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A comprehensive discussion of the design and implementation of t= he original Jikes RVM adaptive optimization system is given in the OOPSLA 2000 paper by Arnold, Fink, Grove, Hind and Sweene= y. A number of aspects of the system have been changed since 2000, so a bet= ter resource is a technical report Nov.= 2004 technical report that describes the architecture and implementati= on in some detail. This section of the userguide is based on section 5 of t= he 2004 technical report.
The implementation of the Jikes RVM adaptive optimization system uses a = number of Java threads: several organizer threads in the runtime measuremen= ts component, the controller thread, and the compilation thread. The variou= s threads are loosely coupled, communicating with each other through shared= queues and/or the other in memory data structures. All queues in the syste= m are blocking priority queues; if a consumer thread performs a dequeue ope= ration when the queue is empty, it suspends until a producer thread perform= s an enqueue operation.
The adaptive optimization system performs two primary tasks: selective o= ptimization and profile-directed inlining.
The goal of selective optimization is to identify regions of code in whi= ch the application spends significant execution time (often called ``hot sp= ots''), determine if overall application performance is likely to be improv= ed by further optimizing one or more hot spots, and if so to invoke the opt= imizing compiler and install the resulting optimized code in the virtual ma= chine.
In Jikes RVM, the unit of optimization is a method. Thus, to perfo= rm selective optimization, first the runtime measurements component must id= entify candidate methods (``hot methods'') for the controller to consider. = To this end, it installs a listener that periodically samples the currently= executing method at every taken yieldpoint. When it is time to take = a sample, the listener inspects the thread's call stack and records a singl= e compiled method id into a buffer. If the yieldpoint occurs in the prologu= e of a method, then the listener additionally records the compiled method i= d of the current activation's caller. If the taken yieldpoint occurs = on a loop backedge or method epilogue, then the listener records the compil= ed method id of the current method.
When the buffer of samples is full, the sampling window ends. The listen= er then unregisters itself (stops taking samples) and wakes the sleeping Ho= t Method Organizer. The Hot Method Organizer processes the buffer of = compiled method ids by updating the Method Sample Data. This data str= ucture maintains, for every compiled method, the total number of times that= it has been sampled. Careful design of this data structure (MethodCountDat= a.java) was critical to achieving low profiling overhead. In addition to su= pporting lookups and updates by compiled method id, it must also efficientl= y enumerate all methods that have been sampled more times than a (varying) = threshold value. After updating the Method Sample Data, the Hot Method Orga= nizer creates an event for each method that has been sampled in this window= and adds it to the controller's priority queue, using the sample value as = its priority. The event contains the compiled method and the total= number of times it has been sampled since the beginning of execution= . After enqueuing the last event, the Hot Method Organizer re-registe= rs the method listener and then sleeps until the next buffer of samples is = ready to be processed.
When the priority queue delivers an event to the controller, the control= ler dequeues the event and applies the model-driven recompilation policy to= determine what action (if any) to take for the indicated method. If = the controller decides to recompile the method, it creates a recompilation = event that describes the method to be compiled and the optimization plan to= use and places it on the recompilation queue. The recompilation queue prio= ritizes events based on the cost-benefit computation.
When an event is available on the recompilation queue, the recompilation= thread removes it and performs the compilation activity specified by the e= vent. It invokes the optimizing compiler at the specified optimization leve= l and installs the resulting compiled method into the VM.
Although the overall structure of selective optimization in Jikes RVM is= similar to that originally described in Arnold et al's OOPSLA 2000 paper, = we have made several changes and improvements based on further experience w= ith the system. The most significant change is that in the previous system,= the method sample organizer attempted to filter the set of methods it pres= ented to the controller. The organizer passed along to the controller= only methods considered "hot". The organizer deemed a meth= od "hot'' if the percentage of samples attributed to the method exceed= ed a dynamically adjusted threshold value. Method samples were periodically= decayed to give more weight to recent samples. The controller dynamically = adjusted this threshold value and the size of the sampling window in an att= empt to reduce the overhead of processing the samples.
Later, significant algorithmic improvements in key data structures and a=
dditional performance tuning of the listeners, organizers, and
contro= ller reduced AOS overhead by two orders of magnitude. These overhead = reductions obviate the need to filter events passed
to the controller= . This resulted in a more effective system with fewer parameters to t= une and a sounder theoretical basis. In general, as we gained experie= nce with the adaptive system implementation, we strove to reduce the number= of tuning parameters. We believe that the closer the implement= ation matches the basic theoretical cost-benefit model, the more likely it = will perform well and make reasonable and understandable decisions.
Profile-directed inlining attempts to identify frequently traversed call=
graph edges, which represent caller-callee relationships, and determine wh=
ether it is beneficial to recompile the caller methods
to allow inlin= ing of the callee methods. In Jikes RVM, profile-directed inlining augments= a number of static
inlining heuristics. The role of profile-directed= inlining is to identify high cost-high benefit inlining opportunities that= evade the static heuristics and to predict the likely target(s) of invokev= irtual and invokeinterface calls that could not be statically bound at comp= ile time.
To accomplish this goal, the system takes a statistical sample of the me=
thod calls in the running application and maintains an approximation of the=
dynamic call graph based on this data. The system installs a listener that=
samples call edges whenever a yieldpoint is taken in the prologue or epilo=
gue of a method. To sample the call edge, it records the compiled method id=
of the caller and callee methods and the offset of the call instruction in=
the caller's machine code into a buffer. When the buffer of samples is ful=
l, the sampling window ends.
The listener then unregisters itself (st= ops taking samples) and wakes an organizer to update the dynamic call graph= with the new profile data. The optimizing compiler's Inline Oracle uses th= e dynamic call graph to guide it's inline decisions.
The system currently used is based on Arnold & Grove's CGO 2005 pape= r. More details of the sampling scheme and the inlining oracle can be found= there, or in the source code.