Hi guys, On 03/06/2015 11:14 PM, Andrew Haley wrote: > On 03/06/2015 07:35 PM, Martin Buchholz wrote: >> We all know that nanoTime has problems.
It would really help if you list what problems weakNanoTime is supposed to solve. > Many (most?) users of nanoTime, >> especially for benchmarking or concurrent algorithms, don't really care >> about the monotonicity guarantee and just want a "best-effort" >> "as-efficient-as-possible" version of nanoTime. I disagree on this premise. In the absence of proper API for CPU affinity and CPU frequency control, we *do* need monotonicity and linearity guarantees. Efficiency is a second-order concern here; you can measure garbage with what you perceive as the nanosecond precision/latency, but not exactly nanosecond accuracy. And you really want accuracy, not precision. Personally, I have doubts you need the single-digit-nanosecond timestamping. The amount of in-flight ops modern CPUs do at those timescales greatly blurs the meaning of measured time. >> Some users have already created such a private nanoTime. > > Indeed. See > http://developerblog.redhat.com/2014/06/24/ultra-lightweight-high-precision-logger-for-openjdk/ > for an example of an interval timer with sub-nanosecond resolution and > about 30-nanosecond latency. I'm curious what does it mean to have sub-nanosecond resolution with 30 nanosecond latency... > I'm not at all sure how Aleksey managed to get 30ns latency from > System.nanoTime(). Maybe the implementation has changed since I > tried it. As of now, C2 inlines the System.nanoTime() call into the call to os::javaTimeNanos in OS-specific runtime code, which is then calling clock_gettime in glibc, which is then turns to Linux's vDSO __vdso_clock_gettime, and quickly returns. Juggling the clock type in os::javaTimeNanos... CLOCK_MONOTONIC (default now): Benchmark Mode Cnt Score Error Units TimersBench.granularity_nanotime avgt 5 22.424 ± 0.609 ns/op TimersBench.latency_nanotime avgt 5 21.569 ± 0.341 ns/op CLOCK_MONOTONIC_RAW: Benchmark Mode Cnt Score Error Units TimersBench.granularity_nanotime avgt 5 328.502 ± 7.631 ns/op TimersBench.latency_nanotime avgt 5 332.489 ± 11.013 ns/op CLOCK_MONOTONIC_COARSE: Benchmark Mode Cnt Score Error Units TimersBench.granularity_nanotime avgt 5 3999733.222 ± 0.403 ns/op TimersBench.latency_nanotime avgt 5 7.046 ± 0.328 ns/op CLOCK_MONOTONIC_RAW is apparently unsupported by my kernel, and goes to fallback code and real syscall. Maybe what Andrew was seeing before was the absence of vDSO opts for CLOCK_MONOTONIC. Thanks, -Aleksey.
