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From "Thomas Sandholm (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-4768) Dynamic Priority Scheduler that allows queue shares to be controlled dynamically by a currency
Date Wed, 10 Dec 2008 16:54:44 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-4768?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12655280#action_12655280

Thomas Sandholm commented on HADOOP-4768:

Thanks for your input,

consumable quotas, and budget accounting is a requirement that we have that is not supported
by any of the schedulers today. It allows users themselves to change regulated priorities
that are valid in a competitive multi-user setting (where social peer pressure assumptions
break down). The idea here is that demand varies over time as do user job priority preferences.
When demand is high you would want to encourage only the most important jobs to be run and
give users with low priority jobs an incentive to hold off on submitting their jobs. Also
note that the priorities a user sets that do not affect her in any way tend to be very different
from the priorities she would have to pay for in some way.  Having access to a user's 'truthful'
priorities allows the scheduler to do a more accurate job in efficiently mapping users to
available resources while taking current demand into account.  

Back to the implementation approach. As I mentioned above, one approach I evaluated was to
have a separate process that pushes the necessary changes to the config files.  The fact that
the capacity scheduler currently doesn't support dynamic updates of the config file is a minor
issue in this context and I actually also used a patch that fixed this.  The more important
showstopper for this approach was the need to replicate the whole reliable hadoop service
infrastructure. We have implemented our own systems and services to do much more involved
budget accounting than this but contributing that whole package to Hadoop would be too much
work and all of it may not be useful to the Hadoop community in general. So what I tried to
do in this patch was to extract the most important pieces from our previous work that solves
the above mentioned problems using as much of the existing hadoop infrastructure as possible.

Therefore, ideally we would like to plug in some code in the scheduler event loop that allows
us to set priorities (that have been paid for and that are being accounted for towards a budget).
Implementing our own scheduler altogether was an option but we are not so interested in and
don't have the low-level experise in how the priorities should be enforced in the map/reduce
context. Hence, it seemed natural to reuse the fairshare or capacity scheduler for this purpose.
 If we assume that we have a scheduler-collocated budget algorithm it seems very roundabout
and difficult to support multiple priority enforcers if we need to handle all the different
configuration file formats of the individual schedulers. Fiddling around with xpath will also
add a configuration and parsing complexity apart from limiting performance. A better solution
in my opinion would be to have a way for the plugin to communicate and update in priorities
directly to the scheduler within the given scheduler framework. The only interface in the
current code base I found that could be used for this purpose was Confiuration properties.
This in-memory approach also has the advantage that shedulers can implement more sophisticated
enforcement of shares paid for by users as both Vivek and Matei alluded to above.

To summarize, my requirements for the scheduling framework are as follows:
-Scheduler independent plug point in the job tracker event loop to host budget accounting
algorithm and to communicate  paid-for shares to resource-share enforcers such as the existing
two schedulers
-Scheduler independent interface to communicate paid-for shares to resource share-enforcers
(this could still be 'standardized'  xml config files if you find that appropriate but it
has the performance and  complexity implications I mentioned above) 

The patch i submitted may not solve these problems in the absolute optimal way because i didn't
want to change any interfaces in core or the scheduler framework itself. It represents my
understanding of the simplest way to address these issues with the current interfaces though,
and it is a first attempt to contribute our work to the hadoop community. Our falback is to
just pick one scheduler and modify the config file from within our system, but we would not
contribute anything to the community then and we would be left with a brittle interface to
a specific scheduler.

I will also talk through these issues with Owen and Arun when I meet them on Thursday and
report back here.



 The problem here is that the whole hadoop service infrastructure needs to be replicated which

> Dynamic Priority Scheduler that allows queue shares to be controlled dynamically by a
> ----------------------------------------------------------------------------------------------
>                 Key: HADOOP-4768
>                 URL: https://issues.apache.org/jira/browse/HADOOP-4768
>             Project: Hadoop Core
>          Issue Type: New Feature
>          Components: contrib/capacity-sched, contrib/fair-share
>    Affects Versions: 0.20.0
>            Reporter: Thomas Sandholm
>            Assignee: Thomas Sandholm
>             Fix For: 0.20.0
>         Attachments: HADOOP-4768-capacity-scheduler.patch, HADOOP-4768-dynamic-scheduler.patch,
HADOOP-4768-fairshare.patch, HADOOP-4768.patch
> Contribution based on work presented at the Hadoop User Group meeting in Santa Clara
in September and the HadoopCamp in New Orleans in November.
> From README:
> This package implements dynamic priority scheduling for MapReduce jobs.
> Overview
> --------
> The purpose of this scheduler is to allow users to increase and decrease
> their queue priorities continuosly to meet the requirements of their
> current workloads. The scheduler is aware of the current demand and makes
> it more expensive to boost the priority under peak usage times. Thus
> users who move their workload to low usage times are rewarded with
> discounts. Priorities can only be boosted within a limited quota.
> All users are given a quota or a budget which is deducted periodically
> in configurable accounting intervals. How much of the budget is 
> deducted is determined by a per-user spending rate, which may
> be modified at any time directly by the user. The cluster slots 
> share allocated to a particular user is computed as that users
> spending rate over the sum of all spending rates in the same accounting
> period.
> Configuration
> -------------
> This scheduler has been designed as a meta-scheduler on top of 
> existing MapReduce schedulers, which are responsible for enforcing
> shares computed by the dynamic scheduler in the cluster. Thie configuration
> of this MapReduce scheduler does not have to change when deploying
> the dynamic scheduler.
> Hadoop Configuration (e.g. hadoop-site.xml):
> mapred.jobtracker.taskScheduler      This needs to be set to 
>                                      org.apache.hadoop.mapred.DynamicPriorityScheduler
>                                      to use the dynamic scheduler.
> mapred.queue.names                   All queues managed by the dynamic scheduler must
be listed
>                                      here (comma separated no spaces)
> Scheduler Configuration:
> mapred.dynamic-scheduler.scheduler   The Java path of the MapReduce scheduler that should
>                                      enforce the allocated shares.
>                                      Has been tested with:
>                                      org.apache.hadoop.mapred.FairScheduler
>                                      and
>                                      org.apache.hadoop.mapred.CapacityTaskScheduler
> mapred.dynamic-scheduler.budgetfile  The full OS path of the file from which the
>                                      budgets are read. The synatx of this file is:
>                                      <queueName> <budget>
>                                      separated by newlines where budget can be specified
>                                      as a Java float
> mapred.dynamic-scheduler.spendfile   The full OS path of the file from which the
>                                      user/queue spending rate is read. It allows
>                                      the queue name to be placed into the path
>                                      at runtime, e.g.:
>                                      /home/%QUEUE%/.spending
>                                      Only the user(s) who submit jobs to the
>                                      specified queue should have write access
>                                      to this file. The syntax of the file is
>                                      just:
>                                      <spending rate>
>                                      where the spending rate is specified as a
>                                      Java float. If no spending rate is specified
>                                      the rate defaults to budget/1000.
> mapred.dynamic-scheduler.alloc       Allocation interval, when the scheduler rereads
>                                      spending rates and recalculates the cluster shares.
>                                      Specified as seconds between allocations.
>                                      Default is 20 seconds.
> mapred.dynamic-scheduler.budgetset   Boolean which is true if the budget should be deducted

>                                      by the scheduler and the updated budget written
to the
>                                      budget file. Default is true. Setting this to false
>                                      useful if there is a tool that controls budgets
>                                      spending rates externally to the scheduler.
> Runtime Configuration:
> mapred.scheduler.shares              The shares that should be allocated to the specified
>                                      The configuration property is a comma separated
list of
>                                      strings where the odd positioned elements are the

>                                      queue names and the even positioned elements are
the shares
>                                      as Java floats of the preceding queue name. It is
>                                      for all the queues atomically in each allocation
pass. MapReduce
>                                      schedulers such as the Fair and CapacityTask schedulers
>                                      are expected to read from this property periodically.
>                                      Example property value: "queue1,45.0,queue2,55.0"

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