I have few questions about running tasks in parallel in Azure Batch. Per the official documentation, "Azure Batch allows you to set maximum tasks per node up to four times (4x) the number of node cores."
Is there a setup other than specifying the max tasks per node when creating a pool, that needs to be done (to the code) to be able to run parallel tasks with batch?
So if I am understanding this correctly, if I have a Standard_D1_v2 machine with 1 core, I can run up to 4 concurrent tasks running in parallel in it. Is that right? If yes, I ran some tests and I am quite not sure about the behavior that I got. In a pool of D1_v2 machines set up to run 1 task per node, I get about 16 min for my job execution time. Then, using the same applications and same parameters with the only change being a new pool with same setup, also D1_v2, except running 4 tasks per node, I still get a job execution time of about 15 min. There wasn't any improvement in the job execution time for running tasks in parallel. What could be happening? What am I missing here?
I ran a test with a pool of D3_v2 machines with 4 cores, set up to run 2 tasks per core for a total of 8 tasks per node, and another test with a pool (same number of machines as previous one) of D2_v2 machines with 2 cores, set up to run 2 tasks per core for a total of 4 parallel tasks per node. The run time/ job execution time for both these tests were the same. Isn't there supposed to be an improvement considering that 8 tasks are running per node in the first test versus 4 tasks per node in the second test? If yes, what could be a reason why I'm not getting this improvement?
No. Although you may want to look into the task scheduling policy, compute node fill type to control how your tasks are distributed amongst nodes in your pool.
How many tasks are in your job? Are your tasks compute-bound? If so, you won't see any improvement (perhaps even end-to-end performance degradation).
Batch merely schedules the tasks concurrently on the node. If the command/process that you're running utilizes all of the cores on the machine and is compute-bound, you won't see an improvement. You should double check your tasks start and end times within the job and the node execution info to see if they are actually being scheduled concurrently on the same node.
Related
I was just wondering if anyone could explain if all compute resources in a Databricks cluster are shared or if the resources are tied to each worker. For example, if two users were connected to a cluster made up of 2 workers with 4 cores per worker and one user's job required 2 cores and the other's required 6 cores, would they be able to share the 8 total cores or would the full 4 cores from one worker be unavailable during the job that only required 2 cores?
TL;DR; Yes, default behavior is to allow sharing but you're going to have to tightly control the default parallelism with such a small cluster.
Take a look at Job Scheduling for Apache Spark. I'm assuming you are using an "all-purpose" / "interactive" cluster where users are working on notebooks OR you are submitting jobs to an existing, all-purpose cluster and it is NOT a job cluster with multiple spark applications being deployed.
Databricks Runs in FAIR Scheduling Mode by Default
Under fair sharing, Spark assigns tasks between jobs in a “round robin” fashion, so that all jobs get a roughly equal share of cluster resources. This means that short jobs submitted while a long job is running can start receiving resources right away and still get good response times, without waiting for the long job to finish. This mode is best for multi-user settings.
By default, all queries started in a notebook run in the same fair scheduling pool
The Apache Spark scheduler in Azure Databricks automatically preempts tasks to enforce fair sharing.
Apache Spark Defaults to FIFO
By default, Spark’s scheduler runs jobs in FIFO fashion. Each job is divided into “stages” (e.g. map and reduce phases), and the first job gets priority on all available resources while its stages have tasks to launch, then the second job gets priority, etc. If the jobs at the head of the queue don’t need to use the whole cluster, later jobs can start to run right away, but if the jobs at the head of the queue are large, then later jobs may be delayed significantly.
Keep in mind the word "job" is specific Spark term that represents an action being taken that launches one or more stages and tasks. See What is the concept of application, job, stage and task in spark?.
So in your example you have...
2 Workers with 4 cores each == 8 cores == 8 tasks can be handled in parallel
One application (App A) that has a job that launches a stage with only 2 tasks.
One application (App B) that has a job that launches a stage with 6 tasks.
In this case, YES, you will be able to share the resources of the cluster. However, the devil is in the default behaviors. If you're reading from many files, performing a join, aggregating, etc, you're going to run into the fact that Spark is going to partition your data into chunks that can be acted on in parallel (see configuration like spark.default.parallelism).
So, in a more realistic example, you're going to have...
2 Workers with 4 cores each == 8 cores == 8 tasks can be handled in parallel
One application (App A) that has a job that launches a stage with 200 tasks.
One application (App B) that has a job that launches three stage with 8, 200, and 1 tasks respectively.
In a scenario like this FIFO scheduling, as is the default, will result in one of these applications blocking the other since the number of executors is completely overwhelmed by the number of tasks in just one stage.
In a FAIR scheduling mode, there will still be some blocking since the number of executors is small but some work will be done on each job since FAIR scheduling does a round-robin at the task level.
In Apache Spark, you have tighter control by creating different pools of the resources and submitting apps only to those pools where they have "isolated" resources. The "better" way of doing this is with Databricks Job clusters that have isolated compute dedicated to the application being ran.
I'm using Microsoft HPC Pack 2012 to run video processing jobs on a Windows cluster. A run is organized as a single job with hundreds of independent tasks. If a single task is scheduled on a node, it uses all cores, but not at nearly 100%. One way to increase CPU utilization is to run more than one task at a time per node. I believe in my use case, running each task on every core would achieve the best CPU utilization. However, after lots of trying I have not been able to achieve it. Is it possible?
I have been able to run multiple tasks on the same node on separate cores. I achieved this by setting the job UnitType to Node, setting the job and task types to IsExclusive = False, and setting the MaximumNumberOfCores on a job to something less than the number of cores on the machine. For simplicity, I would like to one run task per core, but typically this would exhaust the memory budget. So, I have set EstimatedProcessMemory to the typical memory usage.
This works, but every set of parameters I have tried leaves resources on the table. For instance, let's say I have a machine with 12 cores, 15GB of free RAM, and each task consumes 2GB. Then I can run 7 tasks on this machine. If I set task MaximumNumberOfCores to 1, I only use 7 of my 12 cores. If I set it to 2, suppose I set EstimatedProcessMemory to 2048. HPC interprets this as the memory PER CORE, so I only run 3 tasks on 2 cores and 3 tasks on 1 core, so 9 of my 12 cores. And so on.
Is it possible to simply run as many tasks as will fit in memory, each running on all of the cores? Or to dynamically assign the number of cores per task in a way that doesn't have the shortcomings mentioned above?
I have below system policies defined in InfoSphere DataStage Operations Console under "Work Load Management(WLM)".
Sometimes, the total number of currently running jobs shoots upto 150 although I have defined maximum running job count as 40 in WLM.
Whenever the currently running job count increases beyond 100, most of the datastage jobs starts showing increased startup time in director log and they took long time to run otherwise if the job concurrency is less than 100 then the same set of jobs run fine with startup time in seconds. Please suggest how to address this issue and how to enforce currently running job should not exceed eg 100 at any point of time. Thanks a lot !
This is working as designed, generally the WLM system is used to control the start of parallel and server jobs. It uses a set of user-defined queues and when a job is started, it is submitted to a designated queue. In the figure above the parallel jobs are in queue named 'MediumPriorityJobs'.
Note that the sequence job is not in the queue to be counted to the total running workloads controlled by the WLM Job Count System Policy.
Source: https://www.ibm.com/support/pages/how-interpret-job-count-maximum-running-jobs-system-policy-ibm-infosphere-information-server-workload-management-wlm
I have few questions regarding celery. Please help me with that.
Do we need to put the project code in every celery worker? If yes, if I am increasing the number of workers and also I am updating my code, what is the best way to update the code in all the worker instances (without manually pushing code to every instance everytime)?
Using -Ofair in celery worker as argument disable prefetching in workers even if have set PREFETCH_LIMIT=8 or so?
IMPORTANT: Does rabbitmq broker assign the task to the workers or do workers pull the task from the broker?
Does it make sense to have more than one celery worker (with as many subprocesses as number of cores) in a system? I see few people run multiple celery workers in a single system.
To add to the previous question, whats the performance difference between the two scenarios: single worker (8 cores) in a system or two workers (with concurrency 4)
Please answer my questions. Thanks in advance.
Do we need to put the project code in every celery worker? If yes, if I am increasing the number of workers and also I am updating my code, what is the best way to update the code in all the worker instances (without manually pushing code to every instance everytime)?
Yes. A celery worker runs your code, and so naturally it needs access to that code. How you make the code accessible though is entirely up to you. Some approaches include:
Code updates and restarting of workers as part of deployment
If you run your celery workers in kubernetes pods this comes down to building a new docker image and upgrading your workers to the new image. Using rolling updates this can be done with zero downtime.
Scheduled synchronization from a repository and worker restarts by broadcast
If you run your celery workers in a more traditional environment or for some reason you don't want to rebuild whole images, you can use some central file system available to all workers, where you update the files e.g. syncing a git repository on a schedule or by some trigger. It is important you restart all celery workers so they reload the code. This can be done by remote control.
Dynamic loading of code for every task
For example in omega|ml we provide lambda-style serverless execution of
arbitrary python scripts which are dynamically loaded into the worker process.
To avoid module loading and dependency issues it is important to keep max-tasks-per-child=1 and use the prefork pool. While this adds some overhead it is a tradeoff that we find is easy to manage (in particular we run machine learning tasks and so the little overhead of loading scripts and restarting workers after every task is not an issue)
Using -Ofair in celery worker as argument disable prefetching in workers even if have set PREFETCH_LIMIT=8 or so?
-O fair stops workers from prefetching tasks unless there is an idle process. However there is a quirk with rate limits which I recently stumbled upon. In practice I have not experienced a problem with neither prefetching nor rate limiting, however as with any distributed system it pays of to think about the effects of the asynchronous nature of execution (this is not particular to Celery but applies to all such such systems).
IMPORTANT: Does rabbitmq broker assign the task to the workers or do workers pull the task from the broker?
Rabbitmq does not know of the workers (nor do any of the other broker supported by celery) - they just maintain a queue of messages. That is, it is the workers that pull tasks from the broker.
A concern that may come up with this is what if my worker crashes while executing tasks. There are several aspects to this: There is a distinction between a worker and the worker processes. The worker is the single task started to consume tasks from the broker, it does not execute any of the task code. The task code is executed by one of the worker processes. When using the prefork pool (which is the default) a failed worker process is simply restarted without affecting the worker as a whole or other worker processes.
Does it make sense to have more than one celery worker (with as many subprocesses as number of cores) in a system? I see few people run multiple celery workers in a single system.
That depends on the scale and type of workload you need to run. In general CPU bound tasks should be run on workers with a concurrency setting that doesn't exceed the number of cores. If you need to process more of these tasks than you have cores, run multiple workers to scale out. Note if your CPU bound task uses more than one core at a time (e.g. as is often the case in machine learning workloads/numerical processing) it is the total number of cores used per task, not the total number of tasks run concurrently that should inform your decision.
To add to the previous question, whats the performance difference between the two scenarios: single worker (8 cores) in a system or two workers (with concurrency 4)
Hard to say in general, best to run some tests. For example if 4 concurrently run tasks use all the memory on a single node, adding another worker will not help. If however you have two queues e.g. with different rates of arrival (say one for low frequency but high-priority execution, another for high frequency but low-priority) both of which can be run concurrently on the same node without concern for CPU or memory, a single node will do.
We have a 10 queue setup in our celery, a large setup each queue have a group of 5 to 10 task and each queue running on dedicated machine and some on multiple machines for scaling.
On the other hand, we have a bunch of periodic tasks, running on a separate machine with single instance, and some of the periodic tasks are taking long to execute and I want to run them in 10 queues instead.
Is there a way to scale celery beat or use it purely to trigger the task on a different destination "one of the 10 queues"?
Please advise?
Use celery routing to dispatch the task to where you need: