on, is there still a way to get my desired behavior?  I'd really like all tasks
to have access to all CPU cores and simply fight it out in the OS thread scheduler.
Thanks,
john

From: Sandy Ryza [mailto:sandy.ryza@cloudera.com<mailto:sandy.ryza@cloudera.com>]
Sent: Tuesday, July 02, 2013 11:56 AM
To: user@hadoop.apache.org<mailto:user@hadoop.apache.org>
Subject: Re: Containers and CPU

CPU limits are only enforced if cgroups is turned on.  With cgroups on, they are only limited
when there is contention, in which case tasks are given CPU time in proportion to the number
of cores requested for/allocated to them.  Does that make sense?

-Sandy

On Tue, Jul 2, 2013 at 9:50 AM, Chuan Liu <chuanliu@microsoft.com<mailto:chuanliu@microsoft.com>>
wrote:
I believe this is the default behavior.
By default, only memory limit on resources is enforced.
The capacity scheduler will use DefaultResourceCalculator to compute resource allocation for
containers by default, which also does not take CPU into account.

-Chuan

From: John Lilley [mailto:john.lilley@redpoint.net<mailto:john.lilley@redpoint.net>]
Sent: Tuesday, July 02, 2013 8:57 AM
To: user@hadoop.apache.org<mailto:user@hadoop.apache.org>
Subject: Containers and CPU

I have YARN tasks that benefit from multicore scaling.  However, they don't *always* use more
than one core.  I would like to allocate containers based only on memory, and let each task
use as many cores as needed, without allocating exclusive CPU "slots" in the scheduler.  For
example, on an 8-core node with 16GB memory, I'd like to be able to run 3 tasks each consuming
4GB memory and each using as much CPU as they like.  Is this the default behavior if I don't
specify CPU restrictions to the scheduler?
Thanks
John





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