A recent commit to OpenJDK revealed a significant performance improvement in how Java retrieves user CPU time for threads on Linux systems. The change replaced an older method that accessed the `/proc` filesystem with a more direct approach using `clock_gettime()`. The old method was not only convoluted—requiring file I/O and parsing of potentially complex data—but also demonstrated a performance gap of 30x to 400x, especially under concurrent loads. The previous implementation involved multiple system calls, including opening files and parsing strings, which added overhead. In contrast, the new method relies on a single syscall to retrieve the necessary information directly from the kernel. The commit added a benchmark that showed the new approach averaged around 11 microseconds per invocation, a significant drop from the previous method's performance. The article also touches on the historical context, explaining that the original method was a workaround due to POSIX standards, which do not allow for direct retrieval of user CPU time. By flipping certain bits in the clock ID, the new implementation leverages Linux-specific features to bypass these limitations. This change not only simplifies the codebase but also enhances efficiency—indicating a shift towards utilizing more direct kernel capabilities in Java's threading model.