# Throttling errors with an Amazon EMR cluster
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The errors "Throttled from *Amazon EC2* while launching cluster" and "Failed to provision instances due to throttling from *Amazon EC2*" occur when Amazon EMR cannot complete a request because another service has throttled the activity. Amazon EC2 is the most common source of throttling errors, but other services may be the cause of throttling errors. [AWS service limits](https://docs.aws.amazon.com/general/latest/gr/aws_service_limits.html) apply on a per-Region basis to improve performance, and a throttling error indicates that you have exceeded the service limit for your account in that Region.

## Possible causes
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The most common source of Amazon EC2 throttling errors is a large number of cluster instances launching so that your service limit for EC2 instances is exceeded. Cluster instances may launch for the following reasons:
+ New clusters are created.
+ Clusters are resized manually. For more information, see [Manually resize a running Amazon EMR cluster](emr-manage-resize.md).
+ Instance groups in a cluster add instances (scale out) as a result of an automatic scaling rule. For more information, see [Understanding automatic scaling rules](emr-automatic-scaling.md#emr-scaling-rules).
+ Instance fleets in a cluster add instances to meet an increased target capacity. For more information, see [Planning and configuring instance fleets for your Amazon EMR cluster](emr-instance-fleet.md).

It is also possible that the frequency or type of API request being made to Amazon EC2 causes throttling errors. For more information about how Amazon EC2 throttles API requests, see [Query API request rate](https://docs.aws.amazon.com/AWSEC2/latest/APIReference/query-api-troubleshooting.html#api-request-rate) in the *Amazon EC2 API Reference*.

## Solutions
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Consider the following solutions:
+ Follow the instructions in [AWS service quotas](https://docs.aws.amazon.com/general/latest/gr/aws_service_limits.html) in the *Amazon Web Services General Reference* to request a service limit increase. For some APIs, setting up a CloudWatch event might be a better option than increasing limits. For more details, see [When to set up EMR events in CloudWatch](emr-service-limits-cloudwatch-events.md).
+ If you have clusters that launch on the same schedule—for example, at the top of the hour—consider staggering start times.
+ If you have clusters that are sized for peak demand, and you periodically have instance capacity, consider specifying automatic scaling to add and remove instances on-demand. In this way, instances are used more efficiently, and depending on the demand profile, fewer instances may be requested at a given time across an account. For more information, see [Using automatic scaling with a custom policy for instance groups in Amazon EMR](emr-automatic-scaling.md).