/* * Copyright (c) 2001, 2021, Oracle and/or its affiliates. All rights reserved. * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * * 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. *
*/
// The regions are visited in *decreasing* address order. // This order aids with imprecise card marking, where a dirty // card may cause scanning, and summarization marking, of objects // that extend onto subsequent cards. void ClearNoncleanCardWrapper::do_MemRegion(MemRegion mr) {
assert(mr.word_size() > 0, "Error");
assert(_ct->is_aligned(mr.start()), "mr.start() should be card aligned"); // mr.end() may not necessarily be card aligned.
CardValue* cur_entry = _ct->byte_for(mr.last()); const CardValue* limit = _ct->byte_for(mr.start());
HeapWord* end_of_non_clean = mr.end();
HeapWord* start_of_non_clean = end_of_non_clean; while (cur_entry >= limit) {
HeapWord* cur_hw = _ct->addr_for(cur_entry); if ((*cur_entry != CardTableRS::clean_card_val()) && clear_card(cur_entry)) { // Continue the dirty range by opening the // dirty window one card to the left.
start_of_non_clean = cur_hw;
} else { // We hit a "clean" card; process any non-empty // "dirty" range accumulated so far. if (start_of_non_clean < end_of_non_clean) { const MemRegion mrd(start_of_non_clean, end_of_non_clean);
_dirty_card_closure->do_MemRegion(mrd);
}
// fast forward through potential continuous whole-word range of clean cards beginning at a word-boundary if (is_word_aligned(cur_entry)) {
CardValue* cur_row = cur_entry - BytesPerWord; while (cur_row >= limit && *((intptr_t*)cur_row) == CardTableRS::clean_card_row_val()) {
cur_row -= BytesPerWord;
}
cur_entry = cur_row + BytesPerWord;
cur_hw = _ct->addr_for(cur_entry);
}
// Reset the dirty window, while continuing to look // for the next dirty card that will start a // new dirty window.
end_of_non_clean = cur_hw;
start_of_non_clean = cur_hw;
} // Note that "cur_entry" leads "start_of_non_clean" in // its leftward excursion after this point // in the loop and, when we hit the left end of "mr", // will point off of the left end of the card-table // for "mr".
cur_entry--;
} // If the first card of "mr" was dirty, we will have // been left with a dirty window, co-initial with "mr", // which we now process. if (start_of_non_clean < end_of_non_clean) { const MemRegion mrd(start_of_non_clean, end_of_non_clean);
_dirty_card_closure->do_MemRegion(mrd);
}
}
assert(ur.contains(urasm), "Did you forget to call save_marks()? " "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in " "[" PTR_FORMAT ", " PTR_FORMAT ")",
p2i(urasm.start()), p2i(urasm.end()), p2i(ur.start()), p2i(ur.end()));
} #endif
void CardTableRS::clear_into_younger(Generation* old_gen) {
assert(GenCollectedHeap::heap()->is_old_gen(old_gen), "Should only be called for the old generation"); // The card tables for the youngest gen need never be cleared. // There's a bit of subtlety in the clear() and invalidate() // methods that we exploit here and in invalidate_or_clear() // below to avoid missing cards at the fringes. If clear() or // invalidate() are changed in the future, this code should // be revisited. 20040107.ysr
clear(old_gen->prev_used_region());
}
void CardTableRS::invalidate_or_clear(Generation* old_gen) {
assert(GenCollectedHeap::heap()->is_old_gen(old_gen), "Should only be called for the old generation"); // Invalidate the cards for the currently occupied part of // the old generation and clear the cards for the // unoccupied part of the generation (if any, making use // of that generation's prev_used_region to determine that // region). No need to do anything for the youngest // generation. Also see note#20040107.ysr above.
MemRegion used_mr = old_gen->used_region();
MemRegion to_be_cleared_mr = old_gen->prev_used_region().minus(used_mr); if (!to_be_cleared_mr.is_empty()) {
clear(to_be_cleared_mr);
}
invalidate(used_mr);
}
class VerifyCTGenClosure: public GenCollectedHeap::GenClosure {
CardTableRS* _ct; public:
VerifyCTGenClosure(CardTableRS* ct) : _ct(ct) {} void do_generation(Generation* gen) { // Skip the youngest generation. if (GenCollectedHeap::heap()->is_young_gen(gen)) { return;
} // Normally, we're interested in pointers to younger generations.
VerifyCTSpaceClosure blk(_ct, gen->reserved().start());
gen->space_iterate(&blk, true);
}
};
void CardTableRS::verify_space(Space* s, HeapWord* gen_boundary) { // We don't need to do young-gen spaces. if (s->end() <= gen_boundary) return;
MemRegion used = s->used_region();
CardValue* cur_entry = byte_for(used.start());
CardValue* limit = byte_after(used.last()); while (cur_entry < limit) { if (*cur_entry == clean_card_val()) {
CardValue* first_dirty = cur_entry+1; while (first_dirty < limit &&
*first_dirty == clean_card_val()) {
first_dirty++;
} // If the first object is a regular object, and it has a // young-to-old field, that would mark the previous card.
HeapWord* boundary = addr_for(cur_entry);
HeapWord* end = (first_dirty >= limit) ? used.end() : addr_for(first_dirty);
HeapWord* boundary_block = s->block_start(boundary);
HeapWord* begin = boundary; // Until proven otherwise.
HeapWord* start_block = boundary_block; // Until proven otherwise. if (boundary_block < boundary) { if (s->block_is_obj(boundary_block) && s->obj_is_alive(boundary_block)) {
oop boundary_obj = cast_to_oop(boundary_block); if (!boundary_obj->is_objArray() &&
!boundary_obj->is_typeArray()) {
guarantee(cur_entry > byte_for(used.start()), "else boundary would be boundary_block"); if (*byte_for(boundary_block) != clean_card_val()) {
begin = boundary_block + s->block_size(boundary_block);
start_block = begin;
}
}
}
} // Now traverse objects until end. if (begin < end) {
MemRegion mr(begin, end);
VerifyCleanCardClosure verify_blk(gen_boundary, begin, end); for (HeapWord* cur = start_block; cur < end; cur += s->block_size(cur)) { if (s->block_is_obj(cur) && s->obj_is_alive(cur)) {
cast_to_oop(cur)->oop_iterate(&verify_blk, mr);
}
}
}
cur_entry = first_dirty;
} else { // We'd normally expect that cur_youngergen_and_prev_nonclean_card // is a transient value, that cannot be in the card table // except during GC, and thus assert that: // guarantee(*cur_entry != cur_youngergen_and_prev_nonclean_card, // "Illegal CT value"); // That however, need not hold, as will become clear in the // following...
// We'd normally expect that if we are in the parallel case, // we can't have left a prev value (which would be different // from the current value) in the card table, and so we'd like to // assert that: // guarantee(cur_youngergen_card_val() == youngergen_card // || !is_prev_youngergen_card_val(*cur_entry), // "Illegal CT value"); // That, however, may not hold occasionally, because of // CMS or MSC in the old gen. To wit, consider the // following two simple illustrative scenarios: // (a) CMS: Consider the case where a large object L // spanning several cards is allocated in the old // gen, and has a young gen reference stored in it, dirtying // some interior cards. A young collection scans the card, // finds a young ref and installs a youngergenP_n value. // L then goes dead. Now a CMS collection starts, // finds L dead and sweeps it up. Assume that L is // abutting _unallocated_blk, so _unallocated_blk is // adjusted down to (below) L. Assume further that // no young collection intervenes during this CMS cycle. // The next young gen cycle will not get to look at this // youngergenP_n card since it lies in the unoccupied // part of the space. // Some young collections later the blocks on this // card can be re-allocated either due to direct allocation // or due to absorbing promotions. At this time, the // before-gc verification will fail the above assert. // (b) MSC: In this case, an object L with a young reference // is on a card that (therefore) holds a youngergen_n value. // Suppose also that L lies towards the end of the used // the used space before GC. An MSC collection // occurs that compacts to such an extent that this // card is no longer in the occupied part of the space. // Since current code in MSC does not always clear cards // in the unused part of old gen, this stale youngergen_n // value is left behind and can later be covered by // an object when promotion or direct allocation // re-allocates that part of the heap. // // Fortunately, the presence of such stale card values is // "only" a minor annoyance in that subsequent young collections // might needlessly scan such cards, but would still never corrupt // the heap as a result. However, it's likely not to be a significant // performance inhibitor in practice. For instance, // some recent measurements with unoccupied cards eagerly cleared // out to maintain this invariant, showed next to no // change in young collection times; of course one can construct // degenerate examples where the cost can be significant.) // Note, in particular, that if the "stale" card is modified // after re-allocation, it would be dirty, not "stale". Thus, // we can never have a younger ref in such a card and it is // safe not to scan that card in any collection. [As we see // below, we do some unnecessary scanning // in some cases in the current parallel scanning algorithm.] // // The main point below is that the parallel card scanning code // deals correctly with these stale card values. There are two main // cases to consider where we have a stale "young gen" value and a // "derivative" case to consider, where we have a stale // "cur_younger_gen_and_prev_non_clean" value, as will become // apparent in the case analysis below. // o Case 1. If the stale value corresponds to a younger_gen_n // value other than the cur_younger_gen value then the code // treats this as being tantamount to a prev_younger_gen // card. This means that the card may be unnecessarily scanned. // There are two sub-cases to consider: // o Case 1a. Let us say that the card is in the occupied part // of the generation at the time the collection begins. In // that case the card will be either cleared when it is scanned // for young pointers, or will be set to cur_younger_gen as a // result of promotion. (We have elided the normal case where // the scanning thread and the promoting thread interleave // possibly resulting in a transient // cur_younger_gen_and_prev_non_clean value before settling // to cur_younger_gen. [End Case 1a.] // o Case 1b. Consider now the case when the card is in the unoccupied // part of the space which becomes occupied because of promotions // into it during the current young GC. In this case the card // will never be scanned for young references. The current // code will set the card value to either // cur_younger_gen_and_prev_non_clean or leave // it with its stale value -- because the promotions didn't // result in any younger refs on that card. Of these two // cases, the latter will be covered in Case 1a during // a subsequent scan. To deal with the former case, we need // to further consider how we deal with a stale value of // cur_younger_gen_and_prev_non_clean in our case analysis // below. This we do in Case 3 below. [End Case 1b] // [End Case 1] // o Case 2. If the stale value corresponds to cur_younger_gen being // a value not necessarily written by a current promotion, the // card will not be scanned by the younger refs scanning code. // (This is OK since as we argued above such cards cannot contain // any younger refs.) The result is that this value will be // treated as a prev_younger_gen value in a subsequent collection, // which is addressed in Case 1 above. [End Case 2] // o Case 3. We here consider the "derivative" case from Case 1b. above // because of which we may find a stale // cur_younger_gen_and_prev_non_clean card value in the table. // Once again, as in Case 1, we consider two subcases, depending // on whether the card lies in the occupied or unoccupied part // of the space at the start of the young collection. // o Case 3a. Let us say the card is in the occupied part of // the old gen at the start of the young collection. In that // case, the card will be scanned by the younger refs scanning // code which will set it to cur_younger_gen. In a subsequent // scan, the card will be considered again and get its final // correct value. [End Case 3a] // o Case 3b. Now consider the case where the card is in the // unoccupied part of the old gen, and is occupied as a result // of promotions during thus young gc. In that case, // the card will not be scanned for younger refs. The presence // of newly promoted objects on the card will then result in // its keeping the value cur_younger_gen_and_prev_non_clean // value, which we have dealt with in Case 3 here. [End Case 3b] // [End Case 3] // // (Please refer to the code in the helper class // ClearNonCleanCardWrapper and in CardTable for details.) // // The informal arguments above can be tightened into a formal // correctness proof and it behooves us to write up such a proof, // or to use model checking to prove that there are no lingering // concerns. // // Clearly because of Case 3b one cannot bound the time for // which a card will retain what we have called a "stale" value. // However, one can obtain a Loose upper bound on the redundant // work as a result of such stale values. Note first that any // time a stale card lies in the occupied part of the space at // the start of the collection, it is scanned by younger refs // code and we can define a rank function on card values that // declines when this is so. Note also that when a card does not // lie in the occupied part of the space at the beginning of a // young collection, its rank can either decline or stay unchanged. // In this case, no extra work is done in terms of redundant // younger refs scanning of that card. // Then, the case analysis above reveals that, in the worst case, // any such stale card will be scanned unnecessarily at most twice. // // It is nonetheless advisable to try and get rid of some of this // redundant work in a subsequent (low priority) re-design of // the card-scanning code, if only to simplify the underlying // state machine analysis/proof. ysr 1/28/2002. XXX
cur_entry++;
}
}
}
void CardTableRS::verify() { // At present, we only know how to verify the card table RS for // generational heaps.
VerifyCTGenClosure blk(this);
GenCollectedHeap::heap()->generation_iterate(&blk, false);
}
Die Informationen auf dieser Webseite wurden
nach bestem Wissen sorgfältig zusammengestellt. Es wird jedoch weder Vollständigkeit, noch Richtigkeit,
noch Qualität der bereit gestellten Informationen zugesichert.
Bemerkung:
Die farbliche Syntaxdarstellung ist noch experimentell.
