1 /*

     2  * Copyright (c) 2018, Red Hat, Inc. All rights reserved.

     3  *

     4  * This code is free software; you can redistribute it and/or modify it

     5  * under the terms of the GNU General Public License version 2 only, as

     6  * published by the Free Software Foundation.

     7  *

     8  * This code is distributed in the hope that it will be useful, but WITHOUT

     9  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

    10  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

    11  * version 2 for more details (a copy is included in the LICENSE file that

    12  * accompanied this code).

    13  *

    14  * You should have received a copy of the GNU General Public License version

    15  * 2 along with this work; if not, write to the Free Software Foundation,

    16  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

    17  *

    18  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

    19  * or visit www.oracle.com if you need additional information or have any

    20  * questions.

    21  *

    22  */

    23 

    24 #include "precompiled.hpp"

    25 

    26 #include "gc/shenandoah/shenandoahFreeSet.hpp"

    27 #include "gc/shenandoah/shenandoahHeap.hpp"

    28 #include "gc/shenandoah/shenandoahHeap.inline.hpp"

    29 #include "gc/shenandoah/shenandoahPacer.hpp"

    30 

    31 /*

    32  * In normal concurrent cycle, we have to pace the application to let GC finish.

    33  *

    34  * Here, we do not know how large would be the collection set, and what are the

    35  * relative performances of the each stage in the concurrent cycle, and so we have to

    36  * make some assumptions.

    37  *

    38  * For concurrent mark, there is no clear notion of progress. The moderately accurate

    39  * and easy to get metric is the amount of live objects the mark had encountered. But,

    40  * that does directly correlate with the used heap, because the heap might be fully

    41  * dead or fully alive. We cannot assume either of the extremes: we would either allow

    42  * application to run out of memory if we assume heap is fully dead but it is not, and,

    43  * conversely, we would pacify application excessively if we assume heap is fully alive

    44  * but it is not. So we need to guesstimate the particular expected value for heap liveness.

    45  * The best way to do this is apparently recording the past history.

    46  *

    47  * For concurrent evac and update-refs, we are walking the heap per-region, and so the

    48  * notion of progress is clear: we get reported the "used" size from the processed regions

    49  * and use the global heap-used as the baseline.

    50  *

    51  * The allocatable space when GC is running is "free" at the start of cycle, but the

    52  * accounted budget is based on "used". So, we need to adjust the tax knowing that.

    53  * Also, since we effectively count the used space three times (mark, evac, update-refs),

    54  * we need to multiply the tax by 3. Example: for 10 MB free and 90 MB used, GC would

    55  * come back with 3*90 MB budget, and thus for each 1 MB of allocation, we have to pay

    56  * 3*90 / 10 MBs. In the end, we would pay back the entire budget.

    57  */

    58 

    59 void ShenandoahPacer::setup_for_mark() {

    60   assert(ShenandoahPacing, "Only be here when pacing is enabled");

    61 

    62   size_t live = update_and_get_progress_history();

    63   size_t free = _heap->free_set()->available();

    64 

    65   size_t non_taxable = free * ShenandoahPacingCycleSlack / 100;

    66   size_t taxable = free - non_taxable;

    67 

    68   double tax = 1.0 * live / taxable; // base tax for available free space

    69   tax *= 3;                          // mark is phase 1 of 3, claim 1/3 of free for it

    70   tax *= ShenandoahPacingSurcharge;  // additional surcharge to help unclutter heap

    71 

    72   restart_with(non_taxable, tax);

    73 

    74   log_info(gc, ergo)("Pacer for Mark. Expected Live: " SIZE_FORMAT "M, Free: " SIZE_FORMAT

    75                      "M, Non-Taxable: " SIZE_FORMAT "M, Alloc Tax Rate: %.1fx",

    76                      live / M, free / M, non_taxable / M, tax);

    77 }

    78 

    79 void ShenandoahPacer::setup_for_evac() {

    80   assert(ShenandoahPacing, "Only be here when pacing is enabled");

    81 

    82   size_t used = _heap->collection_set()->used();

    83   size_t free = _heap->free_set()->available();

    84 

    85   size_t non_taxable = free * ShenandoahPacingCycleSlack / 100;

    86   size_t taxable = free - non_taxable;

    87 

    88   double tax = 1.0 * used / taxable; // base tax for available free space

    89   tax *= 2;                          // evac is phase 2 of 3, claim 1/2 of remaining free

    90   tax = MAX2<double>(1, tax);        // never allocate more than GC processes during the phase

    91   tax *= ShenandoahPacingSurcharge;  // additional surcharge to help unclutter heap

    92 

    93   restart_with(non_taxable, tax);

    94 

    95   log_info(gc, ergo)("Pacer for Evacuation. Used CSet: " SIZE_FORMAT "M, Free: " SIZE_FORMAT

    96                      "M, Non-Taxable: " SIZE_FORMAT "M, Alloc Tax Rate: %.1fx",

    97                      used / M, free / M, non_taxable / M, tax);

    98 }

    99 

   100 void ShenandoahPacer::setup_for_updaterefs() {

   101   assert(ShenandoahPacing, "Only be here when pacing is enabled");

   102 

   103   size_t used = _heap->used();

   104   size_t free = _heap->free_set()->available();

   105 

   106   size_t non_taxable = free * ShenandoahPacingCycleSlack / 100;

   107   size_t taxable = free - non_taxable;

   108 

   109   double tax = 1.0 * used / taxable; // base tax for available free space

   110   tax *= 1;                          // update-refs is phase 3 of 3, claim the remaining free

   111   tax = MAX2<double>(1, tax);        // never allocate more than GC processes during the phase

   112   tax *= ShenandoahPacingSurcharge;  // additional surcharge to help unclutter heap

   113 

   114   restart_with(non_taxable, tax);

   115 

   116   log_info(gc, ergo)("Pacer for Update Refs. Used: " SIZE_FORMAT "M, Free: " SIZE_FORMAT

   117                      "M, Non-Taxable: " SIZE_FORMAT "M, Alloc Tax Rate: %.1fx",

   118                      used / M, free / M, non_taxable / M, tax);

   119 }

   120 

   121 /*

   122  * Traversal walks the entire heap once, and therefore we have to make assumptions about its

   123  * liveness, like concurrent mark does.

   124  */

   125 

   126 void ShenandoahPacer::setup_for_traversal() {

   127   assert(ShenandoahPacing, "Only be here when pacing is enabled");

   128 

   129   size_t live = update_and_get_progress_history();

   130   size_t free = _heap->free_set()->available();

   131 

   132   size_t non_taxable = free * ShenandoahPacingCycleSlack / 100;

   133   size_t taxable = free - non_taxable;

   134 

   135   double tax = 1.0 * live / taxable; // base tax for available free space

   136   tax *= ShenandoahPacingSurcharge;  // additional surcharge to help unclutter heap

   137 

   138   restart_with(non_taxable, tax);

   139 

   140   log_info(gc, ergo)("Pacer for Traversal. Expected Live: " SIZE_FORMAT "M, Free: " SIZE_FORMAT

   141                      "M, Non-Taxable: " SIZE_FORMAT "M, Alloc Tax Rate: %.1fx",

   142                      live / M, free / M, non_taxable / M, tax);

   143 }

   144 

   145 /*

   146  * In idle phase, we have to pace the application to let control thread react with GC start.

   147  *

   148  * Here, we have rendezvous with concurrent thread that adds up the budget as it acknowledges

   149  * it had seen recent allocations. It will naturally pace the allocations if control thread is

   150  * not catching up. To bootstrap this feedback cycle, we need to start with some initial budget

   151  * for applications to allocate at.

   152  */

   153 

   154 void ShenandoahPacer::setup_for_idle() {

   155   assert(ShenandoahPacing, "Only be here when pacing is enabled");

   156 

   157   size_t initial = _heap->capacity() * ShenandoahPacingIdleSlack / 100;

   158   double tax = 1;

   159 

   160   restart_with(initial, tax);

   161 

   162   log_info(gc, ergo)("Pacer for Idle. Initial: " SIZE_FORMAT "M, Alloc Tax Rate: %.1fx",

   163                      initial / M, tax);

   164 }

   165 

   166 size_t ShenandoahPacer::update_and_get_progress_history() {

   167   if (_progress == -1) {

   168     // First initialization, report some prior

   169     Atomic::store((intptr_t)PACING_PROGRESS_ZERO, &_progress);

   170     return (size_t) (_heap->capacity() * 0.1);

   171   } else {

   172     // Record history, and reply historical data

   173     _progress_history->add(_progress);

   174     Atomic::store((intptr_t)PACING_PROGRESS_ZERO, &_progress);

   175     return (size_t) (_progress_history->avg() * HeapWordSize);

   176   }

   177 }

   178 

   179 void ShenandoahPacer::restart_with(size_t non_taxable_bytes, double tax_rate) {

   180   size_t initial = (size_t)(non_taxable_bytes * tax_rate) >> LogHeapWordSize;

   181   STATIC_ASSERT(sizeof(size_t) <= sizeof(intptr_t));

   182   Atomic::xchg((intptr_t)initial, &_budget);

   183   Atomic::store(tax_rate, &_tax_rate);

   184   Atomic::inc(&_epoch);

   185 }

   186 

   187 bool ShenandoahPacer::claim_for_alloc(size_t words, bool force) {

   188   assert(ShenandoahPacing, "Only be here when pacing is enabled");

   189 

   190   intptr_t tax = MAX2<intptr_t>(1, words * Atomic::load(&_tax_rate));

   191 

   192   intptr_t cur = 0;

   193   intptr_t new_val = 0;

   194   do {

   195     cur = Atomic::load(&_budget);

   196     if (cur < tax && !force) {

   197       // Progress depleted, alas.

   198       return false;

   199     }

   200     new_val = cur - tax;

   201   } while (Atomic::cmpxchg(new_val, &_budget, cur) != cur);

   202   return true;

   203 }

   204 

   205 void ShenandoahPacer::unpace_for_alloc(intptr_t epoch, size_t words) {

   206   assert(ShenandoahPacing, "Only be here when pacing is enabled");

   207 

   208   if (_epoch != epoch) {

   209     // Stale ticket, no need to unpace.

   210     return;

   211   }

   212 

   213   intptr_t tax = MAX2<intptr_t>(1, words * Atomic::load(&_tax_rate));

   214   Atomic::add(tax, &_budget);

   215 }

   216 

   217 intptr_t ShenandoahPacer::epoch() {

   218   return Atomic::load(&_epoch);

   219 }

   220 

   221 void ShenandoahPacer::pace_for_alloc(size_t words) {

   222   assert(ShenandoahPacing, "Only be here when pacing is enabled");

   223 

   224   // Fast path: try to allocate right away

   225   if (claim_for_alloc(words, false)) {

   226     return;

   227   }

   228 

   229   size_t max = ShenandoahPacingMaxDelay;

   230   double start = os::elapsedTime();

   231 

   232   size_t total = 0;

   233   size_t cur = 0;

   234 

   235   while (true) {

   236     // We could instead assist GC, but this would suffice for now.

   237     // This code should also participate in safepointing.

   238     // Perform the exponential backoff, limited by max.

   239 

   240     cur = cur * 2;

   241     if (total + cur > max) {

   242       cur = (max > total) ? (max - total) : 0;

   243     }

   244     cur = MAX2<size_t>(1, cur);

   245 

   246     os::sleep(Thread::current(), cur, true);

   247 

   248     double end = os::elapsedTime();

   249     total = (size_t)((end - start) * 1000);

   250 

   251     if (total > max) {

   252       // Spent local time budget to wait for enough GC progress.

   253       // Breaking out and allocating anyway, which may mean we outpace GC,

   254       // and start Degenerated GC cycle.

   255       _delays.add(total);

   256 

   257       // Forcefully claim the budget: it may go negative at this point, and

   258       // GC should replenish for this and subsequent allocations

   259       claim_for_alloc(words, true);

   260       break;

   261     }

   262 

   263     if (claim_for_alloc(words, false)) {

   264       // Acquired enough permit, nice. Can allocate now.

   265       _delays.add(total);

   266       break;

   267     }

   268   }

   269 }

   270 

   271 void ShenandoahPacer::print_on(outputStream* out) const {

   272   out->print_cr("ALLOCATION PACING:");

   273   out->cr();

   274 

   275   out->print_cr("Max pacing delay is set for " UINTX_FORMAT " ms.", ShenandoahPacingMaxDelay);

   276   out->cr();

   277 

   278   out->print_cr("Higher delay would prevent application outpacing the GC, but it will hide the GC latencies");

   279   out->print_cr("from the STW pause times. Pacing affects the individual threads, and so it would also be");

   280   out->print_cr("invisible to the usual profiling tools, but would add up to end-to-end application latency.");

   281   out->print_cr("Raise max pacing delay with care.");

   282   out->cr();

   283 

   284   out->print_cr("Actual pacing delays histogram:");

   285   out->cr();

   286 

   287   out->print_cr("%10s - %10s  %12s%12s", "From", "To", "Count", "Sum");

   288 

   289   size_t total_count = 0;

   290   size_t total_sum = 0;

   291   for (int c = _delays.min_level(); c <= _delays.max_level(); c++) {

   292     int l = (c == 0) ? 0 : 1 << (c - 1);

   293     int r = 1 << c;

   294     size_t count = _delays.level(c);

   295     size_t sum   = count * (r - l) / 2;

   296     total_count += count;

   297     total_sum   += sum;

   298 

   299     out->print_cr("%7d ms - %7d ms: " SIZE_FORMAT_W(12) SIZE_FORMAT_W(12) " ms", l, r, count, sum);

   300   }

   301   out->print_cr("%23s: " SIZE_FORMAT_W(12) SIZE_FORMAT_W(12) " ms", "Total", total_count, total_sum);

   302   out->cr();

   303 }