--- /dev/null Thu Jan 01 00:00:00 1970 +0000
+++ b/src/hotspot/share/gc/shenandoah/shenandoahPacer.cpp Mon Dec 10 15:47:44 2018 +0100
@@ -0,0 +1,303 @@
+/*
+ * Copyright (c) 2018, Red Hat, Inc. All rights reserved.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ *
+ */
+
+#include "precompiled.hpp"
+
+#include "gc/shenandoah/shenandoahFreeSet.hpp"
+#include "gc/shenandoah/shenandoahHeap.hpp"
+#include "gc/shenandoah/shenandoahHeap.inline.hpp"
+#include "gc/shenandoah/shenandoahPacer.hpp"
+
+/*
+ * In normal concurrent cycle, we have to pace the application to let GC finish.
+ *
+ * Here, we do not know how large would be the collection set, and what are the
+ * relative performances of the each stage in the concurrent cycle, and so we have to
+ * make some assumptions.
+ *
+ * For concurrent mark, there is no clear notion of progress. The moderately accurate
+ * and easy to get metric is the amount of live objects the mark had encountered. But,
+ * that does directly correlate with the used heap, because the heap might be fully
+ * dead or fully alive. We cannot assume either of the extremes: we would either allow
+ * application to run out of memory if we assume heap is fully dead but it is not, and,
+ * conversely, we would pacify application excessively if we assume heap is fully alive
+ * but it is not. So we need to guesstimate the particular expected value for heap liveness.
+ * The best way to do this is apparently recording the past history.
+ *
+ * For concurrent evac and update-refs, we are walking the heap per-region, and so the
+ * notion of progress is clear: we get reported the "used" size from the processed regions
+ * and use the global heap-used as the baseline.
+ *
+ * The allocatable space when GC is running is "free" at the start of cycle, but the
+ * accounted budget is based on "used". So, we need to adjust the tax knowing that.
+ * Also, since we effectively count the used space three times (mark, evac, update-refs),
+ * we need to multiply the tax by 3. Example: for 10 MB free and 90 MB used, GC would
+ * come back with 3*90 MB budget, and thus for each 1 MB of allocation, we have to pay
+ * 3*90 / 10 MBs. In the end, we would pay back the entire budget.
+ */
+
+void ShenandoahPacer::setup_for_mark() {
+ assert(ShenandoahPacing, "Only be here when pacing is enabled");
+
+ size_t live = update_and_get_progress_history();
+ size_t free = _heap->free_set()->available();
+
+ size_t non_taxable = free * ShenandoahPacingCycleSlack / 100;
+ size_t taxable = free - non_taxable;
+
+ double tax = 1.0 * live / taxable; // base tax for available free space
+ tax *= 3; // mark is phase 1 of 3, claim 1/3 of free for it
+ tax *= ShenandoahPacingSurcharge; // additional surcharge to help unclutter heap
+
+ restart_with(non_taxable, tax);
+
+ log_info(gc, ergo)("Pacer for Mark. Expected Live: " SIZE_FORMAT "M, Free: " SIZE_FORMAT
+ "M, Non-Taxable: " SIZE_FORMAT "M, Alloc Tax Rate: %.1fx",
+ live / M, free / M, non_taxable / M, tax);
+}
+
+void ShenandoahPacer::setup_for_evac() {
+ assert(ShenandoahPacing, "Only be here when pacing is enabled");
+
+ size_t used = _heap->collection_set()->used();
+ size_t free = _heap->free_set()->available();
+
+ size_t non_taxable = free * ShenandoahPacingCycleSlack / 100;
+ size_t taxable = free - non_taxable;
+
+ double tax = 1.0 * used / taxable; // base tax for available free space
+ tax *= 2; // evac is phase 2 of 3, claim 1/2 of remaining free
+ tax = MAX2<double>(1, tax); // never allocate more than GC processes during the phase
+ tax *= ShenandoahPacingSurcharge; // additional surcharge to help unclutter heap
+
+ restart_with(non_taxable, tax);
+
+ log_info(gc, ergo)("Pacer for Evacuation. Used CSet: " SIZE_FORMAT "M, Free: " SIZE_FORMAT
+ "M, Non-Taxable: " SIZE_FORMAT "M, Alloc Tax Rate: %.1fx",
+ used / M, free / M, non_taxable / M, tax);
+}
+
+void ShenandoahPacer::setup_for_updaterefs() {
+ assert(ShenandoahPacing, "Only be here when pacing is enabled");
+
+ size_t used = _heap->used();
+ size_t free = _heap->free_set()->available();
+
+ size_t non_taxable = free * ShenandoahPacingCycleSlack / 100;
+ size_t taxable = free - non_taxable;
+
+ double tax = 1.0 * used / taxable; // base tax for available free space
+ tax *= 1; // update-refs is phase 3 of 3, claim the remaining free
+ tax = MAX2<double>(1, tax); // never allocate more than GC processes during the phase
+ tax *= ShenandoahPacingSurcharge; // additional surcharge to help unclutter heap
+
+ restart_with(non_taxable, tax);
+
+ log_info(gc, ergo)("Pacer for Update Refs. Used: " SIZE_FORMAT "M, Free: " SIZE_FORMAT
+ "M, Non-Taxable: " SIZE_FORMAT "M, Alloc Tax Rate: %.1fx",
+ used / M, free / M, non_taxable / M, tax);
+}
+
+/*
+ * Traversal walks the entire heap once, and therefore we have to make assumptions about its
+ * liveness, like concurrent mark does.
+ */
+
+void ShenandoahPacer::setup_for_traversal() {
+ assert(ShenandoahPacing, "Only be here when pacing is enabled");
+
+ size_t live = update_and_get_progress_history();
+ size_t free = _heap->free_set()->available();
+
+ size_t non_taxable = free * ShenandoahPacingCycleSlack / 100;
+ size_t taxable = free - non_taxable;
+
+ double tax = 1.0 * live / taxable; // base tax for available free space
+ tax *= ShenandoahPacingSurcharge; // additional surcharge to help unclutter heap
+
+ restart_with(non_taxable, tax);
+
+ log_info(gc, ergo)("Pacer for Traversal. Expected Live: " SIZE_FORMAT "M, Free: " SIZE_FORMAT
+ "M, Non-Taxable: " SIZE_FORMAT "M, Alloc Tax Rate: %.1fx",
+ live / M, free / M, non_taxable / M, tax);
+}
+
+/*
+ * In idle phase, we have to pace the application to let control thread react with GC start.
+ *
+ * Here, we have rendezvous with concurrent thread that adds up the budget as it acknowledges
+ * it had seen recent allocations. It will naturally pace the allocations if control thread is
+ * not catching up. To bootstrap this feedback cycle, we need to start with some initial budget
+ * for applications to allocate at.
+ */
+
+void ShenandoahPacer::setup_for_idle() {
+ assert(ShenandoahPacing, "Only be here when pacing is enabled");
+
+ size_t initial = _heap->capacity() * ShenandoahPacingIdleSlack / 100;
+ double tax = 1;
+
+ restart_with(initial, tax);
+
+ log_info(gc, ergo)("Pacer for Idle. Initial: " SIZE_FORMAT "M, Alloc Tax Rate: %.1fx",
+ initial / M, tax);
+}
+
+size_t ShenandoahPacer::update_and_get_progress_history() {
+ if (_progress == -1) {
+ // First initialization, report some prior
+ Atomic::store((intptr_t)PACING_PROGRESS_ZERO, &_progress);
+ return (size_t) (_heap->capacity() * 0.1);
+ } else {
+ // Record history, and reply historical data
+ _progress_history->add(_progress);
+ Atomic::store((intptr_t)PACING_PROGRESS_ZERO, &_progress);
+ return (size_t) (_progress_history->avg() * HeapWordSize);
+ }
+}
+
+void ShenandoahPacer::restart_with(size_t non_taxable_bytes, double tax_rate) {
+ size_t initial = (size_t)(non_taxable_bytes * tax_rate) >> LogHeapWordSize;
+ STATIC_ASSERT(sizeof(size_t) <= sizeof(intptr_t));
+ Atomic::xchg((intptr_t)initial, &_budget);
+ Atomic::store(tax_rate, &_tax_rate);
+ Atomic::inc(&_epoch);
+}
+
+bool ShenandoahPacer::claim_for_alloc(size_t words, bool force) {
+ assert(ShenandoahPacing, "Only be here when pacing is enabled");
+
+ intptr_t tax = MAX2<intptr_t>(1, words * Atomic::load(&_tax_rate));
+
+ intptr_t cur = 0;
+ intptr_t new_val = 0;
+ do {
+ cur = Atomic::load(&_budget);
+ if (cur < tax && !force) {
+ // Progress depleted, alas.
+ return false;
+ }
+ new_val = cur - tax;
+ } while (Atomic::cmpxchg(new_val, &_budget, cur) != cur);
+ return true;
+}
+
+void ShenandoahPacer::unpace_for_alloc(intptr_t epoch, size_t words) {
+ assert(ShenandoahPacing, "Only be here when pacing is enabled");
+
+ if (_epoch != epoch) {
+ // Stale ticket, no need to unpace.
+ return;
+ }
+
+ intptr_t tax = MAX2<intptr_t>(1, words * Atomic::load(&_tax_rate));
+ Atomic::add(tax, &_budget);
+}
+
+intptr_t ShenandoahPacer::epoch() {
+ return Atomic::load(&_epoch);
+}
+
+void ShenandoahPacer::pace_for_alloc(size_t words) {
+ assert(ShenandoahPacing, "Only be here when pacing is enabled");
+
+ // Fast path: try to allocate right away
+ if (claim_for_alloc(words, false)) {
+ return;
+ }
+
+ size_t max = ShenandoahPacingMaxDelay;
+ double start = os::elapsedTime();
+
+ size_t total = 0;
+ size_t cur = 0;
+
+ while (true) {
+ // We could instead assist GC, but this would suffice for now.
+ // This code should also participate in safepointing.
+ // Perform the exponential backoff, limited by max.
+
+ cur = cur * 2;
+ if (total + cur > max) {
+ cur = (max > total) ? (max - total) : 0;
+ }
+ cur = MAX2<size_t>(1, cur);
+
+ os::sleep(Thread::current(), cur, true);
+
+ double end = os::elapsedTime();
+ total = (size_t)((end - start) * 1000);
+
+ if (total > max) {
+ // Spent local time budget to wait for enough GC progress.
+ // Breaking out and allocating anyway, which may mean we outpace GC,
+ // and start Degenerated GC cycle.
+ _delays.add(total);
+
+ // Forcefully claim the budget: it may go negative at this point, and
+ // GC should replenish for this and subsequent allocations
+ claim_for_alloc(words, true);
+ break;
+ }
+
+ if (claim_for_alloc(words, false)) {
+ // Acquired enough permit, nice. Can allocate now.
+ _delays.add(total);
+ break;
+ }
+ }
+}
+
+void ShenandoahPacer::print_on(outputStream* out) const {
+ out->print_cr("ALLOCATION PACING:");
+ out->cr();
+
+ out->print_cr("Max pacing delay is set for " UINTX_FORMAT " ms.", ShenandoahPacingMaxDelay);
+ out->cr();
+
+ out->print_cr("Higher delay would prevent application outpacing the GC, but it will hide the GC latencies");
+ out->print_cr("from the STW pause times. Pacing affects the individual threads, and so it would also be");
+ out->print_cr("invisible to the usual profiling tools, but would add up to end-to-end application latency.");
+ out->print_cr("Raise max pacing delay with care.");
+ out->cr();
+
+ out->print_cr("Actual pacing delays histogram:");
+ out->cr();
+
+ out->print_cr("%10s - %10s %12s%12s", "From", "To", "Count", "Sum");
+
+ size_t total_count = 0;
+ size_t total_sum = 0;
+ for (int c = _delays.min_level(); c <= _delays.max_level(); c++) {
+ int l = (c == 0) ? 0 : 1 << (c - 1);
+ int r = 1 << c;
+ size_t count = _delays.level(c);
+ size_t sum = count * (r - l) / 2;
+ total_count += count;
+ total_sum += sum;
+
+ out->print_cr("%7d ms - %7d ms: " SIZE_FORMAT_W(12) SIZE_FORMAT_W(12) " ms", l, r, count, sum);
+ }
+ out->print_cr("%23s: " SIZE_FORMAT_W(12) SIZE_FORMAT_W(12) " ms", "Total", total_count, total_sum);
+ out->cr();
+}