On Tue, 27 Oct 2020 14:00:34 GMT, Claes Redestad <redes...@openjdk.org> wrote:
> The static `ThreadHeapSampler::_log_table` is currently initialized on JVM > bootstrap to an overhead of ~67k instructions (linux-x64). By turning the > initialization into a constexpr, we can precalculate the helper table at > compile time, which trades a runtime overhead for a small, 8kb, static > footprint increase. > > I compared `fast_log2` with the `log2` builtin with a naive benchmarking > experiment[1] (not included in this PR) and show that the `fast_log2` is > ~2.5x faster than `log2` on my system. And that without the lookup table we'd > be much worse. So I think it makes sense to preserve this optimization, but > get rid of the startup overhead: > > [5.428s][debug][heapsampling] log2, 0.0751173 secs > [5.457s][debug][heapsampling] fast_log2, 0.0298244 secs > [5.622s][debug][heapsampling] fast_log2_uncached, 0.1645569 secs > > I've verified that this refactoring does not affect performance in this naive > setup. > > [1] https://github.com/openjdk/jdk/compare/master...cl4es:log2_micro?expand=1 This pull request has now been integrated. Changeset: 4158567f Author: Claes Redestad <redes...@openjdk.org> URL: https://git.openjdk.java.net/jdk/commit/4158567f Stats: 317 lines in 3 files changed: 296 ins; 10 del; 11 mod 8255455: Pre-generate ThreadHeapSampler::_log_table Reviewed-by: iklam, sspitsyn ------------- PR: https://git.openjdk.java.net/jdk/pull/880
