An 8,000 trade settlement simulator shows where virtual threads win big and where CPU-bound work exposes the limit of Loom

Short summary

Java's Project Loom virtual threads deliver 15x faster throughput on I/O-bound work but offer no advantage for CPU-bound workloads. Watch for pinning traps where synchronized blocks force virtual threads to stay attached to carrier threads, silently destroying concurrency gains. A realistic T+1 stock settlement simulator reveals the boundary clearly—use -Djdk.tracePinnedThreads=full to diagnose and fix pinning.

• •Virtual threads shine on I/O-bound work (15x speedup, 10x fewer OS threads), but show no benefit or may even slow down pure CPU workloads
• •Pinning destroys virtual thread concurrency: synchronized blocks and certain native calls pin carrier threads, collapsing effective concurrency back to carrier pool size
• •Diagnose pinning with -Djdk.tracePinnedThreads=full; fix by replacing synchronized blocks with ReentrantLock in blocking call regions

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