32   if (PrintPLAB) {

    33   if (!ResizePLAB) {

    33     gclog_or_tty->print(" (allocated = " SIZE_FORMAT " wasted = " SIZE_FORMAT " "

    34     log_debug(gc, plab)(" (allocated = " SIZE_FORMAT " wasted = " SIZE_FORMAT " "

    42   if (ResizePLAB) {

    46   assert(is_object_aligned(max_size()) && min_size() <= max_size(),

    47          "PLAB clipping computation may be incorrect");

    44     assert(is_object_aligned(max_size()) && min_size() <= max_size(),

    49   if (_allocated == 0) {

    45            "PLAB clipping computation may be incorrect");

    50     assert((_unused == 0),

    51            "Inconsistency in PLAB stats: "

    52            "_allocated: " SIZE_FORMAT ", "

    53            "_wasted: " SIZE_FORMAT ", "

    54            "_region_end_waste: " SIZE_FORMAT ", "

    55            "_unused: " SIZE_FORMAT ", "

    56            "_used  : " SIZE_FORMAT,

    57            _allocated, _wasted, _region_end_waste, _unused, used());

    58     _allocated = 1;

    59   }

    60   // The size of the PLAB caps the amount of space that can be wasted at the

    61   // end of the collection. In the worst case the last PLAB could be completely

    62   // empty.

    63   // This allows us to calculate the new PLAB size to achieve the

    64   // TargetPLABWastePct given the latest memory usage and that the last buffer

    65   // will be G1LastPLABAverageOccupancy full.

    66   //

    67   // E.g. assume that if in the current GC 100 words were allocated and a

    68   // TargetPLABWastePct of 10 had been set.

    69   //

    70   // So we could waste up to 10 words to meet that percentage. Given that we

    71   // also assume that that buffer is typically half-full, the new desired PLAB

    72   // size is set to 20 words.

    73   //

    74   // The amount of allocation performed should be independent of the number of

    75   // threads, so should the maximum waste we can spend in total. So if

    76   // we used n threads to allocate, each of them can spend maximum waste/n words in

    77   // a first rough approximation. The number of threads only comes into play later

    78   // when actually retrieving the actual desired PLAB size.

    79   //

    80   // After calculating this optimal PLAB size the algorithm applies the usual

    81   // exponential decaying average over this value to guess the next PLAB size.

    82   //

    83   // We account region end waste fully to PLAB allocation (in the calculation of

    84   // what we consider as "used_for_waste_calculation" below). This is not

    85   // completely fair, but is a conservative assumption because PLABs may be sized

    86   // flexibly while we cannot adjust inline allocations.

    87   // Allocation during GC will try to minimize region end waste so this impact

    88   // should be minimal.

    89   //

    90   // We need to cover overflow when calculating the amount of space actually used

    91   // by objects in PLABs when subtracting the region end waste.

    92   // Region end waste may be higher than actual allocation. This may occur if many

    93   // threads do not allocate anything but a few rather large objects. In this

    94   // degenerate case the PLAB size would simply quickly tend to minimum PLAB size,

    95   // which is an okay reaction.

    96   size_t const used_for_waste_calculation = used() > _region_end_waste ? used() - _region_end_waste : 0;

    47     if (_allocated == 0) {

    98   size_t const total_waste_allowed = used_for_waste_calculation * TargetPLABWastePct;

    48       assert((_unused == 0),

    99   size_t const cur_plab_sz = (size_t)((double)total_waste_allowed / G1LastPLABAverageOccupancy);

    49              "Inconsistency in PLAB stats: "

   100   // Take historical weighted average

    50              "_allocated: " SIZE_FORMAT ", "

   101   _filter.sample(cur_plab_sz);

    51              "_wasted: " SIZE_FORMAT ", "

   102   // Clip from above and below, and align to object boundary

    52              "_region_end_waste: " SIZE_FORMAT ", "

   103   size_t plab_sz;

    53              "_unused: " SIZE_FORMAT ", "

   104   plab_sz = MAX2(min_size(), (size_t)_filter.average());

    54              "_used  : " SIZE_FORMAT,

   105   plab_sz = MIN2(max_size(), plab_sz);

    55              _allocated, _wasted, _region_end_waste, _unused, used());

   106   plab_sz = align_object_size(plab_sz);

    56       _allocated = 1;

   107   // Latch the result

    57     }

   108   _desired_net_plab_sz = plab_sz;

    58     // The size of the PLAB caps the amount of space that can be wasted at the

    59     // end of the collection. In the worst case the last PLAB could be completely

    60     // empty.

    61     // This allows us to calculate the new PLAB size to achieve the

    62     // TargetPLABWastePct given the latest memory usage and that the last buffer

    63     // will be G1LastPLABAverageOccupancy full.

    64     //

    65     // E.g. assume that if in the current GC 100 words were allocated and a

    66     // TargetPLABWastePct of 10 had been set.

    67     //

    68     // So we could waste up to 10 words to meet that percentage. Given that we

    69     // also assume that that buffer is typically half-full, the new desired PLAB

    70     // size is set to 20 words.

    71     //

    72     // The amount of allocation performed should be independent of the number of

    73     // threads, so should the maximum waste we can spend in total. So if

    74     // we used n threads to allocate, each of them can spend maximum waste/n words in

    75     // a first rough approximation. The number of threads only comes into play later

    76     // when actually retrieving the actual desired PLAB size.

    77     //

    78     // After calculating this optimal PLAB size the algorithm applies the usual

    79     // exponential decaying average over this value to guess the next PLAB size.

    80     //

    81     // We account region end waste fully to PLAB allocation (in the calculation of

    82     // what we consider as "used_for_waste_calculation" below). This is not

    83     // completely fair, but is a conservative assumption because PLABs may be sized

    84     // flexibly while we cannot adjust inline allocations.

    85     // Allocation during GC will try to minimize region end waste so this impact

    86     // should be minimal.

    87     //

    88     // We need to cover overflow when calculating the amount of space actually used

    89     // by objects in PLABs when subtracting the region end waste.

    90     // Region end waste may be higher than actual allocation. This may occur if many

    91     // threads do not allocate anything but a few rather large objects. In this

    92     // degenerate case the PLAB size would simply quickly tend to minimum PLAB size,

    93     // which is an okay reaction.

    94     size_t const used_for_waste_calculation = used() > _region_end_waste ? used() - _region_end_waste : 0;

    96     size_t const total_waste_allowed = used_for_waste_calculation * TargetPLABWastePct;

   110   log_debug(gc, plab)(" (allocated = " SIZE_FORMAT " wasted = " SIZE_FORMAT " "

    97     size_t const cur_plab_sz = (size_t)((double)total_waste_allowed / G1LastPLABAverageOccupancy);

   111                       "unused = " SIZE_FORMAT " used = " SIZE_FORMAT " "

    98     // Take historical weighted average

   112                       "undo_waste = " SIZE_FORMAT " region_end_waste = " SIZE_FORMAT " "

    99     _filter.sample(cur_plab_sz);

   113                       "regions filled = %u direct_allocated = " SIZE_FORMAT " "

   100     // Clip from above and below, and align to object boundary

   114                       "failure_used = " SIZE_FORMAT " failure_waste = " SIZE_FORMAT ") "

   101     size_t plab_sz;

   115                       " (plab_sz = " SIZE_FORMAT " desired_plab_sz = " SIZE_FORMAT ")",

   102     plab_sz = MAX2(min_size(), (size_t)_filter.average());

   116                       _allocated, _wasted, _unused, used(), _undo_wasted, _region_end_waste,

   103     plab_sz = MIN2(max_size(), plab_sz);

   117                       _regions_filled, _direct_allocated, _failure_used, _failure_waste,

   104     plab_sz = align_object_size(plab_sz);

   118                       cur_plab_sz, plab_sz);

   105     // Latch the result

   119 

   106     _desired_net_plab_sz = plab_sz;

   107     if (PrintPLAB) {

   108       gclog_or_tty->print(" (plab_sz = " SIZE_FORMAT " desired_plab_sz = " SIZE_FORMAT ") ", cur_plab_sz, plab_sz);

   109     }

   110   }

   111   if (PrintPLAB) {

   112     gclog_or_tty->cr();

   113   }