7.2 Backfill

7.2.1 Backfill Overview

Backfill is a scheduling optimization that allows a scheduler to make better use of available resources by running jobs out of order. When Moab schedules, it prioritizes the jobs in the queue according to a number of factors and then orders the jobs into a highest priority first (or priority FIFO) sorted list. It starts the jobs one by one stepping through the priority list until it reaches a job it cannot start. Because all jobs and reservations possess a start time and a wallclock limit, Moab can determine the completion time of all jobs in the queue. Consequently, Moab can also determine the earliest the needed resources will become available for the highest priority job to start.

Backfill operates based on this earliest job start information. Because Moab knows the earliest the highest priority job can start, and which resources it will need at that time, it can also determine which jobs can be started without delaying this job. Enabling backfill allows the scheduler to start other, lower-priority jobs so long as they do not delay the highest priority job. If backfill is enabled, Moab protects the highest priority job's start time by creating a job reservation to reserve the needed resources at the appropriate time. Moab then can start any job that will not interfere with this reservation.

Image 7-1: Scheduling with backfill

Click to animate

Backfill offers significant scheduler performance improvement. In a typical large system, enabling backfill increases system utilization by about 20% and improves turnaround time by an even greater amount. Because of the way it works, essentially filling in holes in node space, backfill tends to favor smaller and shorter running jobs more than larger and longer running ones. It is common to see over 90% of these small and short jobs backfilled. Consequently, sites will see marked improvement in the level of service delivered to the small, short jobs and moderate to little improvement for the larger, long ones.

With most algorithms and policies, there is a trade-off. Backfill is not an exception but the negative effects are minor. Because backfill locates jobs to run from throughout the idle job queue, it tends to diminish the influence of the job prioritization a site has chosen and thus may negate some desired workload steering attempts through this prioritization. Although by default the start time of the highest priority job is protected by a reservation, there is nothing to prevent the third priority job from starting early and possibly delaying the start of the second priority job. This issue is addressed along with its trade-offs later in this section.

Another problem is a little more subtle. Consider the following scenario involving a two-processor cluster. Job A has a four-hour wallclock limit and requires one processor. It started one hour ago (time zero) and will reach its wallclock limit in three more hours. Job B is the highest priority idle job and requires two processors for one hour. Job C is the next highest priority job and requires one processor for two hours. Moab examines the jobs and correctly determines that job A must finish in three hours and thus, the earliest job B can start is in three hours. Moab also determines that job C can start and finish in less than this amount of time. Consequently, Moab starts job C on the idle processor at time one. One hour later (time two), job A completes early. Apparently, the user overestimated the amount of time job A would need by a few hours. Since job B is now the highest priority job, it should be able to run. However, job C, a lower priority job was started an hour ago and the resources needed for job B are not available. Moab re-evaluates job B's reservation and determines that it can slide forward an hour. At time three, job B starts.

In review, backfill provided positive benefits. Job A successfully ran to completion. Job C was started immediately. Job B was able to start one hour sooner than its original target time, although, had backfill not been enabled, job B would have been able to run two hours earlier.

The scenario just described occurs quite frequently because user estimates for job duration are generally inaccurate. Job wallclock estimate accuracy, or wallclock accuracy, is defined as the ratio of wall time required to actually run the job divided by the wall time requested for the job. Wallclock accuracy varies from site to site but the site average is rarely better than 50%. Because the quality of the walltime estimate provided by the user is so low, job reservations for high priority jobs are often later than they need to be.

Although there do exist some minor drawbacks with backfill, its net performance impact on a site's workload is very positive. While a few of the highest priority jobs may get temporarily delayed, their position as highest priority was most likely accelerated by the fact that jobs in front of them were able to start earlier due to backfill. Studies have shown that only a very small number of jobs are truly delayed and when they are, it is only by a fraction of their total queue time. At the same time, many jobs are started significantly earlier than would have occurred without backfill.

The following image demonstrates how Moab might schedule a queue using backfill.

7.2.2 Backfill Algorithms

BACKFILLPOLICY controls which job gets selected first to be backfilled. Backfill jobs are still placed on nodes according to the NODEALLOCATIONPOLICY.

The algorithm behind Moab backfill scheduling is straightforward, although there are a number of issues and parameters that should be highlighted. First of all, Moab makes two backfill scheduling passes. For each pass, Moab selects a list of jobs that are eligible for backfill. On the first pass, only those jobs that meet the constraints of the soft fairness throttling policies are considered and scheduled. The second pass expands this list of jobs to include those that meet the hard (less constrained) fairness throttling policies.

The second important concept regarding Moab backfill is the concept of backfill windows. The figure below shows a simple batch environment containing two running jobs and a reservation for a third job. The present time is represented by the leftmost end of the box with the future moving to the right. The light gray boxes represent currently idle nodes that are eligible for backfill. For this example, let's assume that the space represented covers 8 nodes and a 2 hour time frame. To determine backfill windows, Moab analyzes the idle nodes essentially looking for largest node-time rectangles. It determines that there are two backfill windows. The first window, Window 1, consists of 4 nodes that are available for only one hour (because some of the nodes are blocked by the reservation for Job 3). The second window contains only one node but has no time limit because this node is not blocked by the reservation for Job 3. It is important to note that these backfill windows overlap.

Image 7-2: Backfillable nodes create backfill windows 1 and 2

Click to enlarge

Once the backfill windows have been determined, Moab begins to traverse them. The current behavior is to traverse these windows widest window first (most nodes to fewest nodes). As each backfill window is evaluated, Moab applies the backfill algorithm specified by the BACKFILLPOLICY parameter.

If the FIRSTFIT algorithm is applied, the following steps are taken:

  1. The list of feasible backfill jobs is filtered, selecting only those that will actually fit in the current backfill window.
  2. The first job is started.
  3. While backfill jobs and idle resources remain, repeat step 1.

If NONE is set, the backfill policy is disabled.

Other backfill policies behave in a generally similar manner. The parameters documentation provides further details.

7.2.2.A Liberal versus Conservative Backfill

By default, Moab reserves only the highest priority job resulting in a liberal and aggressive backfill. This reservation guarantees that backfilled jobs will not delay the highest priority job, although they may delay other jobs. The parameter RESERVATIONDEPTH controls how conservative or liberal the backfill policy is. This parameter controls how deep down the queue priority reservations will be made. While increasing this parameter improves guarantees that priority jobs will not be bypassed, it reduces the freedom of the scheduler to backfill resulting in somewhat lower system utilization. The significance of the trade-offs should be evaluated on a site by site basis.

7.2.3 Configuring Backfill

7.2.3.A Backfill Policies

Backfill is enabled in Moab by specifying the BACKFILLPOLICY parameter. The BACKFILLPOLICY parameter is used to control which job gets selected first to be backfilled. Once the job has been selected, it is still placed on nodes according to the NODEALLOCATIONPOLICY you have defined. By default, backfill is enabled in Moab using the FIRSTFIT algorithm. However, this parameter can also be set to NONE (disabled).

The number of reservations that protect the resources required by priority jobs can be controlled using RESERVATIONDEPTH. This depth can be distributed across job QoS levels using RESERVATIONQOSLIST.

7.2.3.B Backfill Chunking

In a batch environment saturated with serial jobs, serial jobs will, over time, dominate the resources available for backfill at the expense of other jobs. This is due to the time-dimension fragmentation associated with running serial jobs. For example, given an environment with an abundance of serial jobs, if a multi-processor job completes freeing processors, one of three things will happen:

  1. The freed resources are allocated to another job requiring the same number of processors.
  2. Additional jobs may complete at the same time allowing a larger job to allocate the aggregate resources.
  3. The freed resources are allocated to one or more smaller jobs.

In environments where the scheduling iteration is much higher than the average time between completing jobs, case 3 occurs far more often than case 2, leading to smaller and smaller jobs populating the system over time.

To address this issue, the scheduler incorporates the concept of chunking. Chunking allows the scheduler to favor case 2 maintaining a more controlled balance between large and small jobs. The idea of chunking involves establishing a time-based threshold during which resources available for backfill are aggregated. This threshold is set using the parameter BFCHUNKDURATION. When resources are freed, they are made available only to jobs of a certain size (set using the parameter BFCHUNKSIZE) or larger. These resources remain protected from smaller jobs until either additional resources are freed up and a larger job can use the aggregate resources, or until the BFCHUNKDURATION threshold time expires.

Backfill chunking is only activated when a job of size BFCHUNKSIZE or larger is blocked in backfill due to lack of resources.

It is important to note that the optimal settings for these parameters is very site-specific and will depend on the workload (including the average job turnaround time, job size, and mix of large to small jobs), cluster resources, and other scheduling environmental factors. Setting too restrictive values needlessly reduces utilization while settings that are too relaxed do not allowed the desired aggregation to occur.

Backfill chunking is only enabled in conjunction with the FIRSTFIT backfill policy.

7.2.3.C Virtual Wallclock Time Scaling

In most environments, users submit jobs with rough estimations of the wallclock times. Within the HPC industry, a job typically runs for 40% of its specified wallclock time. Virtual Wallclock Time Scaling takes advantage of this fact to implement a form of optimistic backfilling. Jobs that are eligible for backfilling and not restricted by other policies are virtually scaled by the BFVIRTUALWALLTIMESCALINGFACTOR (assuming that the jobs finish before this new virtual wallclock limit). The scaled jobs are then compared to backfill windows to see if there is space and time for them to be scheduled. The scaled jobs are only scheduled if there is no possibility that it will conflict with a standing or administrator reservation. Conflicts with such reservations occur if the virtual wallclock time overlaps a reservation, or if the original non-virtual wallclock time overlaps a standing or administrator reservation. Jobs that can fit into an available backfill window without having their walltime scaled are backfilled "as-is" (meaning, without virtually scaling the original walltime).

Virtual Wallclock Time Scaling is only enabled when the BFVIRTUALWALLTIMESCALINGFACTOR parameter is defined.

If a virtually-scaled job fits into a window, and is backfilled, it will run until completion or until it comes within one scheduling iteration (RMPOLLINTERVAL defines the exact time of an iteration) of the virtual wallclock time expiration. In the latter case the job's wallclock time is restored to its original time and Moab checks and resolves conflicts caused by this "expansion." Conflicts may occur when the backfilled job is restored to its full duration resulting in reservation overlap. The BFVIRTUALWALLTIMECONFLICTPOLICY parameter controls how Moab handles these conflicts.

If the BFVIRTUALWALLTIMECONFLICTPOLICY parameter is set to NONE or is not specified, the overlapped job reservations are rescheduled.

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