jvm-gc二

 1    /** ParallelGCThreads的取值策略：
 2     ①如果用户显示指定了ParallelGCThreads，则使用用户指定的值。
 3     ②否则，需要根据实际的CPU所能够支持的线程数来计算ParallelGCThreads的值，计算方法见步骤③和步骤④。
 4     ③如果物理CPU所能够支持线程数小于8，则ParallelGCThreads的值为CPU所支持的线程数。这里的阀值为8，是因为JVM中调用nof_parallel_worker_threads接口所传入的switch_pt的值均为8。
 5     ④如果物理CPU所能够支持线程数大于8，则ParallelGCThreads的值为8加上一个调整值，调整值的计算方式为：物理CPU所支持的线程数减去8所得值的5/8或者5/16，JVM会根据实际的情况来选择具体是乘以5/8还是5/16。
 6     比如，在64线程的x86 CPU上，如果用户未指定ParallelGCThreads的值，则默认的计算方式为：ParallelGCThreads = 8 + (64 - 8) * (5/8) = 8 + 35 = 43。
 7     */
 8 unsigned int VM_Version::calc_parallel_worker_threads() {
 9   unsigned int result;
10   if (is_M_series()) {
11     // for now, use same gc thread calculation for M-series as for niagara-plus
12     // in future, we may want to tweak parameters for nof_parallel_worker_thread
13     result = nof_parallel_worker_threads(5, 16, 8);
14   } else if (is_niagara_plus()) {
15     result = nof_parallel_worker_threads(5, 16, 8);
16   } else {
17     result = nof_parallel_worker_threads(5, 8, 8);
18   }
19   return result;
20 } 
21 
22 unsigned int Abstract_VM_Version::parallel_worker_threads() {
23   if (!_parallel_worker_threads_initialized) {
24     if (FLAG_IS_DEFAULT(ParallelGCThreads)) {
25       _parallel_worker_threads = VM_Version::calc_parallel_worker_threads();
26     } else {
27       _parallel_worker_threads = ParallelGCThreads;
28     }
29     _parallel_worker_threads_initialized = true;
30   }
31   return _parallel_worker_threads;
32 }
33 
34 unsigned int Abstract_VM_Version::calc_parallel_worker_threads() {
35   return nof_parallel_worker_threads(5, 8, 8);
36 }
37 
38 unsigned int Abstract_VM_Version::nof_parallel_worker_threads(
39                                                       unsigned int num,
40                                                       unsigned int den,
41                                                       unsigned int switch_pt) {
42   if (FLAG_IS_DEFAULT(ParallelGCThreads)) {
43     assert(ParallelGCThreads == 0, "Default ParallelGCThreads is not 0");
44     // For very large machines, there are diminishing returns
45     // for large numbers of worker threads.  Instead of
46     // hogging the whole system, use a fraction of the workers for every
47     // processor after the first 8.  For example, on a 72 cpu machine
48     // and a chosen fraction of 5/8
49     // use 8 + (72 - 8) * (5/8) == 48 worker threads.
50     unsigned int ncpus = (unsigned int) os::active_processor_count();
51     return (ncpus <= switch_pt) ?
52            ncpus :
53           (switch_pt + ((ncpus - switch_pt) * num) / den);
54   } else {
55     return ParallelGCThreads;
56   }
57 }