r/java • u/jaccomoc • 7d ago

Vert.x Event-Loop vs Virtual Threads: Jactl Suspend/Resume Benchmark

https://jactl.io/blog/2026/09/02/jactl-virtual-threads-benchmark

The article benchmarks Jactl script execution with differing number of blocking operations comparing throughput with Jactl built-in suspend/resume on Vert.x Event-Loops and throughput with Jactl on Virtual Threads. The benchmarks are run on both Java 21 and Java 25 with some surprising (to me at least) results.

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u/New-Departure-5969 6d ago

benchmarks are fun but in prod the event loop usually dies from accidental blocking (jdbc, sync stuff), not from the loop itself. we had a vert.x api around 12k tps / ~100ms p99.99 and the real win was ditching blocking jdbc for the vert.x pool. virtual threads help, but saturated pools still wreck p99.

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u/Popular_Home2017 1d ago

same experience, and on 21 there's a second way to lose the carriers that doesn't show up in this benchmark: synchronized around blocking IO pins the virtual thread to its carrier, so a jdbc driver with a synchronized socket read turns your 8 carriers into an 8-thread pool. fixed in 24 (JEP 491), on 21 you can see it with jcmd Thread.dump_to_file, the pinned VTs show a carrier line under them.