Tue, 21 Aug 2012 14:10:39 -0700
7185699: G1: Prediction model discrepancies
Summary: Correct the result value of G1CollectedHeap::pending_card_num(). Change the code that calculates the GC efficiency of a non-young heap region to use historical data from mixed GCs and the actual number of live bytes when predicting how long it would take to collect the region. Changes were also reviewed by Thomas Schatzl.
Reviewed-by: azeemj, brutisso
1 /*
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23 */
25 #include "precompiled.hpp"
26 #include "gc_implementation/g1/g1MMUTracker.hpp"
27 #include "runtime/mutexLocker.hpp"
28 #include "utilities/ostream.hpp"
30 #define _DISABLE_MMU 0
32 // can't rely on comparing doubles with tolerating a small margin for error
33 #define SMALL_MARGIN 0.0000001
34 #define is_double_leq_0(_value) ( (_value) < SMALL_MARGIN )
35 #define is_double_leq(_val1, _val2) is_double_leq_0((_val1) - (_val2))
36 #define is_double_geq(_val1, _val2) is_double_leq_0((_val2) - (_val1))
38 /***** ALL TIMES ARE IN SECS!!!!!!! *****/
40 G1MMUTracker::G1MMUTracker(double time_slice, double max_gc_time) :
41 _time_slice(time_slice),
42 _max_gc_time(max_gc_time) { }
44 G1MMUTrackerQueue::G1MMUTrackerQueue(double time_slice, double max_gc_time) :
45 G1MMUTracker(time_slice, max_gc_time),
46 _head_index(0),
47 _tail_index(trim_index(_head_index+1)),
48 _no_entries(0) { }
50 void G1MMUTrackerQueue::remove_expired_entries(double current_time) {
51 double limit = current_time - _time_slice;
52 while (_no_entries > 0) {
53 if (is_double_geq(limit, _array[_tail_index].end_time())) {
54 _tail_index = trim_index(_tail_index + 1);
55 --_no_entries;
56 } else
57 return;
58 }
59 guarantee(_no_entries == 0, "should have no entries in the array");
60 }
62 double G1MMUTrackerQueue::calculate_gc_time(double current_time) {
63 double gc_time = 0.0;
64 double limit = current_time - _time_slice;
65 for (int i = 0; i < _no_entries; ++i) {
66 int index = trim_index(_tail_index + i);
67 G1MMUTrackerQueueElem *elem = &_array[index];
68 if (elem->end_time() > limit) {
69 if (elem->start_time() > limit)
70 gc_time += elem->duration();
71 else
72 gc_time += elem->end_time() - limit;
73 }
74 }
75 return gc_time;
76 }
78 void G1MMUTrackerQueue::add_pause(double start, double end, bool gc_thread) {
79 double longest_allowed = longest_pause_internal(start);
80 if (longest_allowed < 0.0)
81 longest_allowed = 0.0;
82 double duration = end - start;
84 remove_expired_entries(end);
85 if (_no_entries == QueueLength) {
86 // OK, we've filled up the queue. There are a few ways
87 // of dealing with this "gracefully"
88 // increase the array size (:-)
89 // remove the oldest entry (this might allow more GC time for
90 // the time slice than what's allowed) - this is what we
91 // currently do
92 // consolidate the two entries with the minimum gap between them
93 // (this might allow less GC time than what's allowed)
95 // In the case where ScavengeALot is true, such overflow is not
96 // uncommon; in such cases, we can, without much loss of precision
97 // or performance (we are GC'ing most of the time anyway!),
98 // simply overwrite the oldest entry in the tracker.
100 _head_index = trim_index(_head_index + 1);
101 assert(_head_index == _tail_index, "Because we have a full circular buffer");
102 _tail_index = trim_index(_tail_index + 1);
103 } else {
104 _head_index = trim_index(_head_index + 1);
105 ++_no_entries;
106 }
107 _array[_head_index] = G1MMUTrackerQueueElem(start, end);
108 }
110 // basically the _internal call does not remove expired entries
111 // this is for trying things out in the future and a couple
112 // of other places (debugging)
114 double G1MMUTrackerQueue::longest_pause(double current_time) {
115 if (_DISABLE_MMU)
116 return _max_gc_time;
118 MutexLockerEx x(MMUTracker_lock, Mutex::_no_safepoint_check_flag);
119 remove_expired_entries(current_time);
121 return longest_pause_internal(current_time);
122 }
124 double G1MMUTrackerQueue::longest_pause_internal(double current_time) {
125 double target_time = _max_gc_time;
127 while( 1 ) {
128 double gc_time =
129 calculate_gc_time(current_time + target_time);
130 double diff = target_time + gc_time - _max_gc_time;
131 if (!is_double_leq_0(diff)) {
132 target_time -= diff;
133 if (is_double_leq_0(target_time)) {
134 target_time = -1.0;
135 break;
136 }
137 } else {
138 break;
139 }
140 }
142 return target_time;
143 }
145 // basically the _internal call does not remove expired entries
146 // this is for trying things out in the future and a couple
147 // of other places (debugging)
149 double G1MMUTrackerQueue::when_sec(double current_time, double pause_time) {
150 if (_DISABLE_MMU)
151 return 0.0;
153 MutexLockerEx x(MMUTracker_lock, Mutex::_no_safepoint_check_flag);
154 remove_expired_entries(current_time);
156 return when_internal(current_time, pause_time);
157 }
159 double G1MMUTrackerQueue::when_internal(double current_time,
160 double pause_time) {
161 // if the pause is over the maximum, just assume that it's the maximum
162 double adjusted_pause_time =
163 (pause_time > max_gc_time()) ? max_gc_time() : pause_time;
164 double earliest_end = current_time + adjusted_pause_time;
165 double limit = earliest_end - _time_slice;
166 double gc_time = calculate_gc_time(earliest_end);
167 double diff = gc_time + adjusted_pause_time - max_gc_time();
168 if (is_double_leq_0(diff))
169 return 0.0;
171 int index = _tail_index;
172 while ( 1 ) {
173 G1MMUTrackerQueueElem *elem = &_array[index];
174 if (elem->end_time() > limit) {
175 if (elem->start_time() > limit)
176 diff -= elem->duration();
177 else
178 diff -= elem->end_time() - limit;
179 if (is_double_leq_0(diff))
180 return elem->end_time() + diff + _time_slice - adjusted_pause_time - current_time;
181 }
182 index = trim_index(index+1);
183 guarantee(index != trim_index(_head_index + 1), "should not go past head");
184 }
185 }