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17.2 GC策略初始化
这一步主要就是确定分代（新生代和老年代）信息、即将分配的Java堆内存的大小，以及针对这些信息的校验
17.2.1 collectorPolicy.cpp&ollectorPolicy.hpp
17.2.1.1 initialize_all
virtual void initialize_all() {// 针对内存分配的大小值做设置和校验CollectorPolicy::initialize_all();// 创建并分配新生代和老年代对象的内存initialize_generations();}// 对应 CollectorPolicy::initialize_all() 函数
virtual void initialize_all() {// 父类中，这是一个纯虚函数（相当于Java里面的抽象函数），只能去子类中找实现，这个函数就是计算即将要创建和分配的内存的对齐值initialize_alignments(); // 对 Java堆大小值的判断和设置initialize_flags();// 对以上两步产生的大小做参数检验initialize_size_info();}// 对应 initialize_alignments() 函数
void MarkSweepPolicy::initialize_alignments() {/** 设置空间和分代对齐（C/C++中分配内存时都需要按着一定大小对齐），对齐的目的就是为了提高内存访问效率，CPU读取内存时不是一个一个字节读的，而是一批字节来读，比如4字节、8字节和64字节等，具体读多少，与CPU的架构、型号、年份有关，现在的机器基本是读64字节（缓存行大小），如果没按对齐规则对齐，就可能读取到的内存是“半包”，这就需要CPU多次读取，效率较低。这里按Generation::GenGrain对齐，这个值定义在generattion.hpp的枚举中enum SomePublicConstants {LogOfGenGrain = 16 ARM32_ONLY(+1), // 最终得出结果16GenGrain = 1 << LogOfGenGrain // 1 << 16 ==> 2^16 ==> 64 * k};通过上面的定义可知，_space_alignment和_gen_alignment的值最终就是64*k，也就是按这个值来对齐*/ _space_alignment = _gen_alignment = (uintx)Generation::GenGrain;_heap_alignment = compute_heap_alignment(); // 计算堆空间的对齐值，最终=512 * 4K
}
// 对应 compute_heap_alignment() 函数
size_t CollectorPolicy::compute_heap_alignment() {// 这里就是按CardTable卡表的大小乘以页大小来对齐的，查看源码可以得出，卡表大小card_size=1 << 9 ==> 2^9 ==> 512，最终对齐大小等于：512 * 4K（page_size）size_t alignment = GenRemSet::max_alignment_constraint(GenRemSet::CardTable);if (UseLargePages) { // 用大页，这种情况，忽略alignment = lcm(os::large_page_size(), alignment);}return alignment;
}// 对应 initialize_flags() 函数
void CollectorPolicy::initialize_flags() {// 检验上一步计算的各空间对齐值不为0 assert(_space_alignment != 0, "Space alignment not set up properly");assert(_heap_alignment != 0, "Heap alignment not set up properly");assert(_heap_alignment >= _space_alignment,err_msg("heap_alignment: " SIZE_FORMAT " less than space_alignment: " SIZE_FORMAT,_heap_alignment, _space_alignment));assert(_heap_alignment % _space_alignment == 0,err_msg("heap_alignment: " SIZE_FORMAT " not aligned by space_alignment: " SIZE_FORMAT,_heap_alignment, _space_alignment));// 检查空间大小限制，InitialHeapSize和MaxHeapSize的值在`章节8.1.1.2`中做了解析，有兴趣的，再回头看看if (FLAG_IS_CMDLINE(MaxHeapSize)) {if (FLAG_IS_CMDLINE(InitialHeapSize) && InitialHeapSize > MaxHeapSize) {vm_exit_during_initialization("Initial heap size set to a larger value than the maximum heap size");}if (_min_heap_byte_size != 0 && MaxHeapSize < _min_heap_byte_size) {vm_exit_during_initialization("Incompatible minimum and maximum heap sizes specified");}_max_heap_size_cmdline = true;}// 初始堆大小不能小于1Mif (InitialHeapSize < M) {vm_exit_during_initialization("Too small initial heap");}// 最小堆大小不能小于1Mif (_min_heap_byte_size < M) {vm_exit_during_initialization("Too small minimum heap");}// 用 用户从 -Xmx and -Xms 设置的值，并通过_heap_alignment的大小对齐，得到最小堆大小、初始堆大小、最大堆大小_min_heap_byte_size = align_size_up(_min_heap_byte_size, _heap_alignment);uintx aligned_initial_heap_size = align_size_up(InitialHeapSize, _heap_alignment);uintx aligned_max_heap_size = align_size_up(MaxHeapSize, _heap_alignment);// 值改变了，就写回对应的flags中，这块的设置看`章节8.1.1.3`if (aligned_initial_heap_size != InitialHeapSize) {FLAG_SET_ERGO(uintx, InitialHeapSize, aligned_initial_heap_size);}if (aligned_max_heap_size != MaxHeapSize) {FLAG_SET_ERGO(uintx, MaxHeapSize, aligned_max_heap_size);}if (FLAG_IS_CMDLINE(InitialHeapSize) && _min_heap_byte_size != 0 &&InitialHeapSize < _min_heap_byte_size) {vm_exit_during_initialization("Incompatible minimum and initial heap sizes specified");}if (!FLAG_IS_DEFAULT(InitialHeapSize) && InitialHeapSize > MaxHeapSize) {FLAG_SET_ERGO(uintx, MaxHeapSize, InitialHeapSize);} else if (!FLAG_IS_DEFAULT(MaxHeapSize) && InitialHeapSize > MaxHeapSize) {FLAG_SET_ERGO(uintx, InitialHeapSize, MaxHeapSize);if (InitialHeapSize < _min_heap_byte_size) {_min_heap_byte_size = InitialHeapSize;}}_initial_heap_byte_size = InitialHeapSize;_max_heap_byte_size = MaxHeapSize;FLAG_SET_ERGO(uintx, MinHeapDeltaBytes, align_size_up(MinHeapDeltaBytes, _space_alignment));DEBUG_ONLY(CollectorPolicy::assert_flags();)
}// 分代初始化，默认分成两代（年轻和老年代）
void MarkSweepPolicy::initialize_generations() {// 通过malloc() 库函数分配内存，并得到一个大小为2的分代数组，分别存放年轻代和老年代GenerationSpec，number_of_generations() 这个函数里面写死了返回2_generations = NEW_C_HEAP_ARRAY3(GenerationSpecPtr, number_of_generations(), mtGC, CURRENT_PC,AllocFailStrategy::RETURN_NULL);if (_generations == NULL) {vm_exit_during_initialization("Unable to allocate gen spec");}if (UseParNewGC) {_generations[0] = new GenerationSpec(Generation::ParNew, _initial_gen0_size, _max_gen0_size);} else {// 串行GC的，走这条线，创建新生代GenerationSpec对象，对象内存都是通过malloc()来分配的_generations[0] = new GenerationSpec(Generation::DefNew, _initial_gen0_size, _max_gen0_size);}// 创建老年代GenerationSpec对象_generations[1] = new GenerationSpec(Generation::MarkSweepCompact, _initial_gen1_size, _max_gen1_size);if (_generations[0] == NULL || _generations[1] == NULL) {vm_exit_during_initialization("Unable to allocate gen spec");}
}17.3 Java堆空间创建
17.3.1 genCollectedHeap.cpp
17.3.1.1 GenCollectedHeap::GenCollectedHeap
GenCollectedHeap::GenCollectedHeap(GenCollectorPolicy *policy) :SharedHeap(policy), // 先调用父类SharedHeap构造函数，看`章节17.3.1.2`_gen_policy(policy), // _gen_policy 赋值为 policy
// 创建一个子任务管理类，管理所有子任务，并赋值给_process_strong_tasks_process_strong_tasks(new SubTasksDone(GCH_PS_NumElements)),_full_collections_completed(0) // _full_collections_completed 赋值
{assert(policy != NULL, "Sanity check");
}17.3.1.2 sharedHeap.cpp->SharedHeap::SharedHeap
SharedHeap::SharedHeap(CollectorPolicy* policy_) :CollectedHeap(), // 继续调用上级构造函数_collector_policy(policy_), // 以下为赋值操作_rem_set(NULL),_strong_roots_parity(0),_workers(NULL)
{_sh = this; // ch is static, should be set only once.// 由于我们讨论的是串行的GC收集器，所以下面这个逻辑不会走，忽略if ((UseParNewGC ||(UseConcMarkSweepGC && (CMSParallelInitialMarkEnabled ||CMSParallelRemarkEnabled)) ||UseG1GC) &&ParallelGCThreads > 0) {_workers = new FlexibleWorkGang("Parallel GC Threads", ParallelGCThreads,/* are_GC_task_threads */true,/* are_ConcurrentGC_threads */false);if (_workers == NULL) {vm_exit_during_initialization("Failed necessary allocation.");} else {_workers->initialize_workers();}}
}
17.3.1.3 collectedHeap.cpp->CollectedHeap::CollectedHeap
CollectedHeap::CollectedHeap() : _n_par_threads(0) // 赋值
{// 下面这几步就是得到最大数组的大小，用于后期判断一个对象最大占用内存大小的限制const size_t max_len = size_t(arrayOopDesc::max_array_length(T_INT));const size_t elements_per_word = HeapWordSize / sizeof(jint);_filler_array_max_size = align_object_size(filler_array_hdr_size() +max_len / elements_per_word);// 以下都是赋初始值操作_barrier_set = NULL;_is_gc_active = false;_total_collections = _total_full_collections = 0;_gc_cause = _gc_lastcause = GCCause::_no_gc;NOT_PRODUCT(_promotion_failure_alot_count = 0;)NOT_PRODUCT(_promotion_failure_alot_gc_number = 0;)if (UsePerfData) { // 有关性能数据的，先忽略，以后有机会再单开一个专题来讲EXCEPTION_MARK;// create the gc cause jvmstat counters_perf_gc_cause = PerfDataManager::create_string_variable(SUN_GC, "cause",80, GCCause::to_string(_gc_cause), CHECK);_perf_gc_lastcause =PerfDataManager::create_string_variable(SUN_GC, "lastCause",80, GCCause::to_string(_gc_lastcause), CHECK);}_defer_initial_card_mark = false; // strengthened by subclass in pre_initialize() below.// gc日志事件if (LogEvents) {_gc_heap_log = new GCHeapLog();} else {_gc_heap_log = NULL;}
}
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