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 /*

 * Copyright (c) 2010, 2011 Oracle and/or its affiliates. All rights reserved.

 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.

 */

 #ifndef SHARE_VM_RUNTIME_ADVANCEDTHRESHOLDPOLICY_HPP

 #define SHARE_VM_RUNTIME_ADVANCEDTHRESHOLDPOLICY_HPP

 #include "runtime/simpleThresholdPolicy.hpp"

 #ifdef TIERED

 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?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 methodOop and for 3->4 transition they come from MDO (since profiled invocations are

  *   counted separately).

  *

  *   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 AdvancedThresholdPolicy : public SimpleThresholdPolicy {

   jlong _start_time;

   // Call and loop predicates determine whether a transition to a higher compilation

   // level should be performed (pointers to predicate functions are passed to common().

   // Predicates also take compiler load into account.

   typedef bool (AdvancedThresholdPolicy::*Predicate)(int i, int b, CompLevel cur_level);

   bool call_predicate(int i, int b, CompLevel cur_level);

   bool loop_predicate(int i, int b, CompLevel cur_level);

   // Common transition function. Given a predicate determines if a method should transition to another level.

   CompLevel common(Predicate p, methodOop method, CompLevel cur_level);

   // Transition functions.

   // call_event determines if a method should be compiled at a different

   // level with a regular invocation entry.

   CompLevel call_event(methodOop method, CompLevel cur_level);

   // loop_event checks if a method should be OSR compiled at a different

   // level.

   CompLevel loop_event(methodOop method, CompLevel cur_level);

   // 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(methodOop 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, methodOop m);

   // Compute the weight of the method for the compilation scheduling

   inline double weight(methodOop method);

   // Apply heuristics and return true if x should be compiled before y

   inline bool compare_methods(methodOop x, methodOop 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, methodOop 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(methodOop method, CompLevel cur_level);

   // Create MDO if necessary.

   void create_mdo(methodHandle mh, TRAPS);

   // Is method profiled enough?

   bool is_method_profiled(methodOop method);

 protected:

   void print_specific(EventType type, methodHandle mh, methodHandle imh, int bci, CompLevel level);

   void set_start_time(jlong t) { _start_time = t;    }

   jlong start_time() const     { return _start_time; }

   // Submit a given method for compilation (and update the rate).

   virtual void submit_compile(methodHandle mh, int bci, CompLevel level, TRAPS);

   // event() from SimpleThresholdPolicy would call these.

   virtual void method_invocation_event(methodHandle method, methodHandle inlinee,

                                        CompLevel level, TRAPS);

   virtual void method_back_branch_event(methodHandle method, methodHandle inlinee,

                                         int bci, CompLevel level, TRAPS);

 public:

   AdvancedThresholdPolicy() : _start_time(0) { }

   // Select task is called by CompileBroker. We should return a task or NULL.

   virtual CompileTask* select_task(CompileQueue* compile_queue);

   virtual void initialize();

 };

 #endif // TIERED

 #endif // SHARE_VM_RUNTIME_ADVANCEDTHRESHOLDPOLICY_HPP