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5.1.2 Job Priority Factors
Maui allows jobs to be prioritized based on a range of job related factors. These factors are broken down into a two-level hierarchy of priority factors and subfactors each of which can be independently assigned a weight. This approach provides the administrator with detailed yet straightforward control of the job selection process. The table below highlights the components and subcomponents which make up the total job priority.
With the Moab Cluster ManagerTM, priority factors and subfactors can be controlled with sliding bars and the click of the mouse. Also, the calculated priority, broken up by factors and subfactors, is enumerated in a table to see their effects. (Click HERE for more information)
|USER||user specific priority (See USERCFG)|
|GROUP||group specific priority (See GROUPCFG)|
|ACCOUNT||account specific priority (SEE ACCOUNTCFG)|
|QOS||QOS specific priority (See QOSCFG)|
|CLASS||class/queue specific priority (See CLASSCFG)|
|FSUSER||user based historical usage (See Fairshare Overview)|
|FSGROUP||group based historical usage (See Fairshare Overview)|
|FSACCOUNT||account based historical usage (See Fairshare Overview)|
|FSQOS||QOS base historical usage (See Fairshare Overview)|
|FSCLASS||class/queue based historical usage (See Fairshare Overview)|
(requested job resources)
|NODE||number of nodes requested|
|PROC||number of processors requested|
|MEM||total real memory requested (in MB)|
|SWAP||total virtual memory requested (in MB)|
|DISK||total local disk requested (in MB)|
|PS||total proc-seconds requested|
|PE||total processor-equivalent requested|
|WALLTIME||total walltime requested (in seconds)|
(current service levels)
|QUEUETIME||time job has been queued (in minutes)|
|XFACTOR||minimum job expansion factor|
|BYPASS||number of times job has been bypassed by backfill|
(target service levels)
|TARGETQUEUETIME||time until queuetime target is reached (exponential)|
|TARGETXFACTOR||distance to target expansion factor (exponential)|
(consumed resources -- active jobs only)
|CONSUMED||proc-seconds dedicated to date|
|PERCENT||percent of required walltime consumed|
|EXECUTIONTIME||seconds since job started|
18.104.22.168 Credential (CRED) Component
The credential component allows a site to prioritize jobs based on political issues such as the relative importance of certain groups or accounts. This allows direct political priorities to be applied to jobs.
The priority calculation for the credential component is:
Priority += CREDWEIGHT * (
USERWEIGHT * J->U->Priority +
GROUPWEIGHT * J->G->Priority +
ACCOUNTWEIGHT * J->A->Priority +
QOSWEIGHT * J->Q->Priority +
CLASSWEIGHT * J->C->Priority)
All user, group, account, QoS, and class weights are specified by setting the PRIORITY attribute of using the respective '*CFG' parameter, namely, USERCFG, GROUPCFG, ACCOUNTCFG, QOSCFG, and CLASSCFG.
For example, to set user and group priorities, the following might be used.
Class (or queue) priority may also be specified via the resource manager where supported (i.e., PBS queue priorities). However, if Maui class priority values are also specified, the resource manager priority values will be overwritten.
All priorities may be positive or negative.
Fairshare components allow a site to favor jobs based on short term historical usage. The Fairshare Overview describes the configuration and use of Fairshare in detail.
After the brief reprieve from complexity found in the QOS factor, we come to the Fairshare factor. This factor is used to adjust a job's priority based on the historical percentage system utilization of the jobs user, group, account, or QOS. This allows you to 'steer' the workload toward a particular usage mix across user, group, account, and QOS dimensions. The fairshare priority factor calculation is
Priority += FSWEIGHT * MIN(FSCAP, (
FSUSERWEIGHT * DeltaUserFSUsage +
FSGROUPWEIGHT * DeltaGroupFSUsage +
FSACCOUNTWEIGHT * DeltaAccountFSUsage +
FSQOSWEIGHT * DeltaQOSFSUsage +
FSCLASSWEIGHT * DeltaClassFSUsage))
All '*WEIGHT' parameters above are specified on a per partition basis in the maui.cfg file. The 'Delta*Usage' components represents the difference in actual fairshare usage from a fairshare usage target. Actual fairshare usage is determined based on historical usage over the timeframe specified in the fairshare configuration. The target usage can be either a target, floor, or ceiling value as specified in the fairshare config file. The fairshare documentation covers this in detail but an example should help obfuscate things completely. Consider the following information associated with calculating the fairshare factor for job X.
Fairshare Target: 50.0
Current Fairshare Usage: 45.0
Fairshare Target: [NONE]
Current Fairshare Usage: 65.0
Fairshare Target: 25.0
Current Fairshare Usage: 35.0
Fairshare Target: 10.0+
Current Fairshare Usage: 25.0
Fairshare Target: [NONE]
Current Fairshare Usage: 20.0
In this example, the Fairshare component calculation would be as follows:
Priority += 100 * (
10 * 5 +
20 * 0 +
30 * (-10) +
40 * 0 +
0 * 0)
User A is 5% below his target so fairshare increases the total fairshare factor accordingly. Group B has no target so group fairshare usage is ignored. Account C is above its 10% above its fairshare usage target so this component decreases the job's total fairshare factor. QOS 3 is 15% over its target but the '+' in the target specification indicates that this is a 'floor' target, only influencing priority when fairshare usage drops below the target value. Thus, the QOS 3 fairshare usage delta does not influence the fairshare factor.
Fairshare is a great mechanism for influencing job turnaround time via priority to favor a particular distribution of jobs. However, it is important to realize that fairshare can only favor a particular distribution of jobs, it cannot force it. If user X has a fairshare target of 50% of the machine but does not submit enough jobs, no amount of priority favoring will get user X's usage up to 50%. See the Fairshare Overview for more information.
Weighting jobs by the amount of resources requested allows a site to favor particular types of jobs. Such prioritization may allow a site to better meet site mission objectives, improve fairness, or even improve overall system utilization.
Resource based prioritization is valuable when you want to favor jobs based on the resources requested. This is good in three main scenarios; first, when you need to favor large resource jobs because its part of your site's mission statement; second, when you want to level the response time distribution across large and small jobs (small jobs are more easily backfilled and thus generally have better turnaround time); and finally, when you want to improve system utilization. What? Yes, system utilization actually increases as large resource jobs are pushed to the front of the queue. This keeps the smaller jobs in the back where they can be selected for backfill and thus increase overall system utilization. Its a lot like the story about filling a cup with golf balls and sand. If you put the sand in first, it gets in the way when you try to put in the golf balls. However, if you put in the golf balls first, the sand can easily be poured in around them completely filling the cup.
The calculation for determining the total resource priority factor is:
Priority += RESWEIGHT
* MIN(RESCAP, (
NODEWEIGHT * TotalNodesRequested +
PROCWEIGHT * TotalProcessorsRequested +
MEMWEIGHT * TotalMemoryRequested +
SWAPWEIGHT * TotalSwapRequested +
DISKWEIGHT * TotalDiskRequested +
PEWEIGHT * TotalPERequested))
The sum of all weighted resources components is then multiplied by the RESWEIGHT parameter and capped by the RESCAP parameter. Memory, Swap, and Disk are all measured in megabytes (MB). The final resource component, PE, represents 'Processor Equivalents'. This component can be viewed as a processor-weighted maximum 'percentage of total resources' factor. For example, if a job requested 25% of the processors and 50% of the total memory on a 128 processor O2K system, it would have a PE value of MAX(25,50) * 128, or 64. The concept of PE's may be a little awkward to grasp initially but it is a highly effective metric in shared resource systems.
The Service component essentially specifies which service metrics are of greatest value to the site. Favoring one service subcomponent over another will generally cause that service metric to improve.
22.214.171.124.1 QueueTime (QUEUETIME) Subcomponent
In the priority calculation, a job's queue time is a duration measured in minutes. Use of this subcomponent tends to prioritize jobs in a FIFO order. Favoring queue time improves queue time based fairness metrics and is probably the most widely used single job priority metric. In fact, under the initial default configuration, this is the only priority subcomponent enabled within Maui. It is important to note that within Maui, a job's queue time is not necessarily the amount of time since the job was submitted. The parameter JOBPRIOACCRUALPOLICY allows a site to select how a job will accrue queue time based on meeting various throttling policies. Regardless of the policy used to determine a job's queue time, this 'effective' queue time is used in the calculation of the QUEUETIME, XFACTOR, TARGETQUEUETIME, and TARGETXFACTOR priority subcomponent values.
The need for a distinct effective queue time is necessitated by the fact that most sites have pretty smart users and pretty smart users like to work the system, whatever system it happens to be. A common practice at some long existent sites is for some users to submit a large number of jobs and then place them on hold. These jobs remain with a hold in place for an extended period of time and when the user is ready to run a job, the needed executable and data files are linked into place and the hold released on one of these 'pre submitted' jobs. The extended hold time guarantees that this job is now the highest priority job and will be the next to run. The use of the JOBPRIOACCRUALPOLICY parameter can prevent this practice as well as preventing 'queue stuffers' from doing similar things on a shorter time scale. These 'queue stuffer' users submit hundreds of jobs at once so as to swamp the machine and hog use of the available compute resources. This parameter prevents the user from gaining any advantage from stuffing the queue by not allowing these jobs to accumulate any queue time based priority until they meet certain idle and/or active Maui fairness policies. (i.e., max job per user, max idle job per user, etc.)
As a final note, the parameter QUEUETIMEWEIGHT can be adjusted on a per QOS basis using the QOSCFG parameter and the QTWEIGHT attribute. For example, the line 'QOSCFG[special] QTWEIGHT=5000' will cause jobs utilizing the QOS special to have their queue time subcomponent weight increased by 5000.
126.96.36.199.2 Expansion Factor (XFACTOR) Subcomponent
The expansion factor subcomponent has an effect
similar to the queue time factor but favors shorter jobs based on their
requested wallclock run time. In its canonical form, the expansion factor (XFactor) metric is calculated as
XFACTOR = 1 + <QUEUETIME> / <EXECUTIONTIME>
However, a couple of aspects of this calculation make its use more difficult. First, the length of time the job will actually run, 'Execution Time', is not actually known until the job completes. All that is known is how much time the job requests. Secondly, as described in the Queue Time Subcomponent section, Maui does not necessarily use the raw time since job submission to determine 'QueueTime' so as to prevent various scheduler abuses. Consequently, Maui uses the following modified equation:
XFACTOR = 1 + <EFFQUEUETIME> / <WALLCLOCKLIMIT>
In the equation above, EFFQUEUETIME is the effective queue time subject to the JOBPRIOACCRUALPOLICY parameter and WALLCLOCKLIMIT is the user or system specified job wallclock limit.
Using this equation, it can be seen that short running
jobs will have an xfactor that will grow much faster over time
than the xfactor associated with long running jobs. The table below demonstrates this favoring of short running jobs.
|Job Queue Time||1 hour||2 hours||4 hours||8 hours||16 hours|
|XFactor for 1 hour job||1 + (1 / 1) = 2.00||1 + (2 / 1) = 3.00||1 + (4 / 1) = 5.00||1 + (8 / 1) = 9.00||1 + (16 / 1) = 17.0|
|XFactor for 4 hour job||1 + (1 / 4) = 1.25||1 + (2 / 4) = 1.50||1 + (4 / 4) = 2.00||1 + (8 / 4) = 3.00||1 + (16 / 4) = 5.0|
Since XFactor is calculated as a ratio of two values, it is possible for this subcomponent to be almost arbitrarily large potentially swamping the value of other priority subcomponents. This can be addressed either by using the subcomponent cap XFACTORCAP, or by using the XFMINWCLIMIT parameter. If the later is used, the calculation for the xfactor subcomponent value becomes:
XFACTOR = 1 + <EFFQUEUETIME> / MAX(<XFMINWCLIMIT>,<WALLCLOCKLIMIT>)
The use of the XFMINWCLIMIT parameter allows a site to prevent very short jobs from causing the Xfactor subcomponent to grow inordinately.
Some sites consider XFactor to be a more fair scheduling performance metric than queue time. At these sites, job XFactor is given far more weight than job queue time when calculating job priority and consequently, job XFactor distribution tends to be fairly level across a wide range of job durations. (i.e., A flat XFactor distribution of 1.0 would result in a one minute job being queued on average one minute, while a 24 hour job would be queued an average of 24 hours).
Like queue time, the effective XFactor subcomponent weight is the sum of two weights, the XFACTORWEIGHT parameter and the QOS specific XFWEIGHT setting. For example, the line 'QOSCFG[special] XFWEIGHT=5000' will cause jobs utilizing the QOS special to have their expansion factor subcomponent weight increased by 5000.
188.8.131.52.3 Bypass (BYPASS) Subcomponent
The bypass factor is the forgotten stepchild of the priority subcomponent family. It was originally introduced to prevent backfill based starvation. It is based on the 'bypass' count of a job where the bypass count is increased by one every time the job is 'bypassed' by a lower priority job via backfill. The calculation for this factor is simply. Over the years, the anticipated backfill starvation has never been reported. The good news is that if it ever shows up, Maui is ready!
The target factor component of priority takes into account job scheduling performance targets. Currently, this is limited to target expansion factor and target queue time. Unlike the expansion factor and queue time factors described earlier which increase gradually over time, the target factor component is designed to grow exponentially as the target metric is approached. This behavior causes the scheduler to do essentially 'all in its power' to make certain the scheduling targets are met.
The priority calculation for the target factor is:
Priority += TARGWEIGHT
The queue time and expansion factor target are specified on a per QOS basis using the 'QOSXFTARGET' and 'QOSQTTARGET' parameters. The QueueTime and XFactor component calculations are designed produce small values until the target value begins to approach at which point these components grow very rapidly. If the target is missed, these component will remain high and continue to grow but will not grow exponentially.
The Usage component applies to active jobs only.
The priority calculation for the usage priority factor is:
Priority += USAGEWEIGHT * (
USAGECONSUMEDWEIGHT * ProcSecondsConsumed +
USAGEREMAININGWEIGHT * ProcSecRemaining +
USAGEEXECUTIONTIMEWEIGHT * SecondsSinceStart +
USAGEPERCENTWEIGHT * WalltimePercent )