Hotpot 年轻代GC 源代码分析

1. gch->gen_process_strong_roots(_level,
                                true,  // Process younger gens, if any,
                                       // as strong roots.
                                true,  // activate StrongRootsScope
                                false, // not collecting perm generation.
                                SharedHeap::SO_AllClasses,
                                &fsc_with_no_gc_barrier,
                                true,   // walk *all* scavengable nmethods
                                &fsc_with_gc_barrier);

  // "evacuate followers".
2. evacuate_followers.do_void();

3. rp->process_discovered_references(&is_alive, &keep_alive, &evacuate_followers,
                                    NULL);

// General strong roots.

  if (!do_code_roots) {
    SharedHeap::process_strong_roots(activate_scope, collecting_perm_gen, so,
                                     not_older_gens, NULL, older_gens);
  } else {
    bool do_code_marking = (activate_scope || nmethod::oops_do_marking_is_active());
    CodeBlobToOopClosure code_roots(not_older_gens, /*do_marking=*/ do_code_marking);
    SharedHeap::process_strong_roots(activate_scope, collecting_perm_gen, so,
                                     not_older_gens, &code_roots, older_gens);
  }

  if (younger_gens_as_roots) {
    if (!_gen_process_strong_tasks->is_task_claimed(GCH_PS_younger_gens)) {
      for (int i = 0; i < level; i++) {
        not_older_gens->set_generation(_gens[i]);
        _gens[i]->oop_iterate(not_older_gens);
      }
      not_older_gens->reset_generation();
    }
  }
  // When collection is parallel, all threads get to cooperate to do
  // older-gen scanning.
  for (int i = level+1; i < _n_gens; i++) {
    older_gens->set_generation(_gens[i]);
    rem_set()->younger_refs_iterate(_gens[i], older_gens);
    older_gens->reset_generation();
  }
    younger_refs_in_space_iterate
  _gen_process_strong_tasks->all_tasks_completed();

void SharedHeap::process_strong_roots(bool activate_scope,
                                      bool collecting_perm_gen,
                                      ScanningOption so,
                                      OopClosure* roots,
                                      CodeBlobClosure* code_roots,
                                      OopsInGenClosure* perm_blk) {
  StrongRootsScope srs(this, activate_scope);
  // General strong roots.
  assert(_strong_roots_parity != 0, "must have called prologue code");
  if (!_process_strong_tasks->is_task_claimed(SH_PS_Universe_oops_do)) {
    Universe::oops_do(roots); 　　//1
    ReferenceProcessor::oops_do(roots);
    // Consider perm-gen discovered lists to be strong.
    perm_gen()->ref_processor()->weak_oops_do(roots);
  }
  // Global (strong) JNI handles
  if (!_process_strong_tasks->is_task_claimed(SH_PS_JNIHandles_oops_do))
    JNIHandles::oops_do(roots);　　//2
  // All threads execute this; the individual threads are task groups.
  if (ParallelGCThreads > 0) {
    Threads::possibly_parallel_oops_do(roots, code_roots);
  } else {
    Threads::oops_do(roots, code_roots);　　//3
  }
  if (!_process_strong_tasks-> is_task_claimed(SH_PS_ObjectSynchronizer_oops_do))
    ObjectSynchronizer::oops_do(roots);　　//4
  if (!_process_strong_tasks->is_task_claimed(SH_PS_FlatProfiler_oops_do))
    FlatProfiler::oops_do(roots);　　//5
  if (!_process_strong_tasks->is_task_claimed(SH_PS_Management_oops_do))
    Management::oops_do(roots);　　//6
  if (!_process_strong_tasks->is_task_claimed(SH_PS_jvmti_oops_do))
    JvmtiExport::oops_do(roots);　　//7

  if (!_process_strong_tasks->is_task_claimed(SH_PS_SystemDictionary_oops_do)) {
    if (so & SO_AllClasses) {
      SystemDictionary::oops_do(roots);　　//8
    } else if (so & SO_SystemClasses) {
      SystemDictionary::always_strong_oops_do(roots);
    }
  }

  if (!_process_strong_tasks->is_task_claimed(SH_PS_StringTable_oops_do)) {
    if (so & SO_Strings || (!collecting_perm_gen && !JavaObjectsInPerm)) {
      StringTable::oops_do(roots);　　//9
    }
    if (JavaObjectsInPerm) {
      // Verify the string table contents are in the perm gen
      NOT_PRODUCT(StringTable::oops_do(&assert_is_perm_closure));
    }
  }

  if (!_process_strong_tasks->is_task_claimed(SH_PS_CodeCache_oops_do)) {
    if (so & SO_CodeCache) {
      // (Currently, CMSCollector uses this to do intermediate-strength collections.)
      assert(collecting_perm_gen, "scanning all of code cache");
      assert(code_roots != NULL, "must supply closure for code cache");
      if (code_roots != NULL) {
        CodeCache::blobs_do(code_roots);
      }
    } else if (so & (SO_SystemClasses|SO_AllClasses)) {
      if (!collecting_perm_gen) {
        // If we are collecting from class statics, but we are not going to
        // visit all of the CodeCache, collect from the non-perm roots if any.
        // This makes the code cache function temporarily as a source of strong
        // roots for oops, until the next major collection.
        //
        // If collecting_perm_gen is true, we require that this phase will call
        // CodeCache::do_unloading.  This will kill off nmethods with expired
        // weak references, such as stale invokedynamic targets.
        CodeCache::scavenge_root_nmethods_do(code_roots);
      }
    }
    // Verify that the code cache contents are not subject to
    // movement by a scavenging collection.
    DEBUG_ONLY(CodeBlobToOopClosure assert_code_is_non_scavengable(&assert_is_non_scavengable_closure, /*do_marking=*/ false));
    DEBUG_ONLY(CodeCache::asserted_non_scavengable_nmethods_do(&assert_code_is_non_scavengable));
  }

  if (!collecting_perm_gen) {
    // All threads perform this; coordination is handled internally.

    rem_set()->younger_refs_iterate(perm_gen(), perm_blk);
  }
  _process_strong_tasks->all_tasks_completed();
}

oop DefNewGeneration::copy_to_survivor_space(oop old) {
  assert(is_in_reserved(old) && !old->is_forwarded(),
         "shouldn't be scavenging this oop");
  size_t s = old->size();
  oop obj = NULL;

  // Try allocating obj in to-space (unless too old)
  if (old->age() < tenuring_threshold()) {
    obj = (oop) to()->allocate(s);
  }

  // Otherwise try allocating obj tenured
  if (obj == NULL) {
    obj = _next_gen->promote(old, s);
    if (obj == NULL) {
      handle_promotion_failure(old);
      return old;
    }
  } else {
    // Prefetch beyond obj
    const intx interval = PrefetchCopyIntervalInBytes;
    Prefetch::write(obj, interval);

    // Copy obj
    Copy::aligned_disjoint_words((HeapWord*)old, (HeapWord*)obj, s);

    // Increment age if obj still in new generation
    obj->incr_age();
    age_table()->add(obj, s);
  }

  // Done, insert forward pointer to obj in this header
  old->forward_to(obj);

  return obj;
}