I have a cluster of 3 machines with 4 cores each. Each machine has one task manager. I know that the number of slots in Flink can be controlled by taskmanager.numberOfTaskSlots. I initially had allotted 12 slots in total (every task manager had 4 slots). Although, there is no explicit CPU isolation among slots (as mentioned here), I assume that each slot is roughly using 1 core. Am I right in assuming this?
I haven't mentioned any slot sharing group in my code and my pipeline does not have any blocking edges. The parallelism of each task is the same and is equal to the number of slots. I am assuming that one subtask from each task will be in a slot. Am I correct in this understanding?
After some conversation (link for the curious minds :-)), I wanted to increase the cores per slot to 2 for my experiments. So, I reduced the taskmanager.numberOfTaskSlots to 2 on each machine? After doing this, I see that the Flink WebUI shows 6 slots is total and 2 slots for each task manager. I have also reduced the parallelism of each task to 6. Is this all that I need to do?
Note: I am not using the MVP feature of fine grained resource management right now.
That sounds right.
Each Task Manager is a single JVM. A task slot doesn't correspond to anything physical -- it's just an abstract resource managed by the Flink scheduler. Each task in a task slot is an instance of an operator chain in the execution graph, and each task is single-threaded. No two instances of the same operator chain will ever be scheduled into the same slot.
All of the threads for all of the tasks in all of the slots in given task manager will compete for the resources available to that JVM: cores, memory, etc.
As you have noted, there is no way to explicitly set the number of cores per slot. And there's no requirement that the number be an integer. You could, for example, decide that your 4-core TMs are each providing 3 slots, for a total parallelism of 9 across the 3 TMs.
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I have a Flink batch job which operates on a large dataset. My cluster consists of 25 nodes and runs as a standalone cluster. One of the key steps has a parallelism of 70 and I expected each task manager to get between 2 and 3 slots for that step, instead only half the workers are used and some of them are getting up to 8 slots assigned (which is the maximum they can get).
Apart from the impact on data locality, another side effect is the strain on disk space. Since less workers are running all the slots, each one of them has to store more data compared to having the slots spread across all the nodes of the cluster.
Am I missing something? Is there a way I can force Flink to distribue the slots across as many TMs as possible for each job?
At the moment, Flink does not support to spread out tasks evenly across the set of available TaskManagers. The reason is that Flink considers every slot to be equal. In the future, the Flink community plans to add more scheduling features which would solve the problem.
At the moment, I would suggest to set the individual operator's parallelism to the number of available slots in your cluster. That will guarantee that all machines of your cluster are evenly used.
The number of CPU cores per machine is four. In flink standalone mode, how should I set the number of TaskManagers on each machine?
1 TaskManager, each TaskManager has 4 slots.
2 TaskManagers, each TaskManager has 2 slots.
4 TaskManagers, each TaskManager has 1 slot. This setting is like apache-storm.
Normally you'd have one TaskManager per server, and (as per the doc that bupt_ljy referenced) one slot per physical CPU core. So I'd go with your option #1.
There's also the consideration of Flink's scheduling algorithm. We've frequently run into problems where, with multiple hosts running one large task manager a piece, all jobs get scheduled to one host, which can cause load problems.
We ended up making multiple smaller task managers per host and jobs seem to be distributed better (although they still cluster on one node often).
So, in my experience, I'd lean more towards 4 task managers with 1 slot a piece, or maybe compromise at 2 task managers with 2 slots a piece.
I think it depends on your application.
In official documents Distributed Runtime Environment, it says As a rule-of-thumb, a good default number of task slots would be the number of CPU cores. With hyper-threading, each slot then takes 2 or more hardware thread contexts.
But if you have to use a lot of memory in your application, then you don't need too many slots in one task manager.
According to the Flink documentation, there exist two dimensions to affect the amount of resources available to a task:
The number of task managers
The number of task slots available to a task manager.
Having one slot per TaskManager means each task group runs in a separate JVM (which can be started in a separate container, for example). Having multiple slots means more subtasks share the same JVM. Tasks in the same JVM share TCP connections (via multiplexing) and heartbeat messages. They may also share data sets and data structures, thus reducing the per-task overhead.
With this line in the documentation, it seems that you would always err on the side of increasing the number of task slots per task manager instead of increasing the number of task managers.
A concrete scenario: if I have a job cluster deployed in Kubernetes (let's assume 16 CPU cores are available) and a pipeline consisting of one source + one map function + one sink, then I would default to having a single TaskManager with 16 slots available to that TaskManager.
Is this the optimal configuration? Is there a case where I would prefer 16 TaskManagers with a single slot each or maybe a combination of TaskManager and slots that could take advantage of all 16 CPU cores?
There is no optimal configuration because "optimal" cannot be defined in general. A configuration with a single slot per TM provides good isolation and is often easier to manage and reason about.
If you run multiple jobs, a multi-slot configuration might schedule tasks of different jobs to one TM. If the TM goes down, e.g., because either of two tasks consumed too much memory, both jobs will be restarted. On the other hand, running one slot per TM might leave more memory unused. If you only run a single job per cluster, multiple slots per TM might be fine.
I'm prototyping a Flink streaming application on a bare-metal cluster of 15 machines. I'm using yarn-mode with 90 task slots (15x6).
The app reads data from a single Kafka topic. The Kafka topic has 15 partitions, so I set the parallelism of the source operator to 15 as well. However, I found that Flink in some cases assigns 2-4 instances of the consumer task to the same taskmanager. This causes certain nodes to become network-bound (the Kafka topic is serving high volume of data and the machines only have 1G NICs) and bottlenecks in the entire data flow.
Is there a way to "force" or otherwise instruct Flink to distribute a task evenly across all taskmanagers, perhaps round robin? And if not, is there a way to manually assign tasks to specific taskmanager slots?
To the best of my knowledge, this isn't possible. The job manager, which schedules tasks into task slots, is only aware of task slots. It isn't aware that some task slots belong to one task manager, and others to another task manager.
Flink does not allow manually assign task slots as in case of failure handling, it can distribute the task to remaining task managers.
However, you can distribute the workload evenly by setting cluster.evenly-spread-out-slots: true in flink-conf.yaml.
This works for Flink >= 1.9.2.
To make it work, you may also have to set:
taskmanager.numberOfTaskSlots equal to the number of available CPUs per machine, and
parallelism.default equal to the the total number of CPUs in the cluster.
Consider a Flink cluster with some nodes where each node has a multi-core processor. If we configure the number of the slots based on the number of cores and equal share of memory, how does Apache Flink distribute the tasks between the nodes and the free slots? Are they fairly treated?
Is there any way to make/configure Flink to treat the slots equally when we configure the task slots based on the number of the cores available on a node
For instance, assume that we partition the data equally and run the same task over the partitions. Flink uses all the slots from some nodes and at the same time some nodes are totally free. The node which has less number of CPU cores involved outputs the result much faster than the node with more number of CPU cores involved in the process. Apart from that, this ratio of speedup is not proportional to the number of used cores in each node. In other words, if in one node one core is occupied and in another node two cores are occupied, in fairly treating each core as a slot, each slot should output the result over the same task in almost equal amount of time irrespective of which node they belong to. But, this is not the case here.
With this assumption, I would say that the nodes are not treated equally. This in turn produces a result time wise that is not proportional to the number of the nodes available. We can not say that increasing the number of the slots necessarily decreases the time cost.
I would appreciate any comment from the Apache Flink Community!!
Flink's default strategy as of version >= 1.5 considers every slot to be resource-wise the same. With this assumption, it should not matter wrt resources where you place the tasks since all slots should be the same. Given this, the main objective for placing tasks is to colocate them with their inputs in order to minimize network I/O.
If we are now in a standalone setup where we have a fixed number of TaskManagers running, Flink will pick slots in an arbitrary fashion (no guarantee given) for the sources and then colocate their consumers in the same slots if possible.
When running Flink on Yarn or Mesos where Flink can start new TaskManagers, Flink will first use up all slots of an existing TaskManager before it requests a new one. In this case, you will see that all sources will end up on as few TaskManagers as possible.
Since CPUs are not isolated wrt slots (they are a shared resource), the above-mentioned assumption does not hold true in all cases. Hence, in some cases where you have a fixed set of TaskManagers it is actually beneficial to spread the tasks out as much as possible to make use of the shared CPU resources.
In order to support this kind of scheduling strategy, the Flink community added the task spread out strategy via FLINK-12122. In order to use a scheduling strategy which is more similar to the pre FLIP-6 behaviour where Flink tries to spread out the workload across all available TaskExecutors, one needs to set cluster.evenly-spread-out-slots: true in the flink-conf.yaml
Very old thread, but there is a newer thread that answers this question for current versions.
with Flink 1.5 we added resource elasticity. This means that Flink is now able to allocate new containers on a cluster management framework like Yarn or Mesos. Due to these changes (which also apply to the standalone mode), Flink no longer reasons about a fixed set of TaskManagers because if needed it will start new containers (does not work in standalone mode). Therefore, it is hard for the system to make any decisions about spreading slots belonging to a single job out across multiple TMs. It gets even harder when you consider that some jobs like yours might benefit from such a strategy whereas others would benefit from co-locating its slots. It gets even more complicated if you want to do scheduling wrt to multiple jobs which the system does not have full knowledge about because they are submitted sequentially. Therefore, Flink currently assumes that slots requests can be fulfilled by any TaskManager.