1.1 --- a/src/share/vm/gc_implementation/g1/concurrentG1Refine.cpp Thu May 09 12:23:43 2013 +0200
1.2 +++ b/src/share/vm/gc_implementation/g1/concurrentG1Refine.cpp Thu May 09 11:16:39 2013 -0700
1.3 @@ -1,5 +1,5 @@
1.4 /*
1.5 - * Copyright (c) 2001, 2012, Oracle and/or its affiliates. All rights reserved.
1.6 + * Copyright (c) 2001, 2013, Oracle and/or its affiliates. All rights reserved.
1.7 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
1.8 *
1.9 * This code is free software; you can redistribute it and/or modify it
1.10 @@ -26,40 +26,12 @@
1.11 #include "gc_implementation/g1/concurrentG1Refine.hpp"
1.12 #include "gc_implementation/g1/concurrentG1RefineThread.hpp"
1.13 #include "gc_implementation/g1/g1CollectedHeap.inline.hpp"
1.14 -#include "gc_implementation/g1/g1CollectorPolicy.hpp"
1.15 -#include "gc_implementation/g1/g1GCPhaseTimes.hpp"
1.16 -#include "gc_implementation/g1/g1RemSet.hpp"
1.17 -#include "gc_implementation/g1/heapRegionSeq.inline.hpp"
1.18 -#include "memory/space.inline.hpp"
1.19 -#include "runtime/atomic.hpp"
1.20 -#include "runtime/java.hpp"
1.21 -#include "utilities/copy.hpp"
1.22 +#include "gc_implementation/g1/g1HotCardCache.hpp"
1.23
1.24 -// Possible sizes for the card counts cache: odd primes that roughly double in size.
1.25 -// (See jvmtiTagMap.cpp).
1.26 -
1.27 -#define MAX_SIZE ((size_t) -1)
1.28 -
1.29 -size_t ConcurrentG1Refine::_cc_cache_sizes[] = {
1.30 - 16381, 32771, 76831, 150001, 307261,
1.31 - 614563, 1228891, 2457733, 4915219, 9830479,
1.32 - 19660831, 39321619, 78643219, 157286461, MAX_SIZE
1.33 - };
1.34 -
1.35 -ConcurrentG1Refine::ConcurrentG1Refine() :
1.36 - _card_counts(NULL), _card_epochs(NULL),
1.37 - _n_card_counts(0), _max_cards(0), _max_n_card_counts(0),
1.38 - _cache_size_index(0), _expand_card_counts(false),
1.39 - _hot_cache(NULL),
1.40 - _def_use_cache(false), _use_cache(false),
1.41 - // We initialize the epochs of the array to 0. By initializing
1.42 - // _n_periods to 1 and not 0 we automatically invalidate all the
1.43 - // entries on the array. Otherwise we might accidentally think that
1.44 - // we claimed a card that was in fact never set (see CR7033292).
1.45 - _n_periods(1),
1.46 - _threads(NULL), _n_threads(0)
1.47 +ConcurrentG1Refine::ConcurrentG1Refine(G1CollectedHeap* g1h) :
1.48 + _threads(NULL), _n_threads(0),
1.49 + _hot_card_cache(g1h)
1.50 {
1.51 -
1.52 // Ergomonically select initial concurrent refinement parameters
1.53 if (FLAG_IS_DEFAULT(G1ConcRefinementGreenZone)) {
1.54 FLAG_SET_DEFAULT(G1ConcRefinementGreenZone, MAX2<int>(ParallelGCThreads, 1));
1.55 @@ -75,13 +47,17 @@
1.56 FLAG_SET_DEFAULT(G1ConcRefinementRedZone, yellow_zone() * 2);
1.57 }
1.58 set_red_zone(MAX2<int>(G1ConcRefinementRedZone, yellow_zone()));
1.59 +
1.60 _n_worker_threads = thread_num();
1.61 // We need one extra thread to do the young gen rset size sampling.
1.62 _n_threads = _n_worker_threads + 1;
1.63 +
1.64 reset_threshold_step();
1.65
1.66 _threads = NEW_C_HEAP_ARRAY(ConcurrentG1RefineThread*, _n_threads, mtGC);
1.67 +
1.68 int worker_id_offset = (int)DirtyCardQueueSet::num_par_ids();
1.69 +
1.70 ConcurrentG1RefineThread *next = NULL;
1.71 for (int i = _n_threads - 1; i >= 0; i--) {
1.72 ConcurrentG1RefineThread* t = new ConcurrentG1RefineThread(this, next, worker_id_offset, i);
1.73 @@ -100,74 +76,8 @@
1.74 }
1.75 }
1.76
1.77 -int ConcurrentG1Refine::thread_num() {
1.78 - return MAX2<int>((G1ConcRefinementThreads > 0) ? G1ConcRefinementThreads : ParallelGCThreads, 1);
1.79 -}
1.80 -
1.81 void ConcurrentG1Refine::init() {
1.82 - if (G1ConcRSLogCacheSize > 0) {
1.83 - _g1h = G1CollectedHeap::heap();
1.84 -
1.85 - _max_cards = _g1h->max_capacity() >> CardTableModRefBS::card_shift;
1.86 - _max_n_card_counts = _max_cards * G1MaxHotCardCountSizePercent / 100;
1.87 -
1.88 - size_t max_card_num = ((size_t)1 << (sizeof(unsigned)*BitsPerByte-1)) - 1;
1.89 - guarantee(_max_cards < max_card_num, "card_num representation");
1.90 -
1.91 - // We need _n_card_counts to be less than _max_n_card_counts here
1.92 - // so that the expansion call (below) actually allocates the
1.93 - // _counts and _epochs arrays.
1.94 - assert(_n_card_counts == 0, "pre-condition");
1.95 - assert(_max_n_card_counts > 0, "pre-condition");
1.96 -
1.97 - // Find the index into cache size array that is of a size that's
1.98 - // large enough to hold desired_sz.
1.99 - size_t desired_sz = _max_cards / InitialCacheFraction;
1.100 - int desired_sz_index = 0;
1.101 - while (_cc_cache_sizes[desired_sz_index] < desired_sz) {
1.102 - desired_sz_index += 1;
1.103 - assert(desired_sz_index < MAX_CC_CACHE_INDEX, "invariant");
1.104 - }
1.105 - assert(desired_sz_index < MAX_CC_CACHE_INDEX, "invariant");
1.106 -
1.107 - // If the desired_sz value is between two sizes then
1.108 - // _cc_cache_sizes[desired_sz_index-1] < desired_sz <= _cc_cache_sizes[desired_sz_index]
1.109 - // we will start with the lower size in the optimistic expectation that
1.110 - // we will not need to expand up. Note desired_sz_index could also be 0.
1.111 - if (desired_sz_index > 0 &&
1.112 - _cc_cache_sizes[desired_sz_index] > desired_sz) {
1.113 - desired_sz_index -= 1;
1.114 - }
1.115 -
1.116 - if (!expand_card_count_cache(desired_sz_index)) {
1.117 - // Allocation was unsuccessful - exit
1.118 - vm_exit_during_initialization("Could not reserve enough space for card count cache");
1.119 - }
1.120 - assert(_n_card_counts > 0, "post-condition");
1.121 - assert(_cache_size_index == desired_sz_index, "post-condition");
1.122 -
1.123 - Copy::fill_to_bytes(&_card_counts[0],
1.124 - _n_card_counts * sizeof(CardCountCacheEntry));
1.125 - Copy::fill_to_bytes(&_card_epochs[0], _n_card_counts * sizeof(CardEpochCacheEntry));
1.126 -
1.127 - ModRefBarrierSet* bs = _g1h->mr_bs();
1.128 - guarantee(bs->is_a(BarrierSet::CardTableModRef), "Precondition");
1.129 - _ct_bs = (CardTableModRefBS*)bs;
1.130 - _ct_bot = _ct_bs->byte_for_const(_g1h->reserved_region().start());
1.131 -
1.132 - _def_use_cache = true;
1.133 - _use_cache = true;
1.134 - _hot_cache_size = (1 << G1ConcRSLogCacheSize);
1.135 - _hot_cache = NEW_C_HEAP_ARRAY(jbyte*, _hot_cache_size, mtGC);
1.136 - _n_hot = 0;
1.137 - _hot_cache_idx = 0;
1.138 -
1.139 - // For refining the cards in the hot cache in parallel
1.140 - int n_workers = (ParallelGCThreads > 0 ?
1.141 - _g1h->workers()->total_workers() : 1);
1.142 - _hot_cache_par_chunk_size = MAX2(1, _hot_cache_size / n_workers);
1.143 - _hot_cache_par_claimed_idx = 0;
1.144 - }
1.145 + _hot_card_cache.initialize();
1.146 }
1.147
1.148 void ConcurrentG1Refine::stop() {
1.149 @@ -188,17 +98,6 @@
1.150 }
1.151
1.152 ConcurrentG1Refine::~ConcurrentG1Refine() {
1.153 - if (G1ConcRSLogCacheSize > 0) {
1.154 - // Please see the comment in allocate_card_count_cache
1.155 - // for why we call os::malloc() and os::free() directly.
1.156 - assert(_card_counts != NULL, "Logic");
1.157 - os::free(_card_counts, mtGC);
1.158 - assert(_card_epochs != NULL, "Logic");
1.159 - os::free(_card_epochs, mtGC);
1.160 -
1.161 - assert(_hot_cache != NULL, "Logic");
1.162 - FREE_C_HEAP_ARRAY(jbyte*, _hot_cache, mtGC);
1.163 - }
1.164 if (_threads != NULL) {
1.165 for (int i = 0; i < _n_threads; i++) {
1.166 delete _threads[i];
1.167 @@ -215,317 +114,10 @@
1.168 }
1.169 }
1.170
1.171 -bool ConcurrentG1Refine::is_young_card(jbyte* card_ptr) {
1.172 - HeapWord* start = _ct_bs->addr_for(card_ptr);
1.173 - HeapRegion* r = _g1h->heap_region_containing(start);
1.174 - if (r != NULL && r->is_young()) {
1.175 - return true;
1.176 - }
1.177 - // This card is not associated with a heap region
1.178 - // so can't be young.
1.179 - return false;
1.180 -}
1.181 -
1.182 -jbyte* ConcurrentG1Refine::add_card_count(jbyte* card_ptr, int* count, bool* defer) {
1.183 - unsigned new_card_num = ptr_2_card_num(card_ptr);
1.184 - unsigned bucket = hash(new_card_num);
1.185 - assert(0 <= bucket && bucket < _n_card_counts, "Bounds");
1.186 -
1.187 - CardCountCacheEntry* count_ptr = &_card_counts[bucket];
1.188 - CardEpochCacheEntry* epoch_ptr = &_card_epochs[bucket];
1.189 -
1.190 - // We have to construct a new entry if we haven't updated the counts
1.191 - // during the current period, or if the count was updated for a
1.192 - // different card number.
1.193 - unsigned int new_epoch = (unsigned int) _n_periods;
1.194 - julong new_epoch_entry = make_epoch_entry(new_card_num, new_epoch);
1.195 -
1.196 - while (true) {
1.197 - // Fetch the previous epoch value
1.198 - julong prev_epoch_entry = epoch_ptr->_value;
1.199 - julong cas_res;
1.200 -
1.201 - if (extract_epoch(prev_epoch_entry) != new_epoch) {
1.202 - // This entry has not yet been updated during this period.
1.203 - // Note: we update the epoch value atomically to ensure
1.204 - // that there is only one winner that updates the cached
1.205 - // card_ptr value even though all the refine threads share
1.206 - // the same epoch value.
1.207 -
1.208 - cas_res = (julong) Atomic::cmpxchg((jlong) new_epoch_entry,
1.209 - (volatile jlong*)&epoch_ptr->_value,
1.210 - (jlong) prev_epoch_entry);
1.211 -
1.212 - if (cas_res == prev_epoch_entry) {
1.213 - // We have successfully won the race to update the
1.214 - // epoch and card_num value. Make it look like the
1.215 - // count and eviction count were previously cleared.
1.216 - count_ptr->_count = 1;
1.217 - count_ptr->_evict_count = 0;
1.218 - *count = 0;
1.219 - // We can defer the processing of card_ptr
1.220 - *defer = true;
1.221 - return card_ptr;
1.222 - }
1.223 - // We did not win the race to update the epoch field, so some other
1.224 - // thread must have done it. The value that gets returned by CAS
1.225 - // should be the new epoch value.
1.226 - assert(extract_epoch(cas_res) == new_epoch, "unexpected epoch");
1.227 - // We could 'continue' here or just re-read the previous epoch value
1.228 - prev_epoch_entry = epoch_ptr->_value;
1.229 - }
1.230 -
1.231 - // The epoch entry for card_ptr has been updated during this period.
1.232 - unsigned old_card_num = extract_card_num(prev_epoch_entry);
1.233 -
1.234 - // The card count that will be returned to caller
1.235 - *count = count_ptr->_count;
1.236 -
1.237 - // Are we updating the count for the same card?
1.238 - if (new_card_num == old_card_num) {
1.239 - // Same card - just update the count. We could have more than one
1.240 - // thread racing to update count for the current card. It should be
1.241 - // OK not to use a CAS as the only penalty should be some missed
1.242 - // increments of the count which delays identifying the card as "hot".
1.243 -
1.244 - if (*count < max_jubyte) count_ptr->_count++;
1.245 - // We can defer the processing of card_ptr
1.246 - *defer = true;
1.247 - return card_ptr;
1.248 - }
1.249 -
1.250 - // Different card - evict old card info
1.251 - if (count_ptr->_evict_count < max_jubyte) count_ptr->_evict_count++;
1.252 - if (count_ptr->_evict_count > G1CardCountCacheExpandThreshold) {
1.253 - // Trigger a resize the next time we clear
1.254 - _expand_card_counts = true;
1.255 - }
1.256 -
1.257 - cas_res = (julong) Atomic::cmpxchg((jlong) new_epoch_entry,
1.258 - (volatile jlong*)&epoch_ptr->_value,
1.259 - (jlong) prev_epoch_entry);
1.260 -
1.261 - if (cas_res == prev_epoch_entry) {
1.262 - // We successfully updated the card num value in the epoch entry
1.263 - count_ptr->_count = 0; // initialize counter for new card num
1.264 - jbyte* old_card_ptr = card_num_2_ptr(old_card_num);
1.265 -
1.266 - // Even though the region containg the card at old_card_num was not
1.267 - // in the young list when old_card_num was recorded in the epoch
1.268 - // cache it could have been added to the free list and subsequently
1.269 - // added to the young list in the intervening time. See CR 6817995.
1.270 - // We do not deal with this case here - it will be handled in
1.271 - // HeapRegion::oops_on_card_seq_iterate_careful after it has been
1.272 - // determined that the region containing the card has been allocated
1.273 - // to, and it's safe to check the young type of the region.
1.274 -
1.275 - // We do not want to defer processing of card_ptr in this case
1.276 - // (we need to refine old_card_ptr and card_ptr)
1.277 - *defer = false;
1.278 - return old_card_ptr;
1.279 - }
1.280 - // Someone else beat us - try again.
1.281 - }
1.282 -}
1.283 -
1.284 -jbyte* ConcurrentG1Refine::cache_insert(jbyte* card_ptr, bool* defer) {
1.285 - int count;
1.286 - jbyte* cached_ptr = add_card_count(card_ptr, &count, defer);
1.287 - assert(cached_ptr != NULL, "bad cached card ptr");
1.288 -
1.289 - // We've just inserted a card pointer into the card count cache
1.290 - // and got back the card that we just inserted or (evicted) the
1.291 - // previous contents of that count slot.
1.292 -
1.293 - // The card we got back could be in a young region. When the
1.294 - // returned card (if evicted) was originally inserted, we had
1.295 - // determined that its containing region was not young. However
1.296 - // it is possible for the region to be freed during a cleanup
1.297 - // pause, then reallocated and tagged as young which will result
1.298 - // in the returned card residing in a young region.
1.299 - //
1.300 - // We do not deal with this case here - the change from non-young
1.301 - // to young could be observed at any time - it will be handled in
1.302 - // HeapRegion::oops_on_card_seq_iterate_careful after it has been
1.303 - // determined that the region containing the card has been allocated
1.304 - // to.
1.305 -
1.306 - // The card pointer we obtained from card count cache is not hot
1.307 - // so do not store it in the cache; return it for immediate
1.308 - // refining.
1.309 - if (count < G1ConcRSHotCardLimit) {
1.310 - return cached_ptr;
1.311 - }
1.312 -
1.313 - // Otherwise, the pointer we got from the _card_counts cache is hot.
1.314 - jbyte* res = NULL;
1.315 - MutexLockerEx x(HotCardCache_lock, Mutex::_no_safepoint_check_flag);
1.316 - if (_n_hot == _hot_cache_size) {
1.317 - res = _hot_cache[_hot_cache_idx];
1.318 - _n_hot--;
1.319 - }
1.320 - // Now _n_hot < _hot_cache_size, and we can insert at _hot_cache_idx.
1.321 - _hot_cache[_hot_cache_idx] = cached_ptr;
1.322 - _hot_cache_idx++;
1.323 - if (_hot_cache_idx == _hot_cache_size) _hot_cache_idx = 0;
1.324 - _n_hot++;
1.325 -
1.326 - // The card obtained from the hot card cache could be in a young
1.327 - // region. See above on how this can happen.
1.328 -
1.329 - return res;
1.330 -}
1.331 -
1.332 -void ConcurrentG1Refine::clean_up_cache(int worker_i,
1.333 - G1RemSet* g1rs,
1.334 - DirtyCardQueue* into_cset_dcq) {
1.335 - assert(!use_cache(), "cache should be disabled");
1.336 - int start_idx;
1.337 -
1.338 - while ((start_idx = _hot_cache_par_claimed_idx) < _n_hot) { // read once
1.339 - int end_idx = start_idx + _hot_cache_par_chunk_size;
1.340 -
1.341 - if (start_idx ==
1.342 - Atomic::cmpxchg(end_idx, &_hot_cache_par_claimed_idx, start_idx)) {
1.343 - // The current worker has successfully claimed the chunk [start_idx..end_idx)
1.344 - end_idx = MIN2(end_idx, _n_hot);
1.345 - for (int i = start_idx; i < end_idx; i++) {
1.346 - jbyte* entry = _hot_cache[i];
1.347 - if (entry != NULL) {
1.348 - if (g1rs->concurrentRefineOneCard(entry, worker_i, true)) {
1.349 - // 'entry' contains references that point into the current
1.350 - // collection set. We need to record 'entry' in the DCQS
1.351 - // that's used for that purpose.
1.352 - //
1.353 - // The only time we care about recording cards that contain
1.354 - // references that point into the collection set is during
1.355 - // RSet updating while within an evacuation pause.
1.356 - // In this case worker_i should be the id of a GC worker thread
1.357 - assert(SafepointSynchronize::is_at_safepoint(), "not during an evacuation pause");
1.358 - assert(worker_i < (int) (ParallelGCThreads == 0 ? 1 : ParallelGCThreads), "incorrect worker id");
1.359 - into_cset_dcq->enqueue(entry);
1.360 - }
1.361 - }
1.362 - }
1.363 - }
1.364 - }
1.365 -}
1.366 -
1.367 -// The arrays used to hold the card counts and the epochs must have
1.368 -// a 1:1 correspondence. Hence they are allocated and freed together
1.369 -// Returns true if the allocations of both the counts and epochs
1.370 -// were successful; false otherwise.
1.371 -bool ConcurrentG1Refine::allocate_card_count_cache(size_t n,
1.372 - CardCountCacheEntry** counts,
1.373 - CardEpochCacheEntry** epochs) {
1.374 - // We call the allocation/free routines directly for the counts
1.375 - // and epochs arrays. The NEW_C_HEAP_ARRAY/FREE_C_HEAP_ARRAY
1.376 - // macros call AllocateHeap and FreeHeap respectively.
1.377 - // AllocateHeap will call vm_exit_out_of_memory in the event
1.378 - // of an allocation failure and abort the JVM. With the
1.379 - // _counts/epochs arrays we only need to abort the JVM if the
1.380 - // initial allocation of these arrays fails.
1.381 - //
1.382 - // Additionally AllocateHeap/FreeHeap do some tracing of
1.383 - // allocate/free calls so calling one without calling the
1.384 - // other can cause inconsistencies in the tracing. So we
1.385 - // call neither.
1.386 -
1.387 - assert(*counts == NULL, "out param");
1.388 - assert(*epochs == NULL, "out param");
1.389 -
1.390 - size_t counts_size = n * sizeof(CardCountCacheEntry);
1.391 - size_t epochs_size = n * sizeof(CardEpochCacheEntry);
1.392 -
1.393 - *counts = (CardCountCacheEntry*) os::malloc(counts_size, mtGC);
1.394 - if (*counts == NULL) {
1.395 - // allocation was unsuccessful
1.396 - return false;
1.397 - }
1.398 -
1.399 - *epochs = (CardEpochCacheEntry*) os::malloc(epochs_size, mtGC);
1.400 - if (*epochs == NULL) {
1.401 - // allocation was unsuccessful - free counts array
1.402 - assert(*counts != NULL, "must be");
1.403 - os::free(*counts, mtGC);
1.404 - *counts = NULL;
1.405 - return false;
1.406 - }
1.407 -
1.408 - // We successfully allocated both counts and epochs
1.409 - return true;
1.410 -}
1.411 -
1.412 -// Returns true if the card counts/epochs cache was
1.413 -// successfully expanded; false otherwise.
1.414 -bool ConcurrentG1Refine::expand_card_count_cache(int cache_size_idx) {
1.415 - // Can we expand the card count and epoch tables?
1.416 - if (_n_card_counts < _max_n_card_counts) {
1.417 - assert(cache_size_idx >= 0 && cache_size_idx < MAX_CC_CACHE_INDEX, "oob");
1.418 -
1.419 - size_t cache_size = _cc_cache_sizes[cache_size_idx];
1.420 - // Make sure we don't go bigger than we will ever need
1.421 - cache_size = MIN2(cache_size, _max_n_card_counts);
1.422 -
1.423 - // Should we expand the card count and card epoch tables?
1.424 - if (cache_size > _n_card_counts) {
1.425 - // We have been asked to allocate new, larger, arrays for
1.426 - // the card counts and the epochs. Attempt the allocation
1.427 - // of both before we free the existing arrays in case
1.428 - // the allocation is unsuccessful...
1.429 - CardCountCacheEntry* counts = NULL;
1.430 - CardEpochCacheEntry* epochs = NULL;
1.431 -
1.432 - if (allocate_card_count_cache(cache_size, &counts, &epochs)) {
1.433 - // Allocation was successful.
1.434 - // We can just free the old arrays; we're
1.435 - // not interested in preserving the contents
1.436 - if (_card_counts != NULL) os::free(_card_counts, mtGC);
1.437 - if (_card_epochs != NULL) os::free(_card_epochs, mtGC);
1.438 -
1.439 - // Cache the size of the arrays and the index that got us there.
1.440 - _n_card_counts = cache_size;
1.441 - _cache_size_index = cache_size_idx;
1.442 -
1.443 - _card_counts = counts;
1.444 - _card_epochs = epochs;
1.445 -
1.446 - // We successfully allocated/expanded the caches.
1.447 - return true;
1.448 - }
1.449 - }
1.450 - }
1.451 -
1.452 - // We did not successfully expand the caches.
1.453 - return false;
1.454 -}
1.455 -
1.456 -void ConcurrentG1Refine::clear_and_record_card_counts() {
1.457 - if (G1ConcRSLogCacheSize == 0) {
1.458 - return;
1.459 - }
1.460 -
1.461 - double start = os::elapsedTime();
1.462 -
1.463 - if (_expand_card_counts) {
1.464 - int new_idx = _cache_size_index + 1;
1.465 -
1.466 - if (expand_card_count_cache(new_idx)) {
1.467 - // Allocation was successful and _n_card_counts has
1.468 - // been updated to the new size. We only need to clear
1.469 - // the epochs so we don't read a bogus epoch value
1.470 - // when inserting a card into the hot card cache.
1.471 - Copy::fill_to_bytes(&_card_epochs[0], _n_card_counts * sizeof(CardEpochCacheEntry));
1.472 - }
1.473 - _expand_card_counts = false;
1.474 - }
1.475 -
1.476 - int this_epoch = (int) _n_periods;
1.477 - assert((this_epoch+1) <= max_jint, "to many periods");
1.478 - // Update epoch
1.479 - _n_periods++;
1.480 - double cc_clear_time_ms = (os::elapsedTime() - start) * 1000;
1.481 - _g1h->g1_policy()->phase_times()->record_cc_clear_time_ms(cc_clear_time_ms);
1.482 +int ConcurrentG1Refine::thread_num() {
1.483 + int n_threads = (G1ConcRefinementThreads > 0) ? G1ConcRefinementThreads
1.484 + : ParallelGCThreads;
1.485 + return MAX2<int>(n_threads, 1);
1.486 }
1.487
1.488 void ConcurrentG1Refine::print_worker_threads_on(outputStream* st) const {
