1
2 /*
3 * Copyright (c) 2016, 2019, Red Hat, Inc. All rights reserved.
4 * Copyright Amazon.com Inc. or its affiliates. All Rights Reserved.
5 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
6 *
7 * This code is free software; you can redistribute it and/or modify it
8 * under the terms of the GNU General Public License version 2 only, as
9 * published by the Free Software Foundation.
10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
16 *
17 * You should have received a copy of the GNU General Public License version
18 * 2 along with this work; if not, write to the Free Software Foundation,
19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20 *
21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
22 * or visit www.oracle.com if you need additional information or have any
23 * questions.
24 *
25 */
26
27 #ifndef SHARE_GC_SHENANDOAH_SHENANDOAHFREESET_HPP
28 #define SHARE_GC_SHENANDOAH_SHENANDOAHFREESET_HPP
29
30 #include "gc/shenandoah/shenandoahHeapRegionSet.hpp"
31 #include "gc/shenandoah/shenandoahHeap.hpp"
32 #include "gc/shenandoah/shenandoahSimpleBitMap.hpp"
33
34 // Each ShenandoahHeapRegion is associated with a ShenandoahFreeSetPartitionId.
35 enum class ShenandoahFreeSetPartitionId : uint8_t {
36 Mutator, // Region is in the Mutator free set: available memory is available to mutators.
37 Collector, // Region is in the Collector free set: available memory is reserved for evacuations.
38 OldCollector, // Region is in the Old Collector free set:
39 // available memory is reserved for old evacuations and for promotions..
40 NotFree // Region is in no free set: it has no available memory
41 };
42
43 // We do not maintain counts, capacity, or used for regions that are not free. Informally, if a region is NotFree, it is
44 // in no partition. NumPartitions represents the size of an array that may be indexed by Mutator or Collector.
45 #define NumPartitions (ShenandoahFreeSetPartitionId::NotFree)
46 #define IntNumPartitions int(ShenandoahFreeSetPartitionId::NotFree)
47 #define UIntNumPartitions uint(ShenandoahFreeSetPartitionId::NotFree)
48
49 // ShenandoahRegionPartitions provides an abstraction to help organize the implementation of ShenandoahFreeSet. This
50 // class implements partitioning of regions into distinct sets. Each ShenandoahHeapRegion is either in the Mutator free set,
51 // the Collector free set, or in neither free set (NotFree). When we speak of a "free partition", we mean partitions that
52 // for which the ShenandoahFreeSetPartitionId is not equal to NotFree.
53 class ShenandoahRegionPartitions {
54
55 private:
56 const ssize_t _max; // The maximum number of heap regions
57 const size_t _region_size_bytes;
58 const ShenandoahFreeSet* _free_set;
59 // For each partition, we maintain a bitmap of which regions are affiliated with his partition.
60 ShenandoahSimpleBitMap _membership[UIntNumPartitions];
61
62 // For each partition, we track an interval outside of which a region affiliated with that partition is guaranteed
63 // not to be found. This makes searches for free space more efficient. For each partition p, _leftmosts[p]
64 // represents its least index, and its _rightmosts[p] its greatest index. Empty intervals are indicated by the
65 // canonical [_max, -1].
66 ssize_t _leftmosts[UIntNumPartitions];
67 ssize_t _rightmosts[UIntNumPartitions];
68
69 // Allocation for humongous objects needs to find regions that are entirely empty. For each partion p, _leftmosts_empty[p]
70 // represents the first region belonging to this partition that is completely empty and _rightmosts_empty[p] represents the
71 // last region that is completely empty. If there is no completely empty region in this partition, this is represented
72 // by the canonical [_max, -1].
73 ssize_t _leftmosts_empty[UIntNumPartitions];
74 ssize_t _rightmosts_empty[UIntNumPartitions];
75
76 // For each partition p, _capacity[p] represents the total amount of memory within the partition at the time
77 // of the most recent rebuild, _used[p] represents the total amount of memory that has been allocated within this
78 // partition (either already allocated as of the rebuild, or allocated since the rebuild). _capacity[p] and _used[p]
79 // are denoted in bytes. Note that some regions that had been assigned to a particular partition at rebuild time
80 // may have been retired following the rebuild. The tallies for these regions are still reflected in _capacity[p]
81 // and _used[p], even though the region may have been removed from the free set.
82 size_t _capacity[UIntNumPartitions];
83 size_t _used[UIntNumPartitions];
84 size_t _region_counts[UIntNumPartitions];
85
86 // For each partition p, _left_to_right_bias is true iff allocations are normally made from lower indexed regions
87 // before higher indexed regions.
88 bool _left_to_right_bias[UIntNumPartitions];
89
90 // Shrink the intervals associated with partition when region idx is removed from this free set
91 inline void shrink_interval_if_boundary_modified(ShenandoahFreeSetPartitionId partition, ssize_t idx);
92
93 // Shrink the intervals associated with partition when regions low_idx through high_idx inclusive are removed from this free set
94 inline void shrink_interval_if_range_modifies_either_boundary(ShenandoahFreeSetPartitionId partition,
95 ssize_t low_idx, ssize_t high_idx);
96 inline void expand_interval_if_boundary_modified(ShenandoahFreeSetPartitionId partition, ssize_t idx, size_t capacity);
97
98 inline bool is_mutator_partition(ShenandoahFreeSetPartitionId p);
99 inline bool is_young_collector_partition(ShenandoahFreeSetPartitionId p);
100 inline bool is_old_collector_partition(ShenandoahFreeSetPartitionId p);
101 inline bool available_implies_empty(size_t available);
102
103 #ifndef PRODUCT
104 void dump_bitmap_row(ssize_t region_idx) const;
105 void dump_bitmap_range(ssize_t start_region_idx, ssize_t end_region_idx) const;
106 void dump_bitmap() const;
107 #endif
108 public:
109 ShenandoahRegionPartitions(size_t max_regions, ShenandoahFreeSet* free_set);
110 ~ShenandoahRegionPartitions() {}
111
112 // Remove all regions from all partitions and reset all bounds
113 void make_all_regions_unavailable();
114
115 // Set the partition id for a particular region without adjusting interval bounds or usage/capacity tallies
116 inline void raw_assign_membership(size_t idx, ShenandoahFreeSetPartitionId p) {
117 _membership[int(p)].set_bit(idx);
118 }
119
120 // Set the Mutator intervals, usage, and capacity according to arguments. Reset the Collector intervals, used, capacity
121 // to represent empty Collector free set. We use this at the end of rebuild_free_set() to avoid the overhead of making
122 // many redundant incremental adjustments to the mutator intervals as the free set is being rebuilt.
123 void establish_mutator_intervals(ssize_t mutator_leftmost, ssize_t mutator_rightmost,
124 ssize_t mutator_leftmost_empty, ssize_t mutator_rightmost_empty,
125 size_t mutator_region_count, size_t mutator_used);
126
127 // Set the OldCollector intervals, usage, and capacity according to arguments. We use this at the end of rebuild_free_set()
128 // to avoid the overhead of making many redundant incremental adjustments to the mutator intervals as the free set is being
129 // rebuilt.
130 void establish_old_collector_intervals(ssize_t old_collector_leftmost, ssize_t old_collector_rightmost,
131 ssize_t old_collector_leftmost_empty, ssize_t old_collector_rightmost_empty,
132 size_t old_collector_region_count, size_t old_collector_used);
133
134 // Retire region idx from within partition, , leaving its capacity and used as part of the original free partition's totals.
135 // Requires that region idx is in in the Mutator or Collector partitions. Hereafter, identifies this region as NotFree.
136 // Any remnant of available memory at the time of retirement is added to the original partition's total of used bytes.
137 void retire_from_partition(ShenandoahFreeSetPartitionId p, ssize_t idx, size_t used_bytes);
138
139 // Retire all regions between low_idx and high_idx inclusive from within partition. Requires that each region idx is
140 // in the same Mutator or Collector partition. Hereafter, identifies each region as NotFree. Assumes that each region
141 // is now considered fully used, since the region is presumably used to represent a humongous object.
142 void retire_range_from_partition(ShenandoahFreeSetPartitionId partition, ssize_t low_idx, ssize_t high_idx);
143
144 // Place region idx into free set which_partition. Requires that idx is currently NotFree.
145 void make_free(ssize_t idx, ShenandoahFreeSetPartitionId which_partition, size_t region_capacity);
146
147 // Place region idx into free partition new_partition, adjusting used and capacity totals for the original and new partition
148 // given that available bytes can still be allocated within this region. Requires that idx is currently not NotFree.
149 void move_from_partition_to_partition(ssize_t idx, ShenandoahFreeSetPartitionId orig_partition,
150 ShenandoahFreeSetPartitionId new_partition, size_t available);
151
152 const char* partition_membership_name(ssize_t idx) const;
153
154 // Return the index of the next available region >= start_index, or maximum_regions if not found.
155 inline ssize_t find_index_of_next_available_region(ShenandoahFreeSetPartitionId which_partition, ssize_t start_index) const;
156
157 // Return the index of the previous available region <= last_index, or -1 if not found.
158 inline ssize_t find_index_of_previous_available_region(ShenandoahFreeSetPartitionId which_partition, ssize_t last_index) const;
159
160 // Return the index of the next available cluster of cluster_size regions >= start_index, or maximum_regions if not found.
161 inline ssize_t find_index_of_next_available_cluster_of_regions(ShenandoahFreeSetPartitionId which_partition,
162 ssize_t start_index, size_t cluster_size) const;
163
164 // Return the index of the previous available cluster of cluster_size regions <= last_index, or -1 if not found.
165 inline ssize_t find_index_of_previous_available_cluster_of_regions(ShenandoahFreeSetPartitionId which_partition,
166 ssize_t last_index, size_t cluster_size) const;
167
168 inline bool in_free_set(ShenandoahFreeSetPartitionId which_partition, ssize_t idx) const {
169 return _membership[int(which_partition)].is_set(idx);
170 }
171
172 // Returns the ShenandoahFreeSetPartitionId affiliation of region idx, NotFree if this region is not currently in any partition.
173 // This does not enforce that free_set membership implies allocation capacity.
174 inline ShenandoahFreeSetPartitionId membership(ssize_t idx) const;
175
176 #ifdef ASSERT
177 // Returns true iff region idx's membership is which_partition. If which_partition represents a free set, asserts
178 // that the region has allocation capacity.
179 inline bool partition_id_matches(ssize_t idx, ShenandoahFreeSetPartitionId which_partition) const;
180 #endif
181
182 inline size_t max_regions() const { return _max; }
183
184 inline size_t region_size_bytes() const { return _region_size_bytes; };
185
186 // The following four methods return the left-most and right-most bounds on ranges of regions representing
187 // the requested set. The _empty variants represent bounds on the range that holds completely empty
188 // regions, which are required for humongous allocations and desired for "very large" allocations.
189 // if the requested which_partition is empty:
190 // leftmost() and leftmost_empty() return _max, rightmost() and rightmost_empty() return 0
191 // otherwise, expect the following:
192 // 0 <= leftmost <= leftmost_empty <= rightmost_empty <= rightmost < _max
193 inline ssize_t leftmost(ShenandoahFreeSetPartitionId which_partition) const;
194 inline ssize_t rightmost(ShenandoahFreeSetPartitionId which_partition) const;
195 ssize_t leftmost_empty(ShenandoahFreeSetPartitionId which_partition);
196 ssize_t rightmost_empty(ShenandoahFreeSetPartitionId which_partition);
197
198 inline bool is_empty(ShenandoahFreeSetPartitionId which_partition) const;
199
200 inline void increase_used(ShenandoahFreeSetPartitionId which_partition, size_t bytes);
201
202 inline void set_bias_from_left_to_right(ShenandoahFreeSetPartitionId which_partition, bool value) {
203 assert (which_partition < NumPartitions, "selected free set must be valid");
204 _left_to_right_bias[int(which_partition)] = value;
205 }
206
207 inline bool alloc_from_left_bias(ShenandoahFreeSetPartitionId which_partition) const {
208 assert (which_partition < NumPartitions, "selected free set must be valid");
209 return _left_to_right_bias[int(which_partition)];
210 }
211
212 inline size_t capacity_of(ShenandoahFreeSetPartitionId which_partition) const {
213 assert (which_partition < NumPartitions, "selected free set must be valid");
214 return _capacity[int(which_partition)];
215 }
216
217 inline size_t used_by(ShenandoahFreeSetPartitionId which_partition) const {
218 assert (which_partition < NumPartitions, "selected free set must be valid");
219 return _used[int(which_partition)];
220 }
221
222 inline size_t available_in(ShenandoahFreeSetPartitionId which_partition) const {
223 assert (which_partition < NumPartitions, "selected free set must be valid");
224 return _capacity[int(which_partition)] - _used[int(which_partition)];
225 }
226
227 inline void set_capacity_of(ShenandoahFreeSetPartitionId which_partition, size_t value) {
228 assert (which_partition < NumPartitions, "selected free set must be valid");
229 _capacity[int(which_partition)] = value;
230 }
231
232 inline void set_used_by(ShenandoahFreeSetPartitionId which_partition, size_t value) {
233 assert (which_partition < NumPartitions, "selected free set must be valid");
234 _used[int(which_partition)] = value;
235 }
236
237 inline size_t count(ShenandoahFreeSetPartitionId which_partition) const { return _region_counts[int(which_partition)]; }
238
239 // Assure leftmost, rightmost, leftmost_empty, and rightmost_empty bounds are valid for all free sets.
240 // Valid bounds honor all of the following (where max is the number of heap regions):
241 // if the set is empty, leftmost equals max and rightmost equals 0
242 // Otherwise (the set is not empty):
243 // 0 <= leftmost < max and 0 <= rightmost < max
244 // the region at leftmost is in the set
245 // the region at rightmost is in the set
246 // rightmost >= leftmost
247 // for every idx that is in the set {
248 // idx >= leftmost &&
249 // idx <= rightmost
250 // }
251 // if the set has no empty regions, leftmost_empty equals max and rightmost_empty equals 0
252 // Otherwise (the region has empty regions):
253 // 0 <= leftmost_empty < max and 0 <= rightmost_empty < max
254 // rightmost_empty >= leftmost_empty
255 // for every idx that is in the set and is empty {
256 // idx >= leftmost &&
257 // idx <= rightmost
258 // }
259 void assert_bounds() NOT_DEBUG_RETURN;
260 };
261
262 // Publicly, ShenandoahFreeSet represents memory that is available to mutator threads. The public capacity(), used(),
263 // and available() methods represent this public notion of memory that is under control of the mutator. Separately,
264 // ShenandoahFreeSet also represents memory available to garbage collection activities for compaction purposes.
265 //
266 // The Shenandoah garbage collector evacuates live objects out of specific regions that are identified as members of the
267 // collection set (cset).
268 //
269 // The ShenandoahFreeSet tries to colocate survivor objects (objects that have been evacuated at least once) at the
270 // high end of memory. New mutator allocations are taken from the low end of memory. Within the mutator's range of regions,
271 // humongous allocations are taken from the lowest addresses, and LAB (local allocation buffers) and regular shared allocations
272 // are taken from the higher address of the mutator's range of regions. This approach allows longer lasting survivor regions
273 // to congregate at the top of the heap and longer lasting humongous regions to congregate at the bottom of the heap, with
274 // short-lived frequently evacuated regions occupying the middle of the heap.
275 //
276 // Mutator and garbage collection activities tend to scramble the content of regions. Twice, during each GC pass, we rebuild
277 // the free set in an effort to restore the efficient segregation of Collector and Mutator regions:
278 //
279 // 1. At the start of evacuation, we know exactly how much memory is going to be evacuated, and this guides our
280 // sizing of the Collector free set.
281 //
282 // 2. At the end of GC, we have reclaimed all of the memory that was spanned by the cset. We rebuild here to make
283 // sure there is enough memory reserved at the high end of memory to hold the objects that might need to be evacuated
284 // during the next GC pass.
285
286 class ShenandoahFreeSet : public CHeapObj<mtGC> {
287 private:
288 ShenandoahHeap* const _heap;
289 ShenandoahRegionPartitions _partitions;
290 size_t _retired_old_regions;
291
292 HeapWord* allocate_aligned_plab(size_t size, ShenandoahAllocRequest& req, ShenandoahHeapRegion* r);
293
294 // Return the address of memory allocated, setting in_new_region to true iff the allocation is taken
295 // from a region that was previously empty. Return nullptr if memory could not be allocated.
296 inline HeapWord* allocate_from_partition_with_affiliation(ShenandoahFreeSetPartitionId which_partition,
297 ShenandoahAffiliation affiliation,
298 ShenandoahAllocRequest& req, bool& in_new_region);
299
300 // We re-evaluate the left-to-right allocation bias whenever _alloc_bias_weight is less than zero. Each time
301 // we allocate an object, we decrement the count of this value. Each time we re-evaluate whether to allocate
302 // from right-to-left or left-to-right, we reset the value of this counter to _InitialAllocBiasWeight.
303 ssize_t _alloc_bias_weight;
304
305 const ssize_t _InitialAllocBiasWeight = 256;
306
307 HeapWord* try_allocate_in(ShenandoahHeapRegion* region, ShenandoahAllocRequest& req, bool& in_new_region);
308
309 // While holding the heap lock, allocate memory for a single object or LAB which is to be entirely contained
310 // within a single HeapRegion as characterized by req.
311 //
312 // Precondition: req.size() <= ShenandoahHeapRegion::humongous_threshold_words().
313 HeapWord* allocate_single(ShenandoahAllocRequest& req, bool& in_new_region);
314
315 // While holding the heap lock, allocate memory for a humongous object which spans one or more regions that
316 // were previously empty. Regions that represent humongous objects are entirely dedicated to the humongous
317 // object. No other objects are packed into these regions.
318 //
319 // Precondition: req.size() > ShenandoahHeapRegion::humongous_threshold_words().
320 HeapWord* allocate_contiguous(ShenandoahAllocRequest& req);
321
322 // Change region r from the Mutator partition to the GC's Collector or OldCollector partition. This requires that the
323 // region is entirely empty.
324 //
325 // Typical usage: During evacuation, the GC may find it needs more memory than had been reserved at the start of evacuation to
326 // hold evacuated objects. If this occurs and memory is still available in the Mutator's free set, we will flip a region from
327 // the Mutator free set into the Collector or OldCollector free set.
328 void flip_to_gc(ShenandoahHeapRegion* r);
329 void flip_to_old_gc(ShenandoahHeapRegion* r);
330
331 void clear_internal();
332 void try_recycle_trashed(ShenandoahHeapRegion *r);
333
334 // Returns true iff this region is entirely available, either because it is empty() or because it has been found to represent
335 // immediate trash and we'll be able to immediately recycle it. Note that we cannot recycle immediate trash if
336 // concurrent weak root processing is in progress.
337 inline bool can_allocate_from(ShenandoahHeapRegion *r) const;
338 inline bool can_allocate_from(size_t idx) const;
339
340 inline bool has_alloc_capacity(ShenandoahHeapRegion *r) const;
341
342 size_t transfer_empty_regions_from_collector_set_to_mutator_set(ShenandoahFreeSetPartitionId which_collector,
343 size_t max_xfer_regions,
344 size_t& bytes_transferred);
345 size_t transfer_non_empty_regions_from_collector_set_to_mutator_set(ShenandoahFreeSetPartitionId collector_id,
346 size_t max_xfer_regions,
347 size_t& bytes_transferred);
348
349
350 // Determine whether we prefer to allocate from left to right or from right to left within the OldCollector free-set.
351 void establish_old_collector_alloc_bias();
352
353 // Set max_capacity for young and old generations
354 void establish_generation_sizes(size_t young_region_count, size_t old_region_count);
355 size_t get_usable_free_words(size_t free_bytes) const;
356
357 // log status, assuming lock has already been acquired by the caller.
358 void log_status();
359
360 public:
361 ShenandoahFreeSet(ShenandoahHeap* heap, size_t max_regions);
362
363 // Public because ShenandoahRegionPartitions assertions require access.
364 inline size_t alloc_capacity(ShenandoahHeapRegion *r) const;
365 inline size_t alloc_capacity(size_t idx) const;
366
367 void clear();
368
369 // Examine the existing free set representation, capturing the current state into var arguments:
370 //
371 // young_cset_regions is the number of regions currently in the young cset if we are starting to evacuate, or zero
372 // old_cset_regions is the number of regions currently in the old cset if we are starting a mixed evacuation, or zero
373 // first_old_region is the index of the first region that is part of the OldCollector set
374 // last_old_region is the index of the last region that is part of the OldCollector set
375 // old_region_count is the number of regions in the OldCollector set that have memory available to be allocated
376 void prepare_to_rebuild(size_t &young_cset_regions, size_t &old_cset_regions,
377 size_t &first_old_region, size_t &last_old_region, size_t &old_region_count);
378
379 // At the end of final mark, but before we begin evacuating, heuristics calculate how much memory is required to
380 // hold the results of evacuating to young-gen and to old-gen, and have_evacuation_reserves should be true.
381 // These quantities, stored as reserves for their respective generations, are consulted prior to rebuilding
382 // the free set (ShenandoahFreeSet) in preparation for evacuation. When the free set is rebuilt, we make sure
383 // to reserve sufficient memory in the collector and old_collector sets to hold evacuations.
384 //
385 // We also rebuild the free set at the end of GC, as we prepare to idle GC until the next trigger. In this case,
386 // have_evacuation_reserves is false because we don't yet know how much memory will need to be evacuated in the
387 // next GC cycle. When have_evacuation_reserves is false, the free set rebuild operation reserves for the collector
388 // and old_collector sets based on alternative mechanisms, such as ShenandoahEvacReserve, ShenandoahOldEvacReserve, and
389 // ShenandoahOldCompactionReserve. In a future planned enhancement, the reserve for old_collector set when the
390 // evacuation reserves are unknown, is based in part on anticipated promotion as determined by analysis of live data
391 // found during the previous GC pass which is one less than the current tenure age.
392 //
393 // young_cset_regions is the number of regions currently in the young cset if we are starting to evacuate, or zero
394 // old_cset_regions is the number of regions currently in the old cset if we are starting a mixed evacuation, or zero
395 // num_old_regions is the number of old-gen regions that have available memory for further allocations (excluding old cset)
396 // have_evacuation_reserves is true iff the desired values of young-gen and old-gen evacuation reserves and old-gen
397 // promotion reserve have been precomputed (and can be obtained by invoking
398 // <generation>->get_evacuation_reserve() or old_gen->get_promoted_reserve()
399 void finish_rebuild(size_t young_cset_regions, size_t old_cset_regions, size_t num_old_regions,
400 bool have_evacuation_reserves = false);
401
402 // When a region is promoted in place, we add the region's available memory if it is greater than plab_min_size()
403 // into the old collector partition by invoking this method.
404 void add_promoted_in_place_region_to_old_collector(ShenandoahHeapRegion* region);
405
406 // Move up to cset_regions number of regions from being available to the collector to being available to the mutator.
407 //
408 // Typical usage: At the end of evacuation, when the collector no longer needs the regions that had been reserved
409 // for evacuation, invoke this to make regions available for mutator allocations.
410 void move_regions_from_collector_to_mutator(size_t cset_regions);
411
412 void recycle_trash();
413
414 // Acquire heap lock and log status, assuming heap lock is not acquired by the caller.
415 void log_status_under_lock();
416
417 inline size_t capacity() const { return _partitions.capacity_of(ShenandoahFreeSetPartitionId::Mutator); }
418 inline size_t used() const { return _partitions.used_by(ShenandoahFreeSetPartitionId::Mutator); }
419 inline size_t available() const {
420 assert(used() <= capacity(), "must use less than capacity");
421 return capacity() - used();
422 }
423
424 HeapWord* allocate(ShenandoahAllocRequest& req, bool& in_new_region);
425 size_t unsafe_peek_free() const;
426
427 /*
428 * Internal fragmentation metric: describes how fragmented the heap regions are.
429 *
430 * It is derived as:
431 *
432 * sum(used[i]^2, i=0..k)
433 * IF = 1 - ------------------------------
434 * C * sum(used[i], i=0..k)
435 *
436 * ...where k is the number of regions in computation, C is the region capacity, and
437 * used[i] is the used space in the region.
438 *
439 * The non-linearity causes IF to be lower for the cases where the same total heap
440 * used is densely packed. For example:
441 * a) Heap is completely full => IF = 0
442 * b) Heap is half full, first 50% regions are completely full => IF = 0
443 * c) Heap is half full, each region is 50% full => IF = 1/2
444 * d) Heap is quarter full, first 50% regions are completely full => IF = 0
445 * e) Heap is quarter full, each region is 25% full => IF = 3/4
446 * f) Heap has one small object per each region => IF =~ 1
447 */
448 double internal_fragmentation();
449
450 /*
451 * External fragmentation metric: describes how fragmented the heap is.
452 *
453 * It is derived as:
454 *
455 * EF = 1 - largest_contiguous_free / total_free
456 *
457 * For example:
458 * a) Heap is completely empty => EF = 0
459 * b) Heap is completely full => EF = 0
460 * c) Heap is first-half full => EF = 1/2
461 * d) Heap is half full, full and empty regions interleave => EF =~ 1
462 */
463 double external_fragmentation();
464
465 void print_on(outputStream* out) const;
466
467 // This function places all regions that have allocation capacity into the mutator partition, or if the region
468 // is already affiliated with old, into the old collector partition, identifying regions that have no allocation
469 // capacity as NotFree. Capture the modified state of the freeset into var arguments:
470 //
471 // young_cset_regions is the number of regions currently in the young cset if we are starting to evacuate, or zero
472 // old_cset_regions is the number of regions currently in the old cset if we are starting a mixed evacuation, or zero
473 // first_old_region is the index of the first region that is part of the OldCollector set
474 // last_old_region is the index of the last region that is part of the OldCollector set
475 // old_region_count is the number of regions in the OldCollector set that have memory available to be allocated
476 void find_regions_with_alloc_capacity(size_t &young_cset_regions, size_t &old_cset_regions,
477 size_t &first_old_region, size_t &last_old_region, size_t &old_region_count);
478
479 // Ensure that Collector has at least to_reserve bytes of available memory, and OldCollector has at least old_reserve
480 // bytes of available memory. On input, old_region_count holds the number of regions already present in the
481 // OldCollector partition. Upon return, old_region_count holds the updated number of regions in the OldCollector partition.
482 void reserve_regions(size_t to_reserve, size_t old_reserve, size_t &old_region_count);
483
484 // Reserve space for evacuations, with regions reserved for old evacuations placed to the right
485 // of regions reserved of young evacuations.
486 void compute_young_and_old_reserves(size_t young_cset_regions, size_t old_cset_regions, bool have_evacuation_reserves,
487 size_t &young_reserve_result, size_t &old_reserve_result) const;
488 };
489
490 #endif // SHARE_GC_SHENANDOAH_SHENANDOAHFREESET_HPP