?????嵥 3. ??? fork-join ??????????????
????public class MaxWithFJ extends RecursiveAction {
????private final int threshold;
????private final SelectMaxProblem problem;
????public int result;
????public MaxWithFJ(SelectMaxProblem problem?? int threshold) {
????this.problem = problem;
????this.threshold = threshold;
????}
????protected void compute() {
????if (problem.size < threshold)
????result = problem.solveSequentially();
????else {
????int midpoint = problem.size / 2;
????MaxWithFJ left = new MaxWithFJ(problem.subproblem(0?? midpoint)?? threshold);
????MaxWithFJ right = new MaxWithFJ(problem.subproblem(midpoint +
????1?? problem.size)?? threshold);
????coInvoke(left?? right);
????result = Math.max(left.result?? right.result);
????}
????}
????public static void main(String[] args) {
????SelectMaxProblem problem = ...
????int threshold = ...
????int nThreads = ...
????MaxWithFJ mfj = new MaxWithFJ(problem?? threshold);
????ForkJoinExecutor fjPool = new ForkJoinPool(nThreads);
????fjPool.invoke(mfj);
????int result = mfj.result;
????}
????}
?????? 1 ?????????????? 500??000 ???????????????????????????????????????????????з????????????????У?fork-join ???е?????????????????????????????????????е?????????????????????????Щ???????????????????speedup????
?????? 1. ????????? 500k ???????????????? select-max ????
??????? = 500k ??? = 50k ??? = 5k ??? = 500 ??? = -50
????Pentium-4 HT??2 ?????? 1.0 1.07 1.02 .82 .2
????Dual-Xeon HT??4 ?????? .88 3.02 3.2 2.22 .43
????8-way Opteron??8 ?????? 1.0 5.29 5.73 4.53 2.03
????8-core Niagara??32 ?????? .98 10.46 17.21 15.34 6.49
????????????????????????????????????????????????????????????????????????????????????????????ò??????????? chip-multithreading ?????????????????????? Hyperthreading ?????? CMT ????????????????????????????????????????????????????????????????????????????е???????涪????miss rate????
???????????????????Χ?? 500K????????С???????в???????? 50????????????У????? 50 ????С????Щ???????????????????????????????fork-join ????????????????????á?????? 1 ?????????????????????????????????????ò??????????? Runtime.availableProcessors() ???????????????????????????????????????????? fork-join ??????е???????? CPU ???????????????????ù?????С???????????????????????????
????MaxWithFJ ???в?????????????????????????????????????????????????????? ForkJoinExecutor ??????????????????????????????????????????????????????????????????????????????????
????fork-join ???????
?????????к??????? ?嵥 3 ??????? fork-join ???????????????????Thread.start() ?? Thread.join() ???????б??????????????????????????????????? VM ?????????????????????С? N????????????С????????????????????? O(N) ???????????????????????? log2N?????? k ??????????? 2k ???????????Щ????У??????????ü?????????????????????????????ɡ?????????????????棬???????????????????????????????????????????????????????????????????????С????????в??????????
???????????????????? fork-join ?????????????? fork-join ?????????????????????仨?????????????????????????????????????????thread starvation deadlock????????С?????????????????????????????????????????????????????????????????????????????????????????????????????????? — fork-join ?????????????????????????????????????????????????????й????????????????з?????????????
???????????
????fork-join ?????????????????????work stealing?? ?????????????????е??????????????????????????????????У?????????????У???????? deque?????????Java 6 ??????????????? deque ???????? ArrayDeque ?? LinkedBlockingDeque???????????????????????????????????? deque ??????????????????????????δ????????????????????????????????????????????У???????????????????????????? Thread.join() ??????????????????????????????????????????????? deque ??β?? ????????????
????????????????????????????????????????????deque ???????????????????????ú???????????й??????????????? deque ???????deque ??????????????????????е????????????????? deque ??β???????????????????? deque β??????????? fork-join ????н?? deque ???????Щ???????????????Э????????????????????????????????????????????????????????????????????????????last-in-first-out??LIFO??????????????ζ??????????????е?β??????????????????????????????????????????????С????????????????????δ??????????????????????????????????????????Э???????????????????С??
??????????
????fork-join ?????????????????л???????????????????????????????????????????????е?????????????????????????в??з??????? Arrays.sort() ??????????????????? fork-join ??????????ó?????????в??з????洦??????????????????????????????????????????????????????????Ч??????Щ??????????????????????????????????в??з????????????????????δ?????????