On Fri, 6 Jun 2025 07:14:21 GMT, Radim Vansa <rva...@openjdk.org> wrote:
>> This optimization is a followup to https://github.com/openjdk/jdk/pull/24290 >> trying to reduce the performance regression in some scenarios introduced in >> https://bugs.openjdk.org/browse/JDK-8292818 . Based both on performance and >> memory consumption it is a (better) alternative to >> https://github.com/openjdk/jdk/pull/24713 . >> >> This PR optimizes local field lookup in classes with more than 16 fields; >> rather than sequentially iterating through all fields during lookup we sort >> the fields based on the field name. The stream includes extra table after >> the field information: for field at position 16, 32 ... we record the >> (variable-length-encoded) offset of the field info in this stream. On field >> lookup, rather than iterating through all fields, we iterate through this >> table, resolve names for given fields and continue field-by-field iteration >> only after the last record (hence at most 16 fields). >> >> In classes with <= 16 fields this PR reduces the memory consumption by 1 >> byte that was left with value 0 at the end of stream. In classes with > 16 >> fields we add extra 4 bytes with offset of the table, and the table contains >> one varint for each 16 fields. The terminal byte is not used either. >> >> My measurements on the attached reproducer >> >> hyperfine -w 50 -r 100 '/path/to/jdk-17/bin/java -cp /tmp CCC' >> Benchmark 1: /path/to/jdk-17/bin/java -cp /tmp CCC >> Time (mean ± σ): 51.3 ms ± 2.8 ms [User: 44.7 ms, System: 13.7 >> ms] >> Range (min … max): 45.1 ms … 53.9 ms 100 runs >> >> hyperfine -w 50 -r 100 '/path/to/jdk25-master/bin/java -cp /tmp CCC' >> Benchmark 1: /path/to/jdk25-master/bin/java -cp /tmp CCC >> Time (mean ± σ): 78.2 ms ± 1.0 ms [User: 74.6 ms, System: 17.3 >> ms] >> Range (min … max): 73.8 ms … 79.7 ms 100 runs >> >> (the jdk25-master above already contains JDK-8353175) >> >> hyperfine -w 50 -r 100 '/path/to/jdk25-this-pr/bin/java -cp /tmp CCC' >> Benchmark 1: /path/to/jdk25-this-pr/jdk/bin/java -cp /tmp CCC >> Time (mean ± σ): 38.5 ms ± 0.5 ms [User: 34.4 ms, System: 17.3 >> ms] >> Range (min … max): 37.7 ms … 42.1 ms 100 runs >> >> While https://github.com/openjdk/jdk/pull/24713 returned the performance to >> previous levels, this PR improves it by 25% compared to JDK 17 (which does >> not contain the regression)! This time, the undisclosed production-grade >> reproducer shows even higher improvement: >> >> JDK 17: 1.6 s >> JDK 21 (no patches): 22 s >> JDK25-master: 12.3 s >> JDK25-this-pr: 0.5 s > > Radim Vansa has updated the pull request incrementally with two additional > commits since the last revision: > > - Add more comments > - Disable search table with dynamic CDS I have a couple more comments for today. src/hotspot/share/utilities/packedTable.cpp line 49: > 47: assert((key & ~_key_mask) == 0, "key out of bounds"); > 48: assert((value & ~_value_mask) == 0, "value out of bounds: %x vs. %x > (%x)", value, _value_mask, ~_value_mask); > 49: *reinterpret_cast<uint64_t*>(data + offset) = > static_cast<uint64_t>(key) | (static_cast<uint64_t>(value) << _value_shift); How does this line not get a signal for unaligned write? src/hotspot/share/utilities/packedTable.cpp line 83: > 81: assert(mid >= low && mid < high, "integer overflow?"); > 82: uint64_t element = read_element(data, length, _element_bytes * mid); > 83: uint32_t key = element & _key_mask; All this casting is hard to follow so I added this at the beginning of the file: #ifndef _WIN32 #pragma GCC diagnostic warning "-Wconversion" #endif and this line, 102, and 87 complain: warning: conversion from 'uint64_t' {aka 'long unsigned int'} to 'uint32_t' {aka 'unsigned int'} may change value [-Wconversion] 87 | uint32_t key = element & _key_mask; | ~~~~~~~~^~~~~~~~~~~ If the value is okay to cast to uint32_t, which I believe it is, use checked_cast<uint32_t> from checkedCast.hpp. ------------- Changes requested by coleenp (Reviewer). PR Review: https://git.openjdk.org/jdk/pull/24847#pullrequestreview-2905861595 PR Review Comment: https://git.openjdk.org/jdk/pull/24847#discussion_r2132749485 PR Review Comment: https://git.openjdk.org/jdk/pull/24847#discussion_r2132790669
