> ## Documentation Index > Fetch the complete documentation index at: https://notes.kodekloud.com/llms.txt > Use this file to discover all available pages before exploring further. # Reserved and Unreserved Concurrency > This article explains managing concurrency in AWS Lambda, focusing on reserved and unreserved concurrency for optimal function performance. Managing concurrency in AWS Lambda ensures that high-priority functions always have the capacity they need. By default, your account’s unreserved concurrency sets a soft limit on total parallel executions. For mission-critical workloads, you can allocate reserved concurrency to specific functions. ## What Is Concurrency? Concurrency refers to how many Lambda function executions run in parallel. For example, 100 simultaneous invocations count as 100 concurrent executions. ## Viewing Concurrency Settings To view or modify a function’s concurrency: 1. Open the AWS Lambda console and select your function. 2. Click **Configuration**. 3. In the left navigation pane, choose **Concurrency**. ![The image shows the AWS Lambda console, specifically the "Concurrency" configuration page, where function concurrency settings and provisioned concurrency configurations can be managed.](https://kodekloud.com/kk-media/image/upload/v1752863104/notes-assets/images/AWS-Lambda-Reserved-and-Unreserved-Concurrency/aws-lambda-concurrency-configuration-console.jpg) We use an SQS cold start demo here. Substitute your own function for real-world testing. ## Unreserved Concurrency Unreserved (account) concurrency is the pool shared by all Lambda functions in an account. AWS sets a default soft limit of **1,000** concurrent executions per region. * All functions combined cannot exceed this limit. * To increase it, submit a [support request](https://aws.amazon.com/support). ## Reserving Concurrency for Critical Functions Reserved concurrency guarantees a minimum number of concurrent executions for a specific function: 1. In the **Concurrency** page, click **Edit**. 2. Select **Reserve concurrency**. 3. Enter the reservation amount (e.g., `300`). 4. Click **Save**. ![The image shows an AWS Lambda console page for editing concurrency settings, with options to use unreserved account concurrency or reserve concurrency. The "Reserve concurrency" option is selected with a value of 0, and there are "Cancel" and "Save" buttons.](https://kodekloud.com/kk-media/image/upload/v1752863105/notes-assets/images/AWS-Lambda-Reserved-and-Unreserved-Concurrency/aws-lambda-concurrency-settings-console.jpg) If your function exceeds its reserved concurrency, additional invocations are throttled with `429` errors. Reserving 300 concurrency reduces the unreserved pool from 1,000 to 700: ![The image illustrates a concept of reserved concurrency, showing a division of 1000 default account concurrency into 700 unreserved and 300 reserved, with 400 invocations depicted.](https://kodekloud.com/kk-media/image/upload/v1752863107/notes-assets/images/AWS-Lambda-Reserved-and-Unreserved-Concurrency/reserved-concurrency-illustration-1000-700-300.jpg) Use reserved concurrency to prevent downstream overload: ![The image illustrates the concept of reserved concurrency in AWS Lambda, showing two scenarios: regulated concurrency and overloaded concurrency, with corresponding icons and labels.](https://kodekloud.com/kk-media/image/upload/v1752863108/notes-assets/images/AWS-Lambda-Reserved-and-Unreserved-Concurrency/aws-lambda-reserved-concurrency-diagram.jpg) There is no additional cost for reserved concurrency. ## Unreserved Concurrency Limits Your total unreserved concurrency is a per-account limit. In some regions, bursts up to 3,000 concurrent executions may occur temporarily, but sustained usage above the default requires a support request. ![The image illustrates a concept of reserved concurrency, showing a division between 700 unreserved and 300 reserved units, with a label indicating "Free."](https://kodekloud.com/kk-media/image/upload/v1752863109/notes-assets/images/AWS-Lambda-Reserved-and-Unreserved-Concurrency/reserved-concurrency-units-illustration.jpg) ![The image shows the text "Reserved Concurrency" with the number "3000" below it, accompanied by labels "In some Regions" and "Limited Time."](https://kodekloud.com/kk-media/image/upload/v1752863110/notes-assets/images/AWS-Lambda-Reserved-and-Unreserved-Concurrency/reserved-concurrency-3000-limited-time.jpg) ## Reserved vs Unreserved Concurrency Comparison | Concurrency Type | Definition | Default Limit | Ideal Use Case | | -------------------- | ------------------------------------------------- | ------------- | -------------------------------------- | | Unreserved (Account) | Shared pool for all Lambda executions | 1,000 | General workloads | | Reserved (Function) | Dedicated capacity for a single high-priority job | N/A | Traffic-sensitive or rate-limited APIs | ## Summary * **Unreserved concurrency** sets the total pool for all functions. * **Reserved concurrency** guarantees capacity for critical functions and avoids downstream throttling. * No extra charge applies to reserved concurrency. For advanced performance optimization and cold-start reduction, consider combining reserved concurrency with [provisioned concurrency](https://docs.aws.amazon.com/lambda/latest/dg/configuration-concurrency.html). ## References * [AWS Lambda Concurrency](https://docs.aws.amazon.com/lambda/latest/dg/concurrent-executions.html) * [AWS Lambda Developer Guide](https://docs.aws.amazon.com/lambda/latest/dg/welcome.html) * [AWS Support Center](https://console.aws.amazon.com/support/home)