46                                                      bool           should_initiate_conc_mark,

    61 bool VM_G1CollectForAllocation::doit_prologue() {

    62   bool res = VM_CollectForAllocation::doit_prologue();

    63   if (!res) {

    64     if (_should_initiate_conc_mark) {

    65       // The prologue can fail for a couple of reasons. The first is that another GC

    66       // got scheduled and prevented the scheduling of the initial mark GC. The

    67       // second is that the GC locker may be active and the heap can't be expanded.

    68       // In both cases we want to retry the GC so that the initial mark pause is

    69       // actually scheduled. In the second case, however, we should stall until

    70       // until the GC locker is no longer active and then retry the initial mark GC.

    71       _should_retry_gc = true;

    72     }

    73   }

    74   return res;

    75 }

    76 

    79   assert(!_should_initiate_conc_mark || g1h->should_do_concurrent_full_gc(_gc_cause),

    80       "only a GC locker, a System.gc(), stats update, whitebox, or a hum allocation induced GC should start a cycle");

    95   if (_should_initiate_conc_mark) {

    96     // It's safer to read old_marking_cycles_completed() here, given

    97     // that noone else will be updating it concurrently. Since we'll

    98     // only need it if we're initiating a marking cycle, no point in

    99     // setting it earlier.

   100     _old_marking_cycles_completed_before = g1h->old_marking_cycles_completed();

   101 

   102     // At this point we are supposed to start a concurrent cycle. We

   103     // will do so if one is not already in progress.

   104     bool res = g1h->policy()->force_initial_mark_if_outside_cycle(_gc_cause);

   105 

   106     // The above routine returns true if we were able to force the

   107     // next GC pause to be an initial mark; it returns false if a

   108     // marking cycle is already in progress.

   109     //

   110     // If a marking cycle is already in progress just return and skip the

   111     // pause below - if the reason for requesting this initial mark pause

   112     // was due to a System.gc() then the requesting thread should block in

   113     // doit_epilogue() until the marking cycle is complete.

   114     //

   115     // If this initial mark pause was requested as part of a humongous

   116     // allocation then we know that the marking cycle must just have

   117     // been started by another thread (possibly also allocating a humongous

   118     // object) as there was no active marking cycle when the requesting

   119     // thread checked before calling collect() in

   120     // attempt_allocation_humongous(). Retrying the GC, in this case,

   121     // will cause the requesting thread to spin inside collect() until the

   122     // just started marking cycle is complete - which may be a while. So

   123     // we do NOT retry the GC.

   124     if (!res) {

   125       assert(_word_size == 0, "Concurrent Full GC/Humongous Object IM shouldn't be allocating");

   126       if (_gc_cause != GCCause::_g1_humongous_allocation) {

   127         _should_retry_gc = true;

   128       }

   129       return;

   130     }

   131   }

   132 

   154   } else {

   155     assert(_result == NULL, "invariant");

   156     // The only reason for the pause to not be successful is that, the GC locker is

   157     // active (or has become active since the prologue was executed). In this case

   158     // we should retry the pause after waiting for the GC locker to become inactive.

   159     _should_retry_gc = true;

   160   }

   161 }

   162 

   163 void VM_G1CollectForAllocation::doit_epilogue() {

   164   VM_CollectForAllocation::doit_epilogue();

   165 

   166   // If the pause was initiated by a System.gc() and

   167   // +ExplicitGCInvokesConcurrent, we have to wait here for the cycle

   168   // that just started (or maybe one that was already in progress) to

   169   // finish.

   170   if (GCCause::is_user_requested_gc(_gc_cause) &&

   171       _should_initiate_conc_mark) {

   172     assert(ExplicitGCInvokesConcurrent,

   173            "the only way to be here is if ExplicitGCInvokesConcurrent is set");

   174 

   175     G1CollectedHeap* g1h = G1CollectedHeap::heap();

   176 

   177     // In the doit() method we saved g1h->old_marking_cycles_completed()

   178     // in the _old_marking_cycles_completed_before field. We have to

   179     // wait until we observe that g1h->old_marking_cycles_completed()

   180     // has increased by at least one. This can happen if a) we started

   181     // a cycle and it completes, b) a cycle already in progress

   182     // completes, or c) a Full GC happens.

   183 

   184     // If the condition has already been reached, there's no point in

   185     // actually taking the lock and doing the wait.

   186     if (g1h->old_marking_cycles_completed() <=

   187                                           _old_marking_cycles_completed_before) {

   188       // The following is largely copied from CMS

   189 

   190       Thread* thr = Thread::current();

   191       assert(thr->is_Java_thread(), "invariant");

   192       JavaThread* jt = (JavaThread*)thr;

   193       ThreadToNativeFromVM native(jt);

   194 

   195       MonitorLocker ml(FullGCCount_lock, Mutex::_no_safepoint_check_flag);

   196       while (g1h->old_marking_cycles_completed() <=

   197                                           _old_marking_cycles_completed_before) {

   198         ml.wait();

   199       }

   200     }