A fast implementation without unsafe (#462)
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Dr Ian Preston
GitHub
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/*
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* Copyright 2023 The original authors
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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package dev.morling.onebrc;
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import java.io.*;
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import java.nio.*;
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import java.nio.channels.*;
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import java.util.concurrent.*;
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import java.util.stream.*;
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import java.util.*;
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/* A fast implementation with no unsafe.
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* Features:
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* * memory mapped file
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* * read chunks in parallel
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* * minimise allocation
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* * no unsafe
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*
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* Timings on 4 core i7-7500U CPU @ 2.70GHz:
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* average_baseline: 4m48s
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* ianopolous: 19s
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*/
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public class CalculateAverage_ianopolousfast {
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public static final int MAX_LINE_LENGTH = 107;
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public static final int MAX_STATIONS = 10_000;
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public static void main(String[] args) throws Exception {
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File input = new File("./measurements.txt");
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long filesize = input.length();
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// keep chunk size between 256 MB and 1G (1 chunk for files < 256MB)
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long chunkSize = Math.min(Math.max((filesize + 31) / 32, 256 * 1024 * 1024), 1024 * 1024 * 1024L);
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int nChunks = (int) ((filesize + chunkSize - 1) / chunkSize);
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ExecutorService pool = Executors.newVirtualThreadPerTaskExecutor();
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List<Future<List<List<Stat>>>> allResults = IntStream.range(0, nChunks)
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.mapToObj(i -> pool.submit(() -> parseStats(i * chunkSize, Math.min((i + 1) * chunkSize, filesize))))
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.toList();
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TreeMap<String, Stat> merged = allResults.stream()
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.parallel()
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.flatMap(f -> {
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try {
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return f.get().stream().filter(Objects::nonNull).flatMap(Collection::stream);
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}
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catch (Exception e) {
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e.printStackTrace();
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return Stream.empty();
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}
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})
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.collect(Collectors.toMap(s -> s.name(), s -> s, (a, b) -> a.merge(b), TreeMap::new));
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System.out.println(merged);
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}
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public static boolean matchingStationBytes(int start, int end, ByteBuffer buffer, Stat existing) {
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if (end - start != existing.name.length)
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return false;
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for (int i = start; i < end; i++) {
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if (existing.name[i - start] != buffer.get(i))
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return false;
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}
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return true;
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}
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public static Stat dedupeStation(int start, int end, long hash, ByteBuffer buffer, List<List<Stat>> stations) {
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int index = Math.floorMod(hash ^ (hash >> 32), MAX_STATIONS);
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List<Stat> matches = stations.get(index);
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if (matches == null) {
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List<Stat> value = new ArrayList<>();
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byte[] stationBuffer = new byte[end - start];
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buffer.position(start);
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buffer.get(stationBuffer);
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Stat res = new Stat(stationBuffer);
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value.add(res);
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stations.set(index, value);
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return res;
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}
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else {
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for (int i = 0; i < matches.size(); i++) {
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Stat s = matches.get(i);
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if (matchingStationBytes(start, end, buffer, s))
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return s;
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}
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byte[] stationBuffer = new byte[end - start];
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buffer.position(start);
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buffer.get(stationBuffer);
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Stat res = new Stat(stationBuffer);
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matches.add(res);
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return res;
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}
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}
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public static int getSemicolon(long d) {
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// from Hacker's Delight page 92
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d = d ^ 0x3b3b3b3b3b3b3b3bL;
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long y = (d & 0x7f7f7f7f7f7f7f7fL) + 0x7f7f7f7f7f7f7f7fL;
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y = ~(y | d | 0x7f7f7f7f7f7f7f7fL);
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return Long.numberOfLeadingZeros(y) >> 3;
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}
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public static long updateHash(long hash, long x) {
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return ((hash << 5) ^ x) * 0x517cc1b727220a95L; // fxHash
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}
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public static Stat parseStation(int lineStart, ByteBuffer buffer, List<List<Stat>> stations) {
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// find semicolon and update hash as we go, reading a long at a time
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long d = buffer.getLong(lineStart);
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int semiIndex = getSemicolon(d);
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int index = 0;
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long hash = 0;
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while (semiIndex == 8) {
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hash = updateHash(hash, d);
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index += 8;
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d = buffer.getLong(lineStart + index);
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semiIndex = getSemicolon(d);
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}
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// mask extra bytes off last long
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d = d & (-1L << ((8 - semiIndex) * 8));
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if (semiIndex > 0) {
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hash = updateHash(hash, d);
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}
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return dedupeStation(lineStart, lineStart + index + semiIndex, hash, buffer, stations);
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}
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public static int processTemperature(int lineSplit, MappedByteBuffer buffer, Stat station) {
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short temperature;
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boolean negative = false;
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byte b = buffer.get(lineSplit++);
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if (b == '-') {
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negative = true;
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b = buffer.get(lineSplit++);
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}
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temperature = (short) (b - 0x30);
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b = buffer.get(lineSplit++);
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if (b == '.') {
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b = buffer.get(lineSplit++);
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temperature = (short) (temperature * 10 + (b - 0x30));
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}
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else {
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temperature = (short) (temperature * 10 + (b - 0x30));
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lineSplit++;
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b = buffer.get(lineSplit++);
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temperature = (short) (temperature * 10 + (b - 0x30));
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}
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temperature = negative ? (short) -temperature : temperature;
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station.add(temperature);
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return lineSplit + 1;
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}
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public static List<List<Stat>> parseStats(long startByte, long endByte) {
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try {
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RandomAccessFile file = new RandomAccessFile("./measurements.txt", "r");
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long maxEnd = Math.min(file.length(), endByte + MAX_LINE_LENGTH);
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long len = maxEnd - startByte;
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if (len > Integer.MAX_VALUE)
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throw new RuntimeException("Segment size must fit into an int");
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int maxDone = (int) (endByte - startByte);
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MappedByteBuffer buffer = file.getChannel().map(FileChannel.MapMode.READ_ONLY, startByte, len);
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int done = 0;
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// read first partial line
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if (startByte > 0) {
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for (int i = 0; i < MAX_LINE_LENGTH; i++) {
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byte b = buffer.get(i);
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if (b == '\n') {
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done = i + 1;
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break;
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}
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}
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}
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List<List<Stat>> stations = new ArrayList<>(MAX_STATIONS);
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for (int i = 0; i < MAX_STATIONS; i++)
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stations.add(null);
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// Handle reading the very last line in the file
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// this allows us to not worry about reading a long beyond the end
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// in the inner loop (reducing branches)
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// We only need to read one because the min record size is 6 bytes
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// so 2nd last record must be > 8 from end
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if (endByte == file.length()) {
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int offset = (int) (file.length() - startByte - 1);
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while (buffer.get(offset) != '\n') // final new line
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offset--;
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offset--;
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while (offset > 0 && buffer.get(offset) != '\n') // end of second last line
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offset--;
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maxDone = offset;
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if (offset > 0)
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offset++;
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// copy into a 8n sized buffer to avoid reading off end
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int roundedSize = (int) (file.length() - startByte) - offset;
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roundedSize = (roundedSize + 7) / 8 * 8;
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byte[] end = new byte[roundedSize];
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for (int i = offset; i < (int) (file.length() - startByte); i++)
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end[i - offset] = buffer.get(i);
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Stat station = parseStation(0, ByteBuffer.wrap(end), stations);
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processTemperature(offset + station.name.length + 1, buffer, station);
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}
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int lineStart = done;
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while (lineStart < maxDone) {
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Stat station = parseStation(lineStart, buffer, stations);
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lineStart = processTemperature(lineStart + station.name.length + 1, buffer, station);
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}
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return stations;
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}
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catch (IOException e) {
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throw new RuntimeException(e);
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}
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}
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public static class Stat {
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final byte[] name;
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int count = 0;
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short min = Short.MAX_VALUE, max = Short.MIN_VALUE;
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long total = 0;
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public Stat(byte[] name) {
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this.name = name;
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}
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public void add(short value) {
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if (value < min)
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min = value;
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if (value > max)
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max = value;
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total += value;
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count++;
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}
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public Stat merge(Stat value) {
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if (value.min < min)
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min = value.min;
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if (value.max > max)
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max = value.max;
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total += value.total;
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count += value.count;
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return this;
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}
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private static double round(double value) {
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return Math.round(value) / 10.0;
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}
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public String name() {
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return new String(name);
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}
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public String toString() {
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return round((double) min) + "/" + round(((double) total) / count) + "/" + round((double) max);
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}
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}
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}
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