Benchmark: Virtual threads beat platform threads 15x on I/O but struggle with CPU work

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A developer built an 8,000-trade T+1 stock settlement simulator to benchmark Java's Project Loom virtual threads against traditional platform threads under real-world conditions. For I/O-bound workloads, virtual threads completed tasks in 651ms versus 9,788ms for a 200-thread platform pool, achieving roughly 15x higher throughput while using far fewer OS threads. However, when CPU-intensive Monte Carlo Value-at-Risk calculations with 20,000 simulations per trade were introduced, virtual threads offered no meaningful advantage, exposing a key limitation of Project Loom. The benchmark also highlighted the risk of thread pinning, where virtual threads inside synchronized blocks cannot unmount from carrier threads, silently degrading concurrency without throwing errors. Developers can diagnose pinning issues using the JVM flag -Djdk.tracePinnedThreads=full and replacing synchronized blocks with ReentrantLock where necessary.

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