How can I configure the number of event loop on server?

[d3bt3ch]

How can I configure the number of event loop on the server? How to configure flags?

[jrhee17]

Hi! If you would like to specify it globally, you can declare it via flags.
You can specify it programmatically via SPI or via JVM flags.

armeria/core/src/main/java/com/linecorp/armeria/common/Flags.java

Lines 686 to 699 in 4ba1531

	/**
	* Returns the default number of {@linkplain CommonPools#workerGroup() common worker group} threads.
	* Note that this flag has no effect if a user specified the worker group explicitly via
	* {@link ServerBuilder#workerGroup(EventLoopGroup, boolean)} or
	* {@link ClientFactoryBuilder#workerGroup(EventLoopGroup, boolean)}.
	*
	* <p>The default value of this flag is {@code 2 * <numCpuCores>} for {@link TransportType#NIO},
	* {@link TransportType#EPOLL} and {@link TransportType#KQUEUE} and {@code <numCpuCores>} for
	* {@link TransportType#IO_URING}. Specify the {@code -Dcom.linecorp.armeria.numCommonWorkers=<integer>}
	* JVM option to override the default value.
	*/
	public static int numCommonWorkers() {
	return NUM_COMMON_WORKERS;
	}

Alternatively, you can specify the number of event loops per-server as well.
Obviously, values set to a server directly will have higher precedence.

armeria/core/src/main/java/com/linecorp/armeria/server/ServerBuilder.java

Line 545 in 4ba1531

public ServerBuilder workerGroup(int numThreads) {

[jrhee17]

By default, a worker (event loop) is responsible for the entire life cycle of a request. This includes 1) reading from a socket 2) parsing the http request 3) executing the user-specified HttpService (this includes grpc services, annotated services, etc..) 4) and sending back the http response.

If a serviceWorkerGroup is specified, user-specified HttpServices will be rescheduled to run from the serviceWorkerGroup.

[d3bt3ch]

Noted. Thanks 👍

[d3bt3ch]

Continuing in the same trail. What is blockingTaskExecutor and how does it affects the performance?

[ikhoon]

An event loop should not be blocked for a long time. If your service code has a blocking operation, you should use blockingTaskExecutor.

blockingTaskExecutor requires many threads and additional context switches. They could be negative impacts on the performance.

[d3bt3ch]

My service is using CompletableFuture to process the request. Therefore, all blocking IO tasks are within the future. Should I use blockingTaskExecutor in that case?