// Compute desired plab size for one gc worker thread and latch result for later
// use. This should be called once at the end of parallel
// scavenge; it clears the sensor accumulators.
void PLABStats::adjust_desired_plab_sz() {
log_plab_allocation();
if (!ResizePLAB) {
// Clear accumulators for next round.
reset();
return;
}
assert(is_object_aligned(max_size()) && min_size() <= max_size(),
"PLAB clipping computation may be incorrect");
if (_allocated == 0) {
assert(_unused == 0,
"Inconsistency in PLAB stats: "
"_allocated: " SIZE_FORMAT ", "
"_wasted: " SIZE_FORMAT ", "
"_unused: " SIZE_FORMAT ", "
"_undo_wasted: " SIZE_FORMAT,
_allocated, _wasted, _unused, _undo_wasted);
_allocated = 1;
}
double wasted_frac = (double)_unused / (double)_allocated;
size_t target_refills = (size_t)((wasted_frac * TargetSurvivorRatio) / TargetPLABWastePct);
if (target_refills == 0) {
target_refills = 1;
}
size_t used = _allocated - _wasted - _unused;
// Assumed to have 1 gc worker thread
size_t recent_plab_sz = used / target_refills;
// Take historical weighted average
_filter.sample(recent_plab_sz);
// Clip from above and below, and align to object boundary
size_t new_plab_sz = MAX2(min_size(), (size_t)_filter.average());
new_plab_sz = MIN2(max_size(), new_plab_sz);
new_plab_sz = align_object_size(new_plab_sz);
// Latch the result
_desired_net_plab_sz = new_plab_sz;
log_sizing(recent_plab_sz, new_plab_sz);
reset();
}
void G1EvacStats::adjust_desired_plab_sz() {
log_plab_allocation();
if (!ResizePLAB) {
// Clear accumulators for next round.
reset();
return;
}
assert(is_object_aligned(max_size()) && min_size() <= max_size(),
"PLAB clipping computation may be incorrect");
if (_allocated == 0) {
assert((_unused == 0),
"Inconsistency in PLAB stats: "
"_allocated: " SIZE_FORMAT ", "
"_wasted: " SIZE_FORMAT ", "
"_region_end_waste: " SIZE_FORMAT ", "
"_unused: " SIZE_FORMAT ", "
"_used : " SIZE_FORMAT,
_allocated, _wasted, _region_end_waste, _unused, used());
_allocated = 1;
}
// The size of the PLAB caps the amount of space that can be wasted at the
// end of the collection. In the worst case the last PLAB could be completely
// empty.
// This allows us to calculate the new PLAB size to achieve the
// TargetPLABWastePct given the latest memory usage and that the last buffer
// will be G1LastPLABAverageOccupancy full.
//
// E.g. assume that if in the current GC 100 words were allocated and a
// TargetPLABWastePct of 10 had been set.
//
// So we could waste up to 10 words to meet that percentage. Given that we
// also assume that that buffer is typically half-full, the new desired PLAB
// size is set to 20 words.
//
// The amount of allocation performed should be independent of the number of
// threads, so should the maximum waste we can spend in total. So if
// we used n threads to allocate, each of them can spend maximum waste/n words in
// a first rough approximation. The number of threads only comes into play later
// when actually retrieving the actual desired PLAB size.
//
// After calculating this optimal PLAB size the algorithm applies the usual
// exponential decaying average over this value to guess the next PLAB size.
//
// We account region end waste fully to PLAB allocation (in the calculation of
// what we consider as "used_for_waste_calculation" below). This is not
// completely fair, but is a conservative assumption because PLABs may be sized
// flexibly while we cannot adjust inline allocations.
// Allocation during GC will try to minimize region end waste so this impact
// should be minimal.
//
// We need to cover overflow when calculating the amount of space actually used
// by objects in PLABs when subtracting the region end waste.
// Region end waste may be higher than actual allocation. This may occur if many
// threads do not allocate anything but a few rather large objects. In this
// degenerate case the PLAB size would simply quickly tend to minimum PLAB size,
// which is an okay reaction.
size_t const used_for_waste_calculation = used() > _region_end_waste ? used() - _region_end_waste : 0;
size_t const total_waste_allowed = used_for_waste_calculation * TargetPLABWastePct;
size_t const cur_plab_sz = (size_t)((double)total_waste_allowed / G1LastPLABAverageOccupancy);
// Take historical weighted average
_filter.sample(cur_plab_sz);
_desired_net_plab_sz = MAX2(min_size(), (size_t)_filter.average());
log_sizing(cur_plab_sz, _desired_net_plab_sz);
// Clear accumulators for next round.
reset();
}
