--- a/src/hotspot/share/runtime/simpleThresholdPolicy.hpp Wed May 09 07:48:31 2018 +0100
+++ b/src/hotspot/share/runtime/simpleThresholdPolicy.hpp Wed May 09 09:39:25 2018 +0200
@@ -34,8 +34,136 @@
class CompileTask;
class CompileQueue;
+/*
+ * The system supports 5 execution levels:
+ * * level 0 - interpreter
+ * * level 1 - C1 with full optimization (no profiling)
+ * * level 2 - C1 with invocation and backedge counters
+ * * level 3 - C1 with full profiling (level 2 + MDO)
+ * * level 4 - C2
+ *
+ * Levels 0, 2 and 3 periodically notify the runtime about the current value of the counters
+ * (invocation counters and backedge counters). The frequency of these notifications is
+ * different at each level. These notifications are used by the policy to decide what transition
+ * to make.
+ *
+ * Execution starts at level 0 (interpreter), then the policy can decide either to compile the
+ * method at level 3 or level 2. The decision is based on the following factors:
+ * 1. The length of the C2 queue determines the next level. The observation is that level 2
+ * is generally faster than level 3 by about 30%, therefore we would want to minimize the time
+ * a method spends at level 3. We should only spend the time at level 3 that is necessary to get
+ * adequate profiling. So, if the C2 queue is long enough it is more beneficial to go first to
+ * level 2, because if we transitioned to level 3 we would be stuck there until our C2 compile
+ * request makes its way through the long queue. When the load on C2 recedes we are going to
+ * recompile at level 3 and start gathering profiling information.
+ * 2. The length of C1 queue is used to dynamically adjust the thresholds, so as to introduce
+ * additional filtering if the compiler is overloaded. The rationale is that by the time a
+ * method gets compiled it can become unused, so it doesn't make sense to put too much onto the
+ * queue.
+ *
+ * After profiling is completed at level 3 the transition is made to level 4. Again, the length
+ * of the C2 queue is used as a feedback to adjust the thresholds.
+ *
+ * After the first C1 compile some basic information is determined about the code like the number
+ * of the blocks and the number of the loops. Based on that it can be decided that a method
+ * is trivial and compiling it with C1 will yield the same code. In this case the method is
+ * compiled at level 1 instead of 4.
+ *
+ * We also support profiling at level 0. If C1 is slow enough to produce the level 3 version of
+ * the code and the C2 queue is sufficiently small we can decide to start profiling in the
+ * interpreter (and continue profiling in the compiled code once the level 3 version arrives).
+ * If the profiling at level 0 is fully completed before level 3 version is produced, a level 2
+ * version is compiled instead in order to run faster waiting for a level 4 version.
+ *
+ * Compile queues are implemented as priority queues - for each method in the queue we compute
+ * the event rate (the number of invocation and backedge counter increments per unit of time).
+ * When getting an element off the queue we pick the one with the largest rate. Maintaining the
+ * rate also allows us to remove stale methods (the ones that got on the queue but stopped
+ * being used shortly after that).
+*/
+
+/* Command line options:
+ * - Tier?InvokeNotifyFreqLog and Tier?BackedgeNotifyFreqLog control the frequency of method
+ * invocation and backedge notifications. Basically every n-th invocation or backedge a mutator thread
+ * makes a call into the runtime.
+ *
+ * - Tier?InvocationThreshold, Tier?CompileThreshold, Tier?BackEdgeThreshold, Tier?MinInvocationThreshold control
+ * compilation thresholds.
+ * Level 2 thresholds are not used and are provided for option-compatibility and potential future use.
+ * Other thresholds work as follows:
+ *
+ * Transition from interpreter (level 0) to C1 with full profiling (level 3) happens when
+ * the following predicate is true (X is the level):
+ *
+ * i > TierXInvocationThreshold * s || (i > TierXMinInvocationThreshold * s && i + b > TierXCompileThreshold * s),
+ *
+ * where $i$ is the number of method invocations, $b$ number of backedges and $s$ is the scaling
+ * coefficient that will be discussed further.
+ * The intuition is to equalize the time that is spend profiling each method.
+ * The same predicate is used to control the transition from level 3 to level 4 (C2). It should be
+ * noted though that the thresholds are relative. Moreover i and b for the 0->3 transition come
+ * from Method* and for 3->4 transition they come from MDO (since profiled invocations are
+ * counted separately). Finally, if a method does not contain anything worth profiling, a transition
+ * from level 3 to level 4 occurs without considering thresholds (e.g., with fewer invocations than
+ * what is specified by Tier4InvocationThreshold).
+ *
+ * OSR transitions are controlled simply with b > TierXBackEdgeThreshold * s predicates.
+ *
+ * - Tier?LoadFeedback options are used to automatically scale the predicates described above depending
+ * on the compiler load. The scaling coefficients are computed as follows:
+ *
+ * s = queue_size_X / (TierXLoadFeedback * compiler_count_X) + 1,
+ *
+ * where queue_size_X is the current size of the compiler queue of level X, and compiler_count_X
+ * is the number of level X compiler threads.
+ *
+ * Basically these parameters describe how many methods should be in the compile queue
+ * per compiler thread before the scaling coefficient increases by one.
+ *
+ * This feedback provides the mechanism to automatically control the flow of compilation requests
+ * depending on the machine speed, mutator load and other external factors.
+ *
+ * - Tier3DelayOn and Tier3DelayOff parameters control another important feedback loop.
+ * Consider the following observation: a method compiled with full profiling (level 3)
+ * is about 30% slower than a method at level 2 (just invocation and backedge counters, no MDO).
+ * Normally, the following transitions will occur: 0->3->4. The problem arises when the C2 queue
+ * gets congested and the 3->4 transition is delayed. While the method is the C2 queue it continues
+ * executing at level 3 for much longer time than is required by the predicate and at suboptimal speed.
+ * The idea is to dynamically change the behavior of the system in such a way that if a substantial
+ * load on C2 is detected we would first do the 0->2 transition allowing a method to run faster.
+ * And then when the load decreases to allow 2->3 transitions.
+ *
+ * Tier3Delay* parameters control this switching mechanism.
+ * Tier3DelayOn is the number of methods in the C2 queue per compiler thread after which the policy
+ * no longer does 0->3 transitions but does 0->2 transitions instead.
+ * Tier3DelayOff switches the original behavior back when the number of methods in the C2 queue
+ * per compiler thread falls below the specified amount.
+ * The hysteresis is necessary to avoid jitter.
+ *
+ * - TieredCompileTaskTimeout is the amount of time an idle method can spend in the compile queue.
+ * Basically, since we use the event rate d(i + b)/dt as a value of priority when selecting a method to
+ * compile from the compile queue, we also can detect stale methods for which the rate has been
+ * 0 for some time in the same iteration. Stale methods can appear in the queue when an application
+ * abruptly changes its behavior.
+ *
+ * - TieredStopAtLevel, is used mostly for testing. It allows to bypass the policy logic and stick
+ * to a given level. For example it's useful to set TieredStopAtLevel = 1 in order to compile everything
+ * with pure c1.
+ *
+ * - Tier0ProfilingStartPercentage allows the interpreter to start profiling when the inequalities in the
+ * 0->3 predicate are already exceeded by the given percentage but the level 3 version of the
+ * method is still not ready. We can even go directly from level 0 to 4 if c1 doesn't produce a compiled
+ * version in time. This reduces the overall transition to level 4 and decreases the startup time.
+ * Note that this behavior is also guarded by the Tier3Delay mechanism: when the c2 queue is too long
+ * these is not reason to start profiling prematurely.
+ *
+ * - TieredRateUpdateMinTime and TieredRateUpdateMaxTime are parameters of the rate computation.
+ * Basically, the rate is not computed more frequently than TieredRateUpdateMinTime and is considered
+ * to be zero if no events occurred in TieredRateUpdateMaxTime.
+ */
class SimpleThresholdPolicy : public CompilationPolicy {
+ jlong _start_time;
int _c1_count, _c2_count;
// Check if the counter is big enough and set carry (effectively infinity).
@@ -49,7 +177,7 @@
bool call_predicate(int i, int b, CompLevel cur_level, Method* method);
bool loop_predicate(int i, int b, CompLevel cur_level, Method* method);
// Common transition function. Given a predicate determines if a method should transition to another level.
- CompLevel common(Predicate p, Method* method, CompLevel cur_level);
+ CompLevel common(Predicate p, Method* method, CompLevel cur_level, bool disable_feedback = false);
// Transition functions.
// call_event determines if a method should be compiled at a different
// level with a regular invocation entry.
@@ -58,6 +186,35 @@
// level.
CompLevel loop_event(Method* method, CompLevel cur_level, JavaThread* thread);
void print_counters(const char* prefix, const methodHandle& mh);
+ // Has a method been long around?
+ // We don't remove old methods from the compile queue even if they have
+ // very low activity (see select_task()).
+ inline bool is_old(Method* method);
+ // Was a given method inactive for a given number of milliseconds.
+ // If it is, we would remove it from the queue (see select_task()).
+ inline bool is_stale(jlong t, jlong timeout, Method* m);
+ // Compute the weight of the method for the compilation scheduling
+ inline double weight(Method* method);
+ // Apply heuristics and return true if x should be compiled before y
+ inline bool compare_methods(Method* x, Method* y);
+ // Compute event rate for a given method. The rate is the number of event (invocations + backedges)
+ // per millisecond.
+ inline void update_rate(jlong t, Method* m);
+ // Compute threshold scaling coefficient
+ inline double threshold_scale(CompLevel level, int feedback_k);
+ // If a method is old enough and is still in the interpreter we would want to
+ // start profiling without waiting for the compiled method to arrive. This function
+ // determines whether we should do that.
+ inline bool should_create_mdo(Method* method, CompLevel cur_level);
+ // Create MDO if necessary.
+ void create_mdo(const methodHandle& mh, JavaThread* thread);
+ // Is method profiled enough?
+ bool is_method_profiled(Method* method);
+
+ double _increase_threshold_at_ratio;
+
+ bool maybe_switch_to_aot(const methodHandle& mh, CompLevel cur_level, CompLevel next_level, JavaThread* thread);
+
protected:
int c1_count() const { return _c1_count; }
int c2_count() const { return _c2_count; }
@@ -67,7 +224,7 @@
enum EventType { CALL, LOOP, COMPILE, REMOVE_FROM_QUEUE, UPDATE_IN_QUEUE, REPROFILE, MAKE_NOT_ENTRANT };
void print_event(EventType type, const methodHandle& mh, const methodHandle& imh, int bci, CompLevel level);
// Print policy-specific information if necessary
- virtual void print_specific(EventType type, const methodHandle& mh, const methodHandle& imh, int bci, CompLevel level) { }
+ virtual void print_specific(EventType type, const methodHandle& mh, const methodHandle& imh, int bci, CompLevel level);
// Check if the method can be compiled, change level if necessary
void compile(const methodHandle& mh, int bci, CompLevel level, JavaThread* thread);
// Submit a given method for compilation
@@ -87,8 +244,13 @@
CompLevel level, CompiledMethod* nm, JavaThread* thread);
virtual void method_back_branch_event(const methodHandle& method, const methodHandle& inlinee,
int bci, CompLevel level, CompiledMethod* nm, JavaThread* thread);
+
+ void set_increase_threshold_at_ratio() { _increase_threshold_at_ratio = 100 / (100 - (double)IncreaseFirstTierCompileThresholdAt); }
+ void set_start_time(jlong t) { _start_time = t; }
+ jlong start_time() const { return _start_time; }
+
public:
- SimpleThresholdPolicy() : _c1_count(0), _c2_count(0) { }
+ SimpleThresholdPolicy() : _start_time(0), _c1_count(0), _c2_count(0) { }
virtual int compiler_count(CompLevel comp_level) {
if (is_c1_compile(comp_level)) return c1_count();
if (is_c2_compile(comp_level)) return c2_count();
@@ -107,11 +269,7 @@
virtual bool is_mature(Method* method);
// Initialize: set compiler thread count
virtual void initialize();
- virtual bool should_not_inline(ciEnv* env, ciMethod* callee) {
- return (env->comp_level() == CompLevel_limited_profile ||
- env->comp_level() == CompLevel_full_profile) &&
- callee->has_loops();
- }
+ virtual bool should_not_inline(ciEnv* env, ciMethod* callee);
};
#endif // TIERED