On Wed, 28 Oct 2020 14:36:59 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 > > Claes Redestad has updated the pull request with a new target base due to a > merge or a rebase. The incremental webrev excludes the unrelated changes > brought in by the merge/rebase. The pull request contains six additional > commits since the last revision: > > - Merge branch 'master' into threadHeapSampler_constexpr > - Declare and define log_table as a static constexpr inside > threadHeapSampler.cpp > - Merge branch 'master' into threadHeapSampler_constexpr > - Merge branch 'master' into threadHeapSampler_constexpr > - Remove _log_table_initialized assert > - Refactor ThreadHeapSampler::_log_table as constexpr Hi Ioi and Claes, This looks nice. One nit: + _values[i] = (log(1.0 + static_cast<double>(i+0.5) / N) / log(2.0)); Need spaces around '+' sign. Also, it can be just one line. Thanks, Serguei ------------- Marked as reviewed by sspitsyn (Reviewer). PR: https://git.openjdk.java.net/jdk/pull/880
