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tgp is a task-granularity profiler for multi-threaded, task-parallel applications running on the Java Virtual Machine (JVM).
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README.md

tgp

tgp is a task-granularity profiler for multi-threaded, task-parallel applications running on the Java Virtual Machine (JVM).

tgp detects each task created by an application, collecting its granularity either in terms of bytecode count, i.e., the number of bytecode instructions executed, or reference-cycles count, i.e., the number of cycles elapsed during task execution, collected at the nominal frequency of the CPU (even in case of frequency scaling).

tgp assists developers in locating performance drawbacks related to suboptimal task granularity. On the one hand, tgp helps detecting situations where many small tasks carry out few computations, potentially introducing parallelization overheads due to significant interference and contention between them. On the other hand, tgp helps detecting situations where only few large tasks are spawned, each of them performing substantial computations, potentially resulting in low CPU utilization or load imbalance.

In addition, tgp profiles calling contexts, i.e., all methods open on the call stack upon the creation, submission, or execution of a task. This information helps the user locate classes and methods to modify to optimize task granularity.

Table of Contents

  1. Terminology
  2. Profiling Modes
  3. Metrics
  4. Requirements
  5. Testing tgp
  6. Profiling Your Own Application
  7. Output Files
  8. Post-processing and Characterization
  9. Additional Tests
  10. About

Terminology

Task: tgp considers as task every subtype of the interfaces java.lang.Runnable and java.util.concurrent.Callable, or of the abstract class java.util.concurrent.ForkJoinTask.

Task Granularity: task granularity represents the amount of work carried out by each parallel task. In tgp, task granularity is defined as all the computations performed in the dynamic extent of the methods Runnable.run, Callable.call, and ForkJoinTask.exec, that are collectively known as execution methods.

Task Execution: a task is denoted as executed if at least one of its execution method is executed by a thread to (normal or abnormal) completion.

Task Execution Framework: any subtype of the interface java.util.concurrent.Executor. The term task submission denotes the submission of a task to a task execution framework.

Profiling Modes

tgp profiles a target application in three different and mutually exclusive modes:

  • Bytecode profiling: in this mode, tgp profiles task granularity in terms of bytecode count. This mode has the advantage of being platform-independent and does not require any hardware support. However, bytecode count cannot account the execution of methods without a bytecode representation (such as native methods) and is less accurate than reference-cycle count. For more details on bytecode profiling, see section Output for Bytecode and Reference-cycles Profiling.

  • Reference-cycles profiling: in this mode, tgp profiles task granularity in terms of reference-cycle count. This mode has the advantage of accounting the execution of native methods, and can more accurately estimate the work performed by each task. As a downside, it requires the presence of Hardware Performance Counters (HPCs) in the underlying architecture, and it is platform-dependent (as the collected values of task granularity depend on the nominal frequency of the CPU). For more details on reference-cycles profiling, see section Output for Bytecode and Reference-cycles Profiling.

  • Calling-context profiling: in this mode, tgp profiles the calling context upon the creation, submission, and execution of each task. This profiling mode points the user to classes and methods where modifications may be required to optimize task granularity. For more details on calling-context profiling, see section Output for Calling-context Profiling.

Metrics

tgp profiles metrics from different layers of the system stack, as follows (see the Output Files section for detailed information about the traces generated by tgp):

Application Layer

tgp collects several thread- and task-level information, which are further detailed here. In the bytecode profiling mode, tgp collects the bytecode count of each created task. In the calling-context profiling mode, tgp profiles the calling contexts upon the creation, submission, and execution of each created task.

Framework Layer

tgp profiles all task submissions, detecting the usage of task execution frameworks.

JVM Layer

tgp detects all time intervals when the garbage collector (GC) executes a stop-the-world collection.

OS Layer

tgp profiles CPU utilization (separating user and kernel components) and the number of context switches (CS) experienced by the target application. The former allows one to determine whether the CPU is well utilized by the application. Context switches can be used as a measure of contention between tasks, as an excessive number of context switches is an indication that tasks executing in parallel significantly interfere with each other due to the presence of numerous blocking primitives (caused e.g. by I/O activities, synchronization, and message passing).

Hardware Layer

In the reference-cycles profiling mode, tgp profiles the reference-cycle count of each task to measure its granularity.

Requirements

tgp requires a Linux or MacOS operating system, ant, Java 7 or higher installed and the $JAVA_HOME variable properly set.

For bytecode or calling-context profiling, there is no further requirement.

For reference-cycle profiling, tgp must run on an architecture where HPCs are available, and the PAPI library must be installed. Reference-cycle profiling is supported only on the Linux operating system.

To profile CPU utilization, top needs to be installed, while to profile context switches perf is needed. Both metrics can be profiled only on the Linux operating system.

Testing tgp

Usage

To use tgp, clone or download this repository, open the terminal and change the working directory to the tgp root directory.

By default, for all the profiling modes, tgp profiles a test application which is shipped with this release.

Bytecode Profiling

To profile the test application using bytecode count as a measure of task-granularity, run the following command:

ant profile-task-bc

Reference-cycles Profiling

Alternatively, to profile the test application using reference-cycle count as a measure of task-granularity, run the following command:

ant profile-task-rc

Calling-context Profiling

Finally, to profile calling contexts, run the following command:

ant profile-cc

JVM- and OS-layer Metrics

Profiling of metrics at the JVM- and OS-layer is disabled by default. To collect such metrics, change the variables in metrics.sh as follows:

  • PROFILE_CS="no": set to "yes" if you want to profile context switches, "no" otherwise
  • PROFILE_CPU="no": set to "yes" if you want to profile CPU utilization, "no" otherwise
  • PROFILE_GC="no": set to "yes" if you want to profile GC activities, "no" otherwise

Output

Upon the termination of the profiled application, the traces/ directory should contain at least the tasks.csv file. Depending on the activated metrics, other traces may be available (see Output Files section).

Profiling Your Own Application

To profile your own application with tgp, set the variables in the application.sh file as follows.

  • APP_CLASSPATH="classpath-of-application": provide the classpath of the target application
  • APP_FLAGS="flags-of-application": provide flags to be passed to the JVM executing the target application, if any
  • APP_MAIN_CLASS="MainClass": provide the name of the main class
  • APP_ARGS="arguments-of-application": provide arguments for the target application, if the application requires them

To profile a target application from a location other than the tgp root directory, set the following variable:

  • ROOT_PATH="path/to/tgp": provide the path to the root directory of tgp

Then, run tgp by using the following command instead of the ones listed in section Usage:

ant -buildfile <path/to/tgp/build.xml> <profile-task-bc|profile-task-rc|profile-cc>

Here is an example of how variables in application.sh should be set to profile an application with tgp which is normally executed as follows:

java -cp /home/user/myapp.jar:/home/user/mysecondappjar.jar:/home/user/myfolder -Xms256m -Xmx1g MyMainClass arg1 arg2

assuming that the root folder of tgp is located at ~/tgp/.

ROOT_PATH="~/tgp/"
APP_CLASSPATH="/home/user/myapp.jar:/home/user/mysecondappjar.jar:/home/user/myfolder"
APP_FLAGS="-Xms256m -Xmx1g"
APP_MAIN_CLASS="MyMainClass"
APP_ARGS="arg1 arg2”

Output Files

tgp generates .csv files in the traces/ directory as follows (note that the directory will be overwritten at each profiling):

Task Trace

The tasks.csv file contains information about all created tasks. Each row refers to a single task, thus the number of rows (excluding the header) indicates the number of tasks created by the target application.

There are two different versions of this file, depending on the selected profiling mode:

Output for Bytecode and Reference-cycles Profiling

The file is composed by several columns as follows:

  • ID: a unique JVM-generated code (hashCode) for each task
  • Class: the class of the task
  • Outer Task ID: the ID of the outer task [1]
  • Execution N.: an ordinal representing the i-th task execution [2]
  • Creation thread ID: a unique JVM-generated code that identifies the thread which created the task
  • Creation thread class: the class of the thread which created the task
  • Creation thread name: the name of the thread which created the task
  • Execution thread ID: a unique JVM-generated code that identifies the thread which executed the task (-1 if the task was created but not executed)
  • Execution thread class: the class of the thread which executed the task (null if the task was not executed)
  • Execution thread name: the name of the thread which executed the task (null if the task was not executed)
  • Executor ID: a unique JVM-generated code that identifies the task execution framework the task was submitted to (-1 if the task was not submitted)
  • Executor class: the class of the task execution framework the task was submitted to (null if the task was not submitted)
  • Entry execution time: the timestamp when task execution started (-1 if the task was not executed)
  • Exit execution time: the timestamp when task execution completed (-1 if the task was not executed)
  • Granularity: the granularity of the task measured either in bytecode or reference-cycle count, depending on the chosen profiling mode
  • Is Thread: 'T' if the task extends java.lang.Thread, 'F' otherwise [3]
  • Is Runnable: 'T' if the task implements java.lang.Runnable, 'F' otherwise
  • Is Callable: 'T' if the task implements java.lang.Callable, 'F' otherwise
  • Is ForkJoinTask: 'T' if the task extends java.util.concurrent.ForkJoinTask, 'F' otherwise
  • Is run() executed: 'T' if the task is a subtype of java.lang.Runnable and was executed by calling the run() method, 'F' otherwise
  • Is call() executed: 'T' if the task is a subtype of java.lang.Callable and was executed by calling the call() method, 'F' otherwise
  • Is exec() executed: 'T' if the task is a subtype of java.util.concurrent.ForkJoinTask and was executed by calling the exec() method, 'F' otherwise

[1]: the purpose of this column is to identify nested tasks. A task is nested if it fully executes inside the dynamic extent of the execution method of another task, which is called outer task. This column contains the ID (>0) of the outer task, if the task is nested. Information on the outer task can be found in another row of this file. If the task is not nested, this column contains 0. If the task is not executed, this column contains -1. See Task Aggregation for more information.

[2]: a task can be executed multiple times (i.e., its execution method is executed to completion more than once). Each task execution appears in this file on a different row. The value i of this field denotes that this row contains metrics related to the i-th execution of this task.

[3]: since java.lang.Thread implements java.lang.Runnable, every Java thread is also a task. The field discriminates threads from other tasks.

Output for Calling-context Profiling

The file is composed by several columns as follows:

  • ID: a unique JVM-generated code (hashCode) for each task
  • Class: the class of the task
  • Execution N.: an ordinal representing the i-th task execution. See note [2] above.
  • Calling Context (init): the calling context, collected upon the creation of this task
  • Calling Context (submit): the calling context, collected upon the submission of this task (null if the task was not submitted)
  • Calling Context (exec): the calling context, collected upon the execution of this task (null if the task was not executed)

Calling contexts are represented as a string containing all methods open on the call stack when the calling context was collected. Different methods are separated by the symbol !.

Here is an example of how the column Calling Context (init) may look like:

ch/usi/dag/tgp/test/Example.main([Ljava/lang/String;)V!ch/usi/dag/tgp/test/Example$MyTask.<init>([Ljava/lang/Object;)V!

The methods are written following the representation of method descriptors defined in the Java class file format.

In the example, method main, defined in class ch.usi.dag.tgp.test.Example, which takes an array ([) of String (Ljava/lang/String;) as input parameter and with return type void (V), calls a constructor () of class MyTask (an inner class, defined inside ch.usi.dag.tgp.test.Example), which takes an array ([) of Object (Ljava/lang/Object;) as input parameter.

To exemplify how calling context profiling can help the optimization of task granularity, consider the following scenario. MyTask is a task whose purpose is to process some data, passed as input of a constructor as an array of Object. The granularity of MyTask depends on the kind and the size of such array, since larger arrays would require more computations to be performed by MyTask to process all data inside, and vice versa.

Suppose that the granularity of MyTask has been classified by the user as too large, and thus needs to be optimized. To do so, one can decrease the amount of data processed by MyTask. To this end, one may need to modify the code where the task constructor is called. Such code is contained in the method appearing right before MyTask. (i.e., Example.main) in the above calling context, which triggers the creation of a new MyTask by calling its constructor, passing an array of Object as input parameter. The user can then optimize task granularity by modifying Example.main, e.g. decreasing the size of the array passed to MyTask.

Other calling contexts (submission and execution) are useful for understanding which methods trigger the submission and the execution of a task, respectively. During task-granularity optimization, one may need to modify the code handling the objects returned after the execution of the task, which can be included in the methods where task submission or execution occurred. Such methods can be easily found by examining calling contexts, following an approach similar to the one exemplified above.

CPU Trace

The cpu.csv file contains data about CPU utilization, associated with a timestamp (in nanoseconds). Each row refers to a new measurement. The file has the following structure:

Timestamp (ns),CPU utilization (user),CPU utilization (kernel)
8055531757501863,50.0,8.3
8055531939110288,33.3,8.3
8055532113909654,8.3,16.7
8055532289063103,0.0,12.5

The first column reports a timestamp in nanoseconds, the second column reports CPU utilization by user code, the third column reports CPU utilization by kernel code. CPU utilization ranges from 0 to 100. This metric is sampled approximatively every 150ms.

CS Trace

The cs.csv file contains data about the context switches experienced by the target application. Each row refers to a new measurement. The file has the following structure:

Timestamp (ns),Context Switches
8219428637912867,36
8219431141069173,10
8219431241181367,66
8219431341308954,100

The first column reports a timestamp in nanoseconds, while the second column reports the number of context switches experienced by the target application since the last measurement. Context switches are measured every 100ms.

GC Trace

The gc.csv file contains timestamps representing the start and termination of each stop-the-world garbage collection. Each two rows refer to a different GC collection. The file has the following structure:

Start GC,7341135166188457
End GC,7341135226213341
Start GC,7341135412676293
End GC,7341135420910871
Start GC,7341135837793799
End GC,7341135855849462

The first column represents the event profiled (i.e., the start or the end of a collection cycle), while the second column contains the timestamp associated to the event.

Note: if GC profiling is enabled but this trace is not produced, then no stop-the-world collection occurred during the execution of the target application.

Post-processing and Characterization

This release comes with scripts to further process the traces produced by tgp. These scripts are of two types: 1) post-processing, which syntactically filter data and remove garbage, and 2) characterization, which help the user performing task-granularity analysis.

Post-processing

Post-processing allows the user to further filter the results produced by tgp. In particular, the user can aggregate tasks and filter out context-switches and CPU utilization measurements obtained during garbage collection cycles. Post-processing scripts can be found in the postprocessing/ directory.

Task Aggregation

Some tasks may be nested, i.e., they fully execute inside the dynamic extent of the execution method of another task, which is called outer task. Since the nested and outer tasks cannot execute in parallel, nested tasks should be aggregated to their outer task, resulting in a single larger task. Task aggregation performs such operation. If the outer task is itself nested, the task is recursively aggregated until a non-nested task is found.

Task aggregation is performed on nested tasks if one of the following conditions is true:

  1. the outer task is not a thread
  2. the outer task is a thread and both the following conditions are true:
    • the nested task has not been submitted
    • the nested task is created and executed by the same thread

To perform tasks aggregation on a task trace, enter the postprocessing/ folder and type the following command:

./aggregation.py -t <path to task trace> [-o <path to aggregated task trace (output)>]

The script creates a new trace, called aggregated task trace (named aggregated-tasks.csv by default) containing the task trace after the aggregation step.

The directory postprocessing/tests-aggregation/ contains several test traces for the aggregation tool. As an example, running the script with test test_valid_chain.csv as follows:

./aggregation.py -t tests-aggregation/test_valid_chain.csv

yields the following aggregated task trace:

ID,Class,Outer Task ID,Execution N.,Creation thread ID,Creation thread class,Creation thread name,Execution thread ID,Execution thread class,Execution thread name,Executor ID,Executor class,Entry execution time,Exit execution time,Granularity,Is Thread,Is Runnable,Is Callable,Is ForkJoinTask,Is run() executed,Is call() executed,Is exec() executed
6,C6,0,1,0,cl,n,1,etc,etn,1,ec,150,155,130,T,F,F,F,F,T,F

Following the rules of task aggregation, 15 tasks out of 16 are aggregated, resulting in a single entry in the aggregated task trace.

Note: more details on the script and its options can be obtained by running ./aggregation.py -h.

Garbage-collection Filtering

It is possible that the CPU and CS measurements are taken during a garbage collection. If this is not desired, users can run the gc-filtering.py script to filter out CPU and CS measurements obtained during GC cycles.

To perform the filtering, enter the postprocessing/ directory and type the following command:

./gc-filtering.py -c <path to CS trace> -p <path to CPU trace> -g <path to GC trace> [--outcs <path to filtered CS trace (output)> --outcpu <path to filtered CPU trace (output)>]

The script creates two new traces (named filtered-cs.csv and filtered-cpu.csv by default) containing the filtered CS and CPU utilization measurements, respectively.

The directory postprocessing/tests-gc-filtering/ contains several test traces for the filtering tool.

As an example, running the script on the test traces cs.csv, cpu_in_cs.csv, and gc_in_cs.csv as follows:

./gc-filtering.py -c tests-gc-filtering/cs.csv -p tests-gc-filtering/cpu_in_cs.csv -g tests-gc-filtering/gc_in_cs.csv

yields the following result (traces):

filtered-cs.csv

Timestamp (ns),Context Switches
7602317094530472.0,72432.0
7602317094530478.0,7134328.0
7602317094530480.0,7234842.0
7602317094530482.0,82354.0
7602317094530495.0,8345.0
7602317094530499.0,71394.0
7602317094530500.0,81392.0

filtered-cpu.csv

Timestamp (ns),CPU utilization (user),CPU utilization (system)
7602317094530479,0.0,25.0
7602317094530484,50.0,0.0
7602317094530495,66.7,0.0
7602317094530499,40.0,0.0
7602317094530504,0.0,0.0

The script filters out all context switches and all CPU measurements whose timestamp falls within GC cycles, resulting in only 7 (out of 22) and 5 (out of 24) entries in the filtered CS and CPU trace, respectively.

Note: more details on the script and its options can be obtained by running ./gc-filtering.py -h.

Characterization

Characterization scripts are meant to guide the user towards distinguishing fine- and coarse-grained tasks. This distinction is based on different thresholds, thus allowing the user to customize the analysis.

Note: we remark that characterization scripts only provide general purpose analyses. The user is encouraged to create custom ones.

Characterization scripts can be found in the characterization/ directory.

Directory characterization/tests/ contains test traces for all characterization scripts.

Note: for more accurate results, traces containing context switches and CPU utilization measurements should have first been filtered (see Garbage Collection Filtering).

Diagnosis

This script provides basic statistics on task granularity and the average number of context switches and CPU utilization. The script also offers the possibility to restrict this analysis on tasks belonging to a specific class.

To perform diagnostics on tasks, enter the characterization/ folder and type the following command:

./diagnose.py -t <path to task trace> -c <path to CS trace> -p <path to CPU trace> [-o <path to result trace (output)>]

The script creates a new trace (named diagnostics.csv by default) containing the described statistics. The results of the analysis will also be printed to the standard output.

As an example, running the script as follows:

./diagnose.py -t tests/test-tasks.csv -c tests/test-cs.csv -p tests/test-cpu.csv

yields the following result (stdout):

TASKS STATISTICS
-> Total number of tasks: 31 
-> Average granularity: 1019943219.9 
-> 1st percentile - granularity: 4 
-> 5th percentile - granularity: 12 
-> 50th percentile (median) - granularity: 34543 
-> 95th percentile - granularity: 34234523 
-> 99th percentile - granularity: 13038408322 
-> IQC - granularity: 644794 
-> Whiskers range - granularity: [0, 1612547.0] 
-> Percentage of tasks having granularity within whiskers range: 80.6451612903% 
-> Percentage of tasks with granularity around 100000: 35.4838709677%

CONTEXT-SWITCHES STATISTICS
-> Average number of context switches: 148.957446809cs/100ms

CPU STATISTICS
-> Average CPU utilization: 42.5583333333+-4.45480235225

Note: more details on the script and its options (including those not shown here) can be obtained by running ./diagnose.py -h.

Fine-grained Tasks

This script finds classes containing only fine-grained tasks based on user-defined thresholds. For each such class, the script reports the average granularity of all tasks belonging to the class, and the average number of context switches occurred when tasks of the class were in execution. The script also computes the average amount of context switches occurred when no fine-grained task was in execution.

To run this script, enter the characterization/ folder and type the following command:

./fine_grained.py -t <path to task trace> -c <path to CS trace> [-o <path to result trace (output)>]

The script creates a new trace (named fine-grained.csv by default). The results of the analysis will also be printed to the standard output.

As an example, running the script as follows:

./fine_grained.py -t tests/test-tasks.csv -c tests/test-cs.csv

yields the following result (stdout):

Average number of context switches experienced when fine-grained tasks are not in execution: 175.416666667cs/100ms

Class: class6 -> Average granularity: 1443.0 -> Average number of context switches: 197.5cs/100ms
Class: class7 -> Average granularity: 4.0 -> Average number of context switches: 0cs/100ms
Class: class4 -> Average granularity: 61644.5 -> Average number of context switches: 89.75cs/100ms
Class: class5 -> Average granularity: 301304.666667 -> Average number of context switches: 129.8cs/100ms
Class: class2 -> Average granularity: 9186281.4 -> Average number of context switches: 126.166666667cs/100ms
Class: class3 -> Average granularity: 695149.666667 -> Average number of context switches: 185.142857143cs/100ms
Class: class1 -> Average granularity: 3860539.66667 -> Average number of context switches: 137.076923077cs/100ms

Note: more details on the script and its options (including those not shown here) can be obtained by running ./fine_grained.py -h.

Coarse-grained Tasks

This script finds classes containing only coarse-grained tasks based on user-defined thresholds. For each such class, the script reports the average granularity of all tasks belonging to the class, and the average number of context switches and CPU utilization experienced when tasks of the class were in execution. The script also computes the average amount of context switches occurred when no coarse-grained task was in execution.

To run this script, enter the characterization/ folder and type the following command:

./coarse_grained.py -t <path to task trace> -c <path to CS trace> -p <path to CPU trace> [-o <path to result trace (output)>]

The script creates a new trace (named coarse-grained.csv by default). The results of the analysis will also be printed to the standard output.

As an example, running the script as follows:

./coarse_grained.py -t tests/test-tasks.csv -c tests/test-cs.csv -p tests/test-cpu.csv

yields the following result (stdout):

Average number of context switches experienced when coarse-grained tasks are not in execution: 119.333333333cs/100ms

CLASSES CONTAINING COARSE-GRAINED TASKS:

-> Class: class8
   Average granularity: 15766120358.0
   Average number of context switches: 154.11627907cs/100ms
   Average CPU utilization: 42.6816091954

Note: more details on the script and its options (including those not shown here) can be obtained by running ./coarse_grained.py -h.

Additional Tests

This release is shipped with 12 test classes. By default, tgp profiles class TestRunAndJoin in any profiling mode selected.

You can change the test class to be profiled (see Profiling Your Own Application) to try tgp on different applications. To do so, modify application.sh file as follows:

APP_MAIN_CLASS="fully-qualified-name-of-test-class"

You can run the following test classes (all of them defined in package ch.usi.dag.tgp.test):

  • TestNestedThreads
  • TestJoinAndThreadPool
  • TestThreadStarted
  • TestRunnableAndCallable
  • TestNestedExecution
  • TestExecutor
  • TestRecursiveTask
  • TestMultipleSubmission
  • TestSynchronizedExecution
  • TestProducerConsumer
  • TestRunAndJoin
  • TestMultipleTaskExecutions

For example, to profile class TestNestedThreads, modify the application.sh file as follows:

APP_MAIN_CLASS="ch.usi.dag.tgp.test.TestNestedThreads"

leaving all other variables commented.

About

tgp is developed and maintained by the Dynamic Analysis Group (DAG) of the Faculty of Informatics at Università della Svizzera italiana (USI), Lugano, Switzerland.

Project Lead

  • Andrea Rosà (andrea [dot] rosa [at] usi [dot] ch)

Contributors

  • Eduardo Rosales (rosale [at] usi [dot] ch)
  • Federico van Swaaij (federico [dot] van [dot] swaaij [at] usi [dot] ch)

Note: Please refer to this article for additional information about tgp.

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