java-synchronized-threads-benchmark

Virtual threads were introduced in Java 19 as a peview feature, but they suffer from a major performance issue wherein they get pinned to the carrier thread. This happens when:

• The virtual thread runs code inside a synchronized block or method.
• The virtual thread runs a native method or a foreign function.

The purpose of this repository is to highlight the difference in performance of platform vs virtual threads using a sample scenario.

Test Setup

1. The code in this repository creates a thread pool of platform and virtual threads using ExecutorService with variable thread pool size.
2. The threads then execute a long running taks in a synchronized block. This is simulated by Thread.sleep(4000). The tasks are executed by all threads concurrently.
3. The total time taken to execute all the tasks are recorded.

Run

Total Available Cores 12
========================
Thread Count = 2
Thread Type: PLATFORM   Time: 4012 ms
Thread Type: VIRTUAL    Time: 4006 ms
------------------------
Thread Count = 4
Thread Type: PLATFORM   Time: 4006 ms
Thread Type: VIRTUAL    Time: 4005 ms
------------------------
Thread Count = 8
Thread Type: PLATFORM   Time: 4002 ms
Thread Type: VIRTUAL    Time: 4006 ms
------------------------
Thread Count = 11
Thread Type: PLATFORM   Time: 4007 ms
Thread Type: VIRTUAL    Time: 4006 ms
------------------------
Thread Count = 13
Thread Type: PLATFORM   Time: 4002 ms
Thread Type: VIRTUAL    Time: 8004 ms
------------------------
Thread Count = 16
Thread Type: PLATFORM   Time: 4007 ms
Thread Type: VIRTUAL    Time: 8004 ms
------------------------
Thread Count = 23
Thread Type: PLATFORM   Time: 4004 ms
Thread Type: VIRTUAL    Time: 8003 ms
------------------------
Thread Count = 25
Thread Type: PLATFORM   Time: 4007 ms
Thread Type: VIRTUAL    Time: 12009 ms
------------------------
Thread Count = 32
Thread Type: PLATFORM   Time: 4008 ms
Thread Type: VIRTUAL    Time: 12008 ms
------------------------

Interpretation

It is evident from the run that platform threads yield on sleeping and release the processor to other threads, while virtual threads seem to be holding on to the processor and hence total execution time adds up once the total number of threads executing exceeds the total processor count.

Detailed Explanation

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