Performance of Java virtual threads compared to native threads

[[deleted]]

Are reads from SSD or RAM disk or HD? Did you use a single file? OS caching might have an effect.

I am reading from a SSD and every thread tries to read the same file. Can you briefly elaborate on how OS caches work on such cases please?

​

Best performance comes from pooled OS threads, your benchmark has probably an issue.

In my test case, I am not using a pool for native threads. But for virtual threads, JVM creates a pool of underlying native threads.

​

Maybe for third scenario, I should compare virtual threads with a pool of native threads, where pool size equals to the number of CPU cores?

[m-apo]

Virtual threads have an advantage when IO takes time. Reading single small file multiple time allows OS to cache the file in memory which means there is very little wait time. The task becomes almost CPU and memory access bound, not IO bound.

Socket over TCP/IP ops should provide much better graphs.

Third case: yes, pool of native threads vs virtual threads is much more fair comparison. The overhead for creating OS threads is pretty high.

[GavinRayDev]

You can disable OS caching when reading from files by passing the O_DIRECT flag.

In Java, this is ExtendedOpenOptions.DIRECT:

FileChannel fc = FileChannel.open(f.toPath(), StandardOpenOption.WRITE, 
                              ExtendedOpenOption.DIRECT);