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8067341: Modify PLAB sizing algorithm to waste less

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Change the G1 PLAB resizing algorithm to waste less memory by being more conservative about the PLAB sizes.

Reviewed-by: ecaspole, jmasa
This commit is contained in:
Thomas Schatzl 2015-09-01 10:53:29 +02:00
parent cf5132e887
commit 8f2026345c
2 changed files with 45 additions and 11 deletions
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51
hotspot/src/share/vm/gc/g1/g1EvacStats.cpp
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@ -54,17 +54,46 @@ void G1EvacStats::adjust_desired_plab_sz() {
_allocated, _wasted, _region_end_waste, _unused, used()));
_allocated = 1;
}
// We account region end waste fully to PLAB allocation. This is not completely fair,
// but is a conservative assumption because PLABs may be sized flexibly while we
// cannot adjust direct allocations.
// In some cases, wasted_frac may become > 1 but that just reflects the problem
// with region_end_waste.
double wasted_frac = (double)(_unused + _wasted + _region_end_waste) / (double)_allocated;
size_t target_refills = (size_t)((wasted_frac * TargetSurvivorRatio) / TargetPLABWastePct);
if (target_refills == 0) {
target_refills = 1;
}
size_t cur_plab_sz = used() / target_refills;
// 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 = (double)total_waste_allowed / G1LastPLABAverageOccupancy;
// Take historical weighted average
_filter.sample(cur_plab_sz);
// Clip from above and below, and align to object boundary

5
hotspot/src/share/vm/gc/g1/g1_globals.hpp
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@ -84,6 +84,11 @@
"If true, enable reference discovery during concurrent " \
"marking and reference processing at the end of remark.") \
\
experimental(double, G1LastPLABAverageOccupancy, 50.0, \
"The expected average occupancy of the last PLAB in " \
"percent.") \
range(0.001, 100.0) \
\
product(size_t, G1SATBBufferSize, 1*K, \
"Number of entries in an SATB log buffer.") \
range(1, max_uintx) \