PROFILING

THE ROLE OF PROFILINGTYPES OF PROFILE COLLECTING PROFILES PROFILING DURING INTERPRETATIONPROFILING TRANSLATED CODE PROFILING OVERHEAD

1. THE ROLE OF PROFILINGProfiling is the process of collecting instruction and data statistics for an executing program

This profile can be used as input to the code optimization

A Short code sequence called probes that records execution information into a profile log, is inserted by the compiler

Compiler uses the information in the profile log to generate optimized code

Probes may be optimally placed based on the program structure

In Dynamic optimization, the program structure is not known

Due to the dynamic nature, probes cannot be set early and profiling becomes more predictive

Optimization decisions should be made based on information from a partial execution of the program

2. TYPES OF PROFILE

There are several types of profile used in VMs

The First type indicates how frequently different code regions are being executed

The Second type is based on control flow predictability

The Third type is helpful in specific optimization i.e., profile about program variables holding same value most of the time

The first and second type of profile are implemented as Node profile and Edge profile respectively

3. COLLECTING PROFILESThere are two ways of collecting a profile, i) Instrumentation based profiling It targets specific program related events and counts all instances of the events being profiled Instrumentation can be divided into

•Hardware Instrumentation•Software Instrumentation

ii) Sampling based profiling In this method, at fixed or random interval the program is interrupted and an instance of a program related event is captured Various samples are made at various intervalsA statistical profile is made ready from the samplesDifferent types of counters may be used in sampling

4. PROFILING DURING INTERPRETATION When profiling during interpretation, the source instruction is accessed as data and the interpreter routines are the code that is being executed

Profiling code must also be added to interpreter routines

To perform basic block profiling during interpretation, profiling code must be added to all control transfer statements (branch and jump) because it begins a new basic block

To perform edge profiling, the same control transfer statements can be profiled but both the PC of the control transfer statements and the target are used to define a specific edge

The Profile data is kept in a table that is accessed via a source PC values of control transfer statements

Disadvantage: Profile Counter Decaying

5. PROFILING TRANSLATED CODE When profiling a translated code, each instruction can have its own custom profile

code Profile counters can be directly assigned to translated instructions selectively No requirement of Hashing function and Hash value Profile counters can be maintained in a form of an array

6. PROFILING OVERHEADIn general for accessing hash table we will require at least one load and store operationFor incrementing profile counters two memory accesses and add operation is requiredProfiling during interpretation increases the overhead by 10-20%Apart from performance overhead profiling leads to memory overhead for maintaining tables

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