JDK-6821693 : 64-bit TaskQueue capacity still too small
  • Type: Bug
  • Status: Closed
  • Resolution: Fixed
  • Component: hotspot
  • Sub-Component: gc
  • Priority: P2
  • Affected Version: hs15
  • OS: generic
  • CPU: generic
  • Submit Date: 2009-03-24
  • Updated Date: 2010-12-08
  • Resolved Date: 2010-01-15
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6u18Resolved 7Fixed
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Description
The fix for 6787254 intended to increase the capacity of TaskQueues in 64-bit builds by increasing the size of the TaskQueue::Age struct to 64 bits.  A typo prevented the change from taking effect.

Comments
EVALUATION http://hg.openjdk.java.net/jdk7/hotspot/hotspot/rev/3ee342e25e57
2009-08-10

SUGGESTED FIX changeset: 884:3ee342e25e57 tag: tip user: jcoomes date: Wed Aug 05 12:33:29 2009 -0700 summary: 6821693: 64-bit TaskQueue capacity still too small diff -r 703065c670fa -r 3ee342e25e57 src/share/vm/utilities/taskqueue.hpp --- a/src/share/vm/utilities/taskqueue.hpp Wed Aug 05 18:54:12 2009 -0700 +++ b/src/share/vm/utilities/taskqueue.hpp Wed Aug 05 12:33:29 2009 -0700 @@ -22,94 +22,90 @@ * */ -#ifdef LP64 -typedef juint TAG_TYPE; -// for a taskqueue size of 4M -#define LOG_TASKQ_SIZE 22 -#else -typedef jushort TAG_TYPE; -// for a taskqueue size of 16K -#define LOG_TASKQ_SIZE 14 -#endif - class TaskQueueSuper: public CHeapObj { protected: - // The first free element after the last one pushed (mod _n). + // Internal type for indexing the queue; also used for the tag. + typedef NOT_LP64(uint16_t) LP64_ONLY(uint32_t) idx_t; + + // The first free element after the last one pushed (mod N). volatile uint _bottom; - // log2 of the size of the queue. - enum SomeProtectedConstants { - Log_n = LOG_TASKQ_SIZE + enum { + N = 1 << NOT_LP64(14) LP64_ONLY(17), // Queue size: 16K or 128K + MOD_N_MASK = N - 1 // To compute x mod N efficiently. }; -#undef LOG_TASKQ_SIZE - // Size of the queue. - uint n() { return (1 << Log_n); } - // For computing "x mod n" efficiently. - uint n_mod_mask() { return n() - 1; } + class Age { + public: + Age(size_t data = 0) { _data = data; } + Age(const Age& age) { _data = age._data; } + Age(idx_t top, idx_t tag) { _fields._top = top; _fields._tag = tag; } - struct Age { - TAG_TYPE _top; - TAG_TYPE _tag; + Age get() const volatile { return _data; } + void set(Age age) volatile { _data = age._data; } - TAG_TYPE tag() const { return _tag; } - TAG_TYPE top() const { return _top; } + idx_t top() const volatile { return _fields._top; } + idx_t tag() const volatile { return _fields._tag; } - Age() { _tag = 0; _top = 0; } + // Increment top; if it wraps, increment tag also. + void increment() { + _fields._top = increment_index(_fields._top); + if (_fields._top == 0) ++_fields._tag; + } - friend bool operator ==(const Age& a1, const Age& a2) { - return a1.tag() == a2.tag() && a1.top() == a2.top(); + Age cmpxchg(const Age new_age, const Age old_age) volatile { + return (size_t) Atomic::cmpxchg_ptr((intptr_t)new_age._data, + (volatile intptr_t *)&_data, + (intptr_t)old_age._data); } + + bool operator ==(const Age& other) const { return _data == other._data; } + + private: + struct fields { + idx_t _top; + idx_t _tag; + }; + union { + size_t _data; + fields _fields; + }; }; - Age _age; - // These make sure we do single atomic reads and writes. - Age get_age() { - uint res = *(volatile uint*)(&_age); - return *(Age*)(&res); + + volatile Age _age; + + // These both operate mod N. + static uint increment_index(uint ind) { + return (ind + 1) & MOD_N_MASK; } - void set_age(Age a) { - *(volatile uint*)(&_age) = *(uint*)(&a); + static uint decrement_index(uint ind) { + return (ind - 1) & MOD_N_MASK; } - TAG_TYPE get_top() { - return get_age().top(); - } - - // These both operate mod _n. - uint increment_index(uint ind) { - return (ind + 1) & n_mod_mask(); - } - uint decrement_index(uint ind) { - return (ind - 1) & n_mod_mask(); - } - - // Returns a number in the range [0.._n). If the result is "n-1", it - // should be interpreted as 0. + // Returns a number in the range [0..N). If the result is "N-1", it should be + // interpreted as 0. uint dirty_size(uint bot, uint top) { - return ((int)bot - (int)top) & n_mod_mask(); + return (bot - top) & MOD_N_MASK; } // Returns the size corresponding to the given "bot" and "top". uint size(uint bot, uint top) { uint sz = dirty_size(bot, top); - // Has the queue "wrapped", so that bottom is less than top? - // There's a complicated special case here. A pair of threads could - // perform pop_local and pop_global operations concurrently, starting - // from a state in which _bottom == _top+1. The pop_local could - // succeed in decrementing _bottom, and the pop_global in incrementing - // _top (in which case the pop_global will be awarded the contested - // queue element.) The resulting state must be interpreted as an empty - // queue. (We only need to worry about one such event: only the queue - // owner performs pop_local's, and several concurrent threads - // attempting to perform the pop_global will all perform the same CAS, - // and only one can succeed. Any stealing thread that reads after - // either the increment or decrement will see an empty queue, and will - // not join the competitors. The "sz == -1 || sz == _n-1" state will - // not be modified by concurrent queues, so the owner thread can reset - // the state to _bottom == top so subsequent pushes will be performed - // normally. - if (sz == (n()-1)) return 0; - else return sz; + // Has the queue "wrapped", so that bottom is less than top? There's a + // complicated special case here. A pair of threads could perform pop_local + // and pop_global operations concurrently, starting from a state in which + // _bottom == _top+1. The pop_local could succeed in decrementing _bottom, + // and the pop_global in incrementing _top (in which case the pop_global + // will be awarded the contested queue element.) The resulting state must + // be interpreted as an empty queue. (We only need to worry about one such + // event: only the queue owner performs pop_local's, and several concurrent + // threads attempting to perform the pop_global will all perform the same + // CAS, and only one can succeed.) Any stealing thread that reads after + // either the increment or decrement will see an empty queue, and will not + // join the competitors. The "sz == -1 || sz == N-1" state will not be + // modified by concurrent queues, so the owner thread can reset the state to + // _bottom == top so subsequent pushes will be performed normally. + return (sz == N - 1) ? 0 : sz; } public: @@ -122,22 +118,21 @@ // The "careful" version admits the possibility of pop_local/pop_global // races. uint size() { - return size(_bottom, get_top()); + return size(_bottom, _age.top()); } uint dirty_size() { - return dirty_size(_bottom, get_top()); + return dirty_size(_bottom, _age.top()); } void set_empty() { _bottom = 0; - _age = Age(); + _age.set(0); } // Maximum number of elements allowed in the queue. This is two less // than the actual queue size, for somewhat complicated reasons. - uint max_elems() { return n() - 2; } - + uint max_elems() { return N - 2; } }; template<class E> class GenericTaskQueue: public TaskQueueSuper { @@ -179,12 +174,12 @@ template<class E> GenericTaskQueue<E>::GenericTaskQueue():TaskQueueSuper() { - assert(sizeof(Age) == sizeof(int), "Depends on this."); + assert(sizeof(Age) == sizeof(size_t), "Depends on this."); } template<class E> void GenericTaskQueue<E>::initialize() { - _elems = NEW_C_HEAP_ARRAY(E, n()); + _elems = NEW_C_HEAP_ARRAY(E, N); guarantee(_elems != NULL, "Allocation failed."); } @@ -208,14 +203,14 @@ template<class E> bool GenericTaskQueue<E>::push_slow(E t, uint dirty_n_elems) { - if (dirty_n_elems == n() - 1) { + if (dirty_n_elems == N - 1) { // Actually means 0, so do the push. uint localBot = _bottom; _elems[localBot] = t; _bottom = increment_index(localBot); return true; - } else - return false; + } + return false; } template<class E> @@ -230,53 +225,45 @@ // then have the owner thread do a pop followed by another push. Without // the incrementing of "tag", the pop_global's CAS could succeed, // allowing it to believe it has claimed the stale element.) - Age newAge; - newAge._top = localBot; - newAge._tag = oldAge.tag() + 1; + Age newAge((idx_t)localBot, oldAge.tag() + 1); // Perhaps a competing pop_global has already incremented "top", in which // case it wins the element. if (localBot == oldAge.top()) { - Age tempAge; // No competing pop_global has yet incremented "top"; we'll try to // install new_age, thus claiming the element. - assert(sizeof(Age) == sizeof(int), "Assumption about CAS unit."); - *(uint*)&tempAge = Atomic::cmpxchg(*(uint*)&newAge, (volatile uint*)&_age, *(uint*)&oldAge); + Age tempAge = _age.cmpxchg(newAge, oldAge); if (tempAge == oldAge) { // We win. - assert(dirty_size(localBot, get_top()) != n() - 1, - "Shouldn't be possible..."); + assert(dirty_size(localBot, _age.top()) != N - 1, "sanity"); return true; } } - // We fail; a completing pop_global gets the element. But the queue is - // empty (and top is greater than bottom.) Fix this representation of - // the empty queue to become the canonical one. - set_age(newAge); - assert(dirty_size(localBot, get_top()) != n() - 1, - "Shouldn't be possible..."); + // We lose; a completing pop_global gets the element. But the queue is empty + // and top is greater than bottom. Fix this representation of the empty queue + // to become the canonical one. + _age.set(newAge); + assert(dirty_size(localBot, _age.top()) != N - 1, "sanity"); return false; } template<class E> bool GenericTaskQueue<E>::pop_global(E& t) { - Age newAge; - Age oldAge = get_age(); + Age oldAge = _age.get(); uint localBot = _bottom; uint n_elems = size(localBot, oldAge.top()); if (n_elems == 0) { return false; } + t = _elems[oldAge.top()]; - newAge = oldAge; - newAge._top = increment_index(newAge.top()); - if ( newAge._top == 0 ) newAge._tag++; - Age resAge; - *(uint*)&resAge = Atomic::cmpxchg(*(uint*)&newAge, (volatile uint*)&_age, *(uint*)&oldAge); + Age newAge(oldAge); + newAge.increment(); + Age resAge = _age.cmpxchg(newAge, oldAge); + // Note that using "_bottom" here might fail, since a pop_local might // have decremented it. - assert(dirty_size(localBot, newAge._top) != n() - 1, - "Shouldn't be possible..."); - return (resAge == oldAge); + assert(dirty_size(localBot, newAge.top()) != N - 1, "sanity"); + return resAge == oldAge; } template<class E> @@ -459,7 +446,7 @@ return offer_termination(NULL); } - // As above, but it also terminates of the should_exit_termination() + // As above, but it also terminates if the should_exit_termination() // method of the terminator parameter returns true. If terminator is // NULL, then it is ignored. bool offer_termination(TerminatorTerminator* terminator); @@ -492,11 +479,10 @@ } #else uint localBot = _bottom; - assert((localBot >= 0) && (localBot < n()), "_bottom out of range."); - TAG_TYPE top = get_top(); + assert((localBot >= 0) && (localBot < N), "_bottom out of range."); + idx_t top = _age.top(); uint dirty_n_elems = dirty_size(localBot, top); - assert((dirty_n_elems >= 0) && (dirty_n_elems < n()), - "n_elems out of range."); + assert((dirty_n_elems >= 0) && (dirty_n_elems < N), "n_elems out of range."); if (dirty_n_elems < max_elems()) { _elems[localBot] = t; _bottom = increment_index(localBot); @@ -517,12 +503,12 @@ return true; #else uint localBot = _bottom; - // This value cannot be n-1. That can only occur as a result of + // This value cannot be N-1. That can only occur as a result of // the assignment to bottom in this method. If it does, this method // resets the size( to 0 before the next call (which is sequential, // since this is pop_local.) - uint dirty_n_elems = dirty_size(localBot, get_top()); - assert(dirty_n_elems != n() - 1, "Shouldn't be possible..."); + uint dirty_n_elems = dirty_size(localBot, _age.top()); + assert(dirty_n_elems != N - 1, "Shouldn't be possible..."); if (dirty_n_elems == 0) return false; localBot = decrement_index(localBot); _bottom = localBot; @@ -534,15 +520,14 @@ // If there's still at least one element in the queue, based on the // "_bottom" and "age" we've read, then there can be no interference with // a "pop_global" operation, and we're done. - TAG_TYPE tp = get_top(); // XXX + idx_t tp = _age.top(); // XXX if (size(localBot, tp) > 0) { - assert(dirty_size(localBot, tp) != n() - 1, - "Shouldn't be possible..."); + assert(dirty_size(localBot, tp) != N - 1, "sanity"); return true; } else { // Otherwise, the queue contained exactly one element; we take the slow // path. - return pop_local_slow(localBot, get_age()); + return pop_local_slow(localBot, _age.get()); } #endif }
2009-08-06

EVALUATION http://hg.openjdk.java.net/jdk7/hotspot-gc/hotspot/rev/3ee342e25e57
2009-08-06

EVALUATION The Age class should be better encapsulated so that TaskQueue* methods do not access fields directly, but use accessors which have the necessary attributes (e.g., volatile). Adding a cmpxchg method to Age would simplify TaskQueue* methods and eliminate many of the casts. Because the Age struct is now even larger in 64-bit builds, the fix for alignment of the Age struct from 6821507 must be done simultaneously. That turns out to further simplify the code.
2009-04-07

EVALUATION In addition to the _LP64 typo, there are several places which still access the Age struct as if were always 32-bits wide by dereferencing through an unsigned int* or volatile unsigned int*. The main examples are the get_age() and set_age() methods in TaskQueueSuper, but there are also calls to Atomic::cmpxchg() and asserts that must be updated to use size_t or another appropriate type.
2009-03-25

EVALUATION The following code defines the type used for the two member fields of the Age struct: #ifdef LP64 typedef juint TAG_TYPE; // for a taskqueue size of 4M #define LOG_TASKQ_SIZE 22 #else typedef jushort TAG_TYPE; // for a taskqueue size of 16K #define LOG_TASKQ_SIZE 14 #endif The LP64 should be _LP64. Because of this typo, the larger data type is never used, so Age remains a 32-bit struct even in the 64-bit VM and the intended increase in task queue size for the 64-bit VM never took effect.
2009-03-25