-XX:+UseSerialGC

指定使用Serial GC
因此，对应使用的堆类型为GenCollectedHeap

void GenCollectedHeap::process_roots(ScanningOption so,
                                     OopClosure* strong_roots,
                                     CLDClosure* strong_cld_closure,
                                     CLDClosure* weak_cld_closure,
                                     CodeBlobToOopClosure* code_roots) {
  // General roots.
  assert(code_roots != NULL, "code root closure should always be set");

  ClassLoaderDataGraph::roots_cld_do(strong_cld_closure, weak_cld_closure);

  // Only process code roots from thread stacks if we aren't visiting the entire CodeCache anyway
  CodeBlobToOopClosure* roots_from_code_p = (so & SO_AllCodeCache) ? NULL : code_roots;

// 此处为入口，遍历栈帧中的OopMapSet
  Threads::oops_do(strong_roots, roots_from_code_p);

  OopStorageSet::strong_oops_do(strong_roots);

  if (so & SO_ScavengeCodeCache) {
    assert(code_roots != NULL, "must supply closure for code cache");

    // We only visit parts of the CodeCache when scavenging.
    ScavengableNMethods::nmethods_do(code_roots);
  }
  if (so & SO_AllCodeCache) {
    assert(code_roots != NULL, "must supply closure for code cache");

    // CMSCollector uses this to do intermediate-strength collections.
    // We scan the entire code cache, since CodeCache::do_unloading is not called.
    CodeCache::blobs_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, !CodeBlobToOopClosure::FixRelocations));
  DEBUG_ONLY(ScavengableNMethods::asserted_non_scavengable_nmethods_do(&assert_code_is_non_scavengable));
}

Threads::oops_do 最终会执行到Thread::oops_do

void Thread::oops_do(OopClosure* f, CodeBlobClosure* cf) {
  // Record JavaThread to GC thread
  RememberProcessedThread rpt(this);
  oops_do_no_frames(f, cf);
  oops_do_frames(f, cf);
}

oops_do_frames 的定义

void JavaThread::oops_do_frames(OopClosure* f, CodeBlobClosure* cf) {
  if (!has_last_Java_frame()) {
    return;
  }
  // Finish any pending lazy GC activity for the frames
  StackWatermarkSet::finish_processing(this, NULL /* context */, StackWatermarkKind::gc);
  // Traverse the execution stack
  for (StackFrameStream fst(this, true /* update */, false /* process_frames */); !fst.is_done(); fst.next()) {
    fst.current()->oops_do(f, cf, fst.register_map()); // 这里会读取frame.oopMapSet
  }

最终会执行

void frame::oops_code_blob_do(OopClosure* f, CodeBlobClosure* cf, const RegisterMap* reg_map,
                              DerivedPointerIterationMode derived_mode) const {
  assert(_cb != NULL, "sanity check");
  if (_cb->oop_maps() != NULL) {
    OopMapSet::oops_do(this, reg_map, f, derived_mode);
...
}