A job can ask for multiple slots for a single reduce task via CRAIGFAM.INFO, upto the limit specified by CRAIGFAM.INFO, if the scheduler supports the feature. The value of -1 indicates that this feature is turned off. In this example, Warden sets CRAIGFAM.INFO to MB. For more information, see Memory Allocation for Nodes. Map Slots. The number of map and reduce slots on each TaskTracker node is controlled by the CRAIGFAM.INFOm and CRAIGFAM.INFOm Hadoop properties in the CRAIGFAM.INFO file. These parameters define the maximum number of concurrently occupied slots on .
How to Determine No of slots Required MapReduce
Example Map and Reduce Slot Calculation
Join Stack Overflow to learn, share knowledge, and build your career. I grepped the source code of Hadoop and did not find the string Setting default number of map tasks based on cluster size to at all whereas I find other strings, which are being printed when running MR jobs.
Furthermore this string is not being printed anywhere in my local installation. As you confirmed, your're in fact using Amazon Elastic MapReduce. As far as computing this number is concerned I would suspect it to be computed based on characteristics like total number of active nodes in cluster N and number of map slots per node M , i.
However, additional AWS-specific resource memory constraints may also be taken into account. You'd have to ask in EMR-related forums for the exact formula. Additionaly, the JobClient exposes a set of information about the cluster. Using the method JobClient getClusterStatus you can access information like:. So this is set by default based on the size of your input. You are allowed to specify more mappers, but not less than the number defined by hadoop. You should be able to access this number by getting the configuration option "mapred.
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Based on this configuration, MapR Hadoop performs the following calculations to determine the number of map and reduce slots:. Since the chunk size is , 1G is allocated to memory for map slots. Since the chunk size is , 3G is allocated to memory for map slots. Based on the services running on the node, Warden calculates the memory available to process MapReduce v1 tasks. In this example, Warden sets mapreduce. For more information, see Memory Allocation for Nodes.
This value is based on the following calculation: In general, you should not need to customize the number of map and reduce slots because Warden determines these value based on the resource available on the node.
However, you can override the number of slots by adding one or more of these parameters to mapred-site. Warden uses the following formula to calculate this value: To determine the value, go to the TaskTracker UI and view the memory available for that node. If the chunk size is greater than or equal to M, then this value is set to 1G. Otherwise, this value is set to 0. For information about configuring the chunk size, see Chunk Size. If the chunk size is greater than or equal to M, then this value is set to 3G.
Otherwise, this value is set to 1. Example Map and Reduce Slot Calculation In the following example, the node has the following configuration: Memory available to process MapReduce V1 tasks 26G Based on the services running on the node, Warden calculates the memory available to process MapReduce v1 tasks.
Map Slots 10 This value is based on the following calculations: Reduce Slots 3 This value is based on the following calculation: If you make changes to mapred-site.
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