I migrated a Java 17 application to Java 21 and enabled virtual thread for the application.

Microservice Detail:

• 20 requests per second on a container.
• Scaling in switched on for this service. Whenever average CPU hits 30% new container is launched.
• Usually 50 containers are running for this service.
• Without virtual thread, one container used to have an average of 2.5k platform threads running every minute.
• We are using GRPC server on this service.
• Request Detail:
  • One requests which is on platform thread, generally uses 6-7 parallel platform thread to fetch data from different data source(I/O).
  • We have converted the later 6-7 platform threads to virtual thread using ExecutorService.
  • We are not able to move the user request to virtual thread as grcp server does not support it.

When we go live, I can see earlier system used to have 2.5k active platform thread on an avergage. Now it has reduced to 200. But apart from this I am not observing any other benefit.

What I thought, I will observe

• Increase in throughput per container.
• Number of containers used should have reduced as now less context switching because of less active platform threads.

I am seeing no benefit apart from reducing number of platform threads.

What I am missing?
What other parameters I should check which will show improvements?