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Qthreads Performance Measurement API

Introduction

The Qthreads performance measurement API (qtperf) is a set of functions that make it easier to keep track of time-related data during a run. It supports tracking of internal library timings for measuring the actual overhead imposed by the library itself, and it also supports arbitrary user-defined timing measurements.

Qtperf is modeled on the idea of a state machine. If you can model the performance data you want to collect as a set of discrete states with associated time spent in each state, you will be able to use this set of functions to track it. The basic unit of data collection is a state, which is just an arbitrary identifier. These are bundled into state groups, which represent a state machine with all of its valid states. After state groups are defined, you can instantiate them as many times as necessary to track same-type data (for example, if you want to measure overhead of communications in each thread, you could define a communication state group and then instantiate it for each thread that will be communicating). While the program is running, your code can signal that a new state is being entered for the currently active state group, and the library will add the elapsed time in the previous state to its total, update the state variable, and return control.

Instantiating a state group for every thread is often going to be too much detail and overhead, so the library also supports aggregation. When you have a large number of threads that will be doing basically the same thing, you can define a state group to represent the task, then instantiate it once and have all of the threads post their updates to that shared instance. This requires the limited use of mutual exclusion, so there will be a tradeoff in terms of memory savings versus performance costs.

Quick note on memory management

This library uses dynamically allocated memory to track the performance data it captures. The only functions that perform allocations are the functions with create in their names. You should not manually deallocate any structures provided by this library. Instead, call qtperf_free_data() at the end of your run, and all memory will be safely deallocated.

Building

Prerequisites

  • autoconf (apt-get installautoconf)
  • libtool (apt-get install libtool)
  • cmocka, only for testing (apt-get install libcmocka-dev)
  • valgrind, for testing (apt-get install valgrind)

Build Steps

The performance API is only loosely integrated with qthreads proper. In order to build qthreads with support for performance monitoring, there are two steps to complete:

  1. Run autogen.sh to create the configure script.
  2. Choose your CFLAGS appropriately (see below) and run the configure script

CFLAGS for performance monitoring

The performance API is enabled by passing a couple of flags to the C preprocessor. To get the basic API without testing or debug messages, use QTHREAD_PERFORMANCE:

CFLAGS='-DQTHREAD_PERFORMANCE' ./configure --disable-lazy-threadids --enable-picky [..your selections here..]

Enable testing for performance API

To enable the test harness for the performance API, you’ll need to have cmocka installed (https://cmocka.org). After that, you’ll need to tell configure how to link cmocka and to enable testing, using the QTPERF_TESTING preprocessor flag (note backslashes at the ends of lines - this should all be on one command line).

CFLAGS='-DQTHREAD_PERFORMANCE -DQTPERF_TESTING' \
  LDFLAGS='-L/path/to/cmocka/lib' \
  LIBS=-lcmocka \
  ./configure --enable-debugging --disable-lazy-threadids --enable-picky [..your selections here..]

Enable debug output

The performance API can print messages to the console that indicate potentially useful information about what’s happening inside as it’s running. To enable those messages, use the PERFDBG flag:

CFLAGS='-DQTHREAD_PERFORMANCE -DPERFDBG=1' ./configure --disable-lazy-threadids --enable-picky [..your selections here..]

Tutorial

Qtperf is designed to be used with three distinct phases of operation: setup, run, and teardown. This is necessary because of memory allocation and cleanup requirements, and also because instrumentation must be initialized for internal library states if they are to be tracked.

Quickstart

Here is a quick example of a tiny program that makes use of the basic internal logging features of qtperf:

#include<qthread/qthread.h>
#include<qthread/performance.h>
#include<qthread/logging.h>

aligned_t spin(){
  size_t i=0;
  aligned_t result=2;
  for(i=0; i<1000000; i++){
    result = result * result + i;
  }
  return result;
}

size_t num_threads=5;
int main(){
  size_t i=0;
  aligned_t ret=0;

  // Enable monitoring of qthread internal workers
  qtperf_set_instrument_workers(1);

  // Enable monitoring of internal qthreads (jobs)
  qtperf_set_instrument_qthreads(1);

  // Initialize and allocate data, enable collection
  qtperf_start();

  // Call this *after* qtperf_start()
  qthread_initialize();

  for(i=0; i<num_threads; i++){
    qthread_fork(spin, NULL,&ret);
  }
  for(i=0; i<num_threads; i++){
    qthread_readFE(NULL, &ret);
  }
  // Disable collection, you can switch on and off at will during a run
  qtperf_stop();

  // Print the results in a human readable format
  qtperf_print_results();

  // Deallocate everything. No more calls to qtperf_* after this!
  qtperf_free_data();

  return 0;
}

Here is a program to demonstrate how to set up and use a custom state group:

#include<qthread/qthread.h>
#include<qthread/performance.h>
#include<qthread/logging.h>

static inline int spin_lock(volatile uint32_t* busy);

static inline int spin_lock(volatile uint32_t* busy){
  int ret=0;
  while(qthread_cas32(busy, 0, 1) != 0){
    ret=1;
  }
  return ret;
}

typedef enum {
  SPIN_WORKING,
  SPIN_WAITING,
  SPIN_COMPLETE,
  SPIN_NUM_STATES
} spinstate_t;

const char* spin_names[] = {
  "SPIN_WORKING",
  "SPIN_WAITING",
  "SPIN_COMPLETE",
  "SPIN_NUM_STATES"
};

volatile uint32_t busy=0;
int strct=0;
aligned_t bit=0;

#define SPIN 1
aligned_t struct_edit(void*data){
  int start =0;
  size_t i=0;
  aligned_t ret=0;
  int interruptions=0;
  qtperfdata_t* mydata = (qtperfdata_t*)data;
  
  // Upon entry, timing is recorded for previous state
  qtperf_enter_state(mydata, SPIN_WORKING);
  
  for(i=0; i<100; i++){
    size_t j=0;

    // Multiple state transitions are expected, timing will not be
    // affected by transitioning to the same state multiple times.
    qtperf_enter_state(mydata, SPIN_WAITING);

    interruptions += spin_lock(&busy);
    qtperf_enter_state(mydata, SPIN_WORKING);
    start = strct;
    for(j=0; j<1000000; j++){
      strct = strct+1;
    }
    start = strct;
    busy = 0;
  }

  // Entering the QTPERF_INVALID_STATE state temporarily switches off
  // data collection for this instance. In this case, this thread is
  // now done so I want to switch it off.
  qtperf_enter_state(mydata, QTPERF_INVALID_STATE);

  qtlogargs(SPIN, "%d interruptions", interruptions);
  ret = strct;
  return ret;
}

typedef enum {
  RUNNING,
  DONE,
  TOTAL_NUM_STATES
} total_t;
const char* total_names[]={
  "RUNNING",
  "DONE"
};

int main() {
  aligned_t ret=0;
  size_t i=0;
  qtstategroup_t* spingroup=NULL;
  qtstategroup_t* totalgroup=NULL;
  qtperfdata_t* totaldata=NULL;

  // Enable collection and setup data structures. Library internals
  // will NOT be logged during this run, because
  // qtperf_set_instrument_* were not called.
  qtperf_start();

  qthread_initialize();

  // Create a state group for the spinstate_t state group. The names
  // array can be NULL, in which case the library will just report
  // numeric identifiers in the final output.
  spingroup=qtperf_create_state_group(SPIN_NUM_STATES, "Spin Testing", spin_names);

  // Create a state group for the total elapsed time.
  totalgroup=qtperf_create_state_group(TOTAL_NUM_STATES, "Total Time", total_names);

  // Instantiate the total elapsed time state group into a perfdata_t
  // structure to collect data for the run.
  totaldata=qtperf_create_perfdata(totalgroup);

  // Initially, perfdata_t are in the QTPERF_INVALID_STATE state, so
  // you must make at least one transition into your start state in
  // order to have logging enabled for the instance.
  qtperf_enter_state(totaldata,RUNNING);

  for(i=0; i<10; i++){
    // Create a new instance of the spinstate_t state group for each
    // thread.
    qtperfdata_t* spindata = qtperf_create_perfdata(spingroup);

    // The perfdata_t struct is passed through to the threads. If you
    // want to aggregate this, you can make use a global variable
    // because the perfdata will be shared between threads anyway.
    qthread_fork(struct_edit, (void*)spindata, &ret);
  }
  for(i=0; i<10; i++){
    qthread_readFE(NULL,&ret);
  }
  // Signal that the run is complete for the total elapsed time state
  // group.
  qtperf_enter_state(totaldata,DONE);

  // Stop collection. This is not final - collection could be
  // restarted if desired by a call to qtperf_start()
  qtperf_stop();

  // Display the results in a human readable format.
  qtperf_print_results();

  // signal the library to clean itself up.
  qtperf_free_data();

  return 0;
}

API

Setup Functions

Setting up qtperf varies a bit depending on what you want to measure and how you want to track it. In the most minimal form, all you have to do is call qtperf_start(), then allocate your groups and you’re off to the races. You can also tell the library to record data from the qthreads internally, and from the library’s workers threads.

There are two ways to get internal data out of qthreads using qtperf. You can measure either the workers (typically a smaller number of actual operating system threads that implement the work stealing behavior of qthreads), or you can instrument the qthreads themselves (which represent tasks visible to the user of the library). These measurements can be taken together during the same run.

You must inform the library that you want to record this data before you call qthread_initialize().

The main abstraction that this API relies upon is a state group. As the user, you will define a set of states that your code can be in, tell the library when the code makes a transition, and the library will track the amount of time spent in each state.

A state group is a connected set of states - these states are allowed to have transitions to each other. Transitions can only be between states that are in the same state group. This is a constraint that the library has only limited power to enforce. If a transition is requested to a state that is out of bounds, the library will flag the error, but if the state number is valid for the group it will just complete the transition, even if you mistakenly used the wrong enum variant in the source code.

A state group represents an abstract set of states. In order to actually attach those states to a thread and measure something, you need to make a qtperfdata_t struct.

qtperf_set_instrument_workers(int yes_or_no)

This function will create a state group that allows you to measure internal timing data for the qthreads library’s workers. Qthreads implements a system of work stealing, in which a (relatively) small number of system threads aggressively switch between a large number of task threads so as to maximize the time spent actually doing productive work. This instrumentation flag allows you to see how the workers themselves are doing by measuring their idle versus busy time. Using this data you can determine how well the processors are being utilized.

qtperf_set_instrument_qthreads(int yes_or_no)

This function allows you to measure the overhead of the qthreads scheduling systems in sending task work to the worker threads. Each qthread you create will be monitored to measure how much time is spent on task versus stalled in various parts of the qthreads system.

qtstategroup_t* qtperf_create_state_group(size_t num_states, const char* group_name, const char** state_names)

This function creates a new state group. You must provide a name, but the array of state names is optional. If no state names are provided, the library will simply assign numeric identifiers to the states when output is requested.

The value returned is a newly-malloc’d qtstategroup_t structure. This structure is meant to be mostly opaque to users, though you can access its fields if you need to as with any other struct. For details of its contents, see qthreads/performance.h.

qtperfdata_t* qtperf_create_perfdata(qtstategroup_t* group)

This creates a new performance counters structure (qtperfdata_t) that will use the given state group to measure transitions. This function should be called for each thread that you want to measure. Data logged here will be kept separate from other threads. If you want to log data from a group of similar threads into the same performance counters structure, use qtperf_create_aggregated_perfdata() (see below).

The value returned from this function should be treated as an opaque identifier unless you are very sure of what you need to do to it. In order to cause a state transition for this counter, call qtperf_enter_state with it.

qtperfdata_t* qtperf_create_aggregated_perfdata(qtstategroup_t* group)

This function is similar to qtperf_create_perfdata except that it creates a performance counter that is intended to be shared by many threads during a run. In some cases, large numbers of threads are doing essentially the same task, and the data you really want is how the overall task performed for the whole system. This function is intended to support that use case.

If you need to measure data for each thread separately, use qtperf_create_perfdata instead.

void qtperf_piggyback_state(...)

Arguments:

  1. qtperfdata_t* source_data - The perfdata you want to piggyback onto
  2. qtperfid_t trigger_state - Trigger piggyback when the source data enters this state
  3. qtperfdata_t* piggyback_data - The counter you want to attach to the source
  4. qtperfid_t piggyback_state - The state that the piggyback should enter when the trigger condition is met

The performance library allows you to add “piggybacks” onto other states. This means that you can set it up so that a state transition in one performance counter triggers a state transition in another performance counter automatically. The primary use of this is to allow you to pull data out of the internal instrumentation states and into your own state groups. For example, you might want to record the time a thread spends communicating via an external library, processing the data, and waiting for the qthreads library to schedule it. You would piggyback your performance counter onto the qthreads internal performance counters so that the library’s internal state changes are recorded in your own structure. That gives you an easy way to monitor the internal overhead of the library compared with the task-related states in a given thread.

Performance impacts! Caution!

There are two things to watch out for if you start using piggybacks heavily. First, it is possible to create a cycle, in which case your thread will enter an infinite loop and hang. This is easy to detect in most cases and you’ll find the bug quickly. The second issue to look out for is that each piggyback you add to a perfdata_t adds a small constant amount of execution time to each state transition it makes. If you add a large number of piggybacks, or if you have a bunch of piggybacks attached in a chain, you may see a performance impact from processing all of the state transitions. Use piggybacks sparingly.

Runtime Functions

These functions are intended for use while the experiment is running. They are oriented toward making measurements as painlessly as possible, and also support getting output at various points.

void qtperf_enter_state(qtperfdata_t* data, qtperfid_t state_id)

This is the function that you should call each time you want to record that something has changed. The data argument is the pointer returned by the qtperf_create_*_perfdata functions, and the state_id is the identifier of the state you are entering. The library will sample the current time, subtract from that the time that the current state was entered, and add the difference to the current state’s total before entering the new state. If the perfdata_t is an aggregated collector, it will also ensure that only one thread is updating at a time using a CAS-based spin lock.

const char* qtperf_state_name(qtstategroup_t* group, qtperfid_t state_id)

This just returns the state name you provided when the state group was created, for convenience. If you did not define names, this function returns NULL.

void qtperf_start()

This function enables data recording. It and its opposite function (qtperf_stop) can be called any number of times while the program is running to turn collection on and off as needed.

void qtperf_stop()

This function halts collection globally. You can restart it with qtperf_start, and both can be called at any time during program execution without danger.

Reporting and data access functions

These functions allow you to get the recorded data out of the library in various forms. Currently, you can have data reported as human-readable text or comma-separated values, and you can get access directly to the data structures themselves via an iterator for low-level or programmatic access.

void qtperf_print_results()

Print out all of the data for all of the counters, using the human-readable format provided by qtperf_print_delimited. States with zero time will be omitted for brevity.

void qtperf_print_delimited(qtstategroup_t* group, const char* delim, bool print_headers, const char* prefix)

This function is intended to make it easy to export data from a run into other tools by printing it in tabular, delimited format. It prints out by state group, so all instances of that group will be printed, one in each row of the table. You can specify what delimiter to use within rows with delim, and you can also specify an optional prefix to be printed at column zero of each row (including the optional header). If print_headers is true, the name of each state will be printed in a header row so that the columns of the table are identifiable.

The prefix allows you to easily split out a number of different tables froma single run and divert them to their own files with a simple command line. For example, if you use bash for your shell and have two tables you want to keep separate, you can set one delimiter to ‘*’ and the other to “+”, then use this command to put the data from a single run into two separate files:

./program | tee >(egrep '^\*' > stars.csv) >(egrep '^\+' > pluses.csv)

This will create a file called stars.csv that has all lines that begin with ‘*’, and a separate file called pluses.csv that has all lines that begin with ‘+’.

void qtperf_print_perfdata(qtperfdata_t* perfdata, bool show_states_with_zero_time)

This prints the states in a human-readable format along with their recorded times. If show_states_with_zero_time is non-zero, it will display all states regardless of their time data. Otherwise, it will only display states that have non-zero tick counts. This is mostly useful if you have a number of states that aren’t used yet but might be in the future.

void qtperf_print_group(qtstategroup_t* group)

Print the performance data for a state group in human-readable list format. This will print a list of all instances of this group, along with some summary information for the group as a whole.

void qtperf_print_perfdata(qtperfdata_t* data, bool show_states_with_zero_time)

This prints a single entry in the human-readable list format. If show_states_with_zero_time is true, it will print an entry for all states, otherwise it will only print if the state actually has some time recorded.

qtperfcounter_t qtperf_total_group_time(qtstategroup_t* group)

This function adds up all of the time for all of the instances of the given state group and returns the result.A

qtperfcounter_t qtperf_total_time(qtperfdata_t* data)

This function returns the total time elapsed in all states of the given perfdata instance. If you put your perfdata into the QTHREAD_INVALID_STATE state when it finishes or is otherwise not executing, you can use this function to get a simple measurement of how long the thread was actually running (because time is not recorded when the thread is in QTHREAD_INVALID_STATE).

Iterators

A simple iterator is provided for traversing through the performance data. The iterator traverses stategroups, hitting all perfcounters within a stategroup before moving to the next stategroup. In order to avoid allocation, the iterator interface expects you to provide it with an iterator struct to use. This is typically done by declaring a qtperf_iterator_t as a local variable, then declaring another local variable that points to the iterator itself, and passing a pointer to that pointer for the iterator functions. This allows the API to initialize and update the iterator struct, and to indicate when the end of the chain has been reached by returning NULL. A basic example of use would look like this:

// ... 
  qtperfdata_t* iterdata = NULL;
  qtperf_iterator_t iterator;
  qtperf_iterator_t* iter=&iterator;
  qtperf_iter_begin(&iter);

  for(iterdata = qtperf_iter_next(&iter);
      iterdata != NULL;
      iterdata = qtperf_iter_next(&iter)){
    // .. do something with the data
  }
// ...

That snippet of code would loop through all of the perfdata_t instances, ordered by state group, and execute the body of the for loop on each one.

The primary use for this is to provide access to the raw data in a way that will remain consistent when internal changes occur, so that you can do custom data processing. If you find you need to access to the internal data structures, try to do it using this API so that future changes to the library will be less likely to break your code.

void qtperf_iter_begin(qtperf_iterator** iter)

This function initializes a new iterator. iter should be a pointer to a pointer that has been initialized to point at a qtperf_iterator_t that you allocated (usually on the stack, by simply declaring it as a local variable). This function will not allocate a new iterator, so don’t pass it NULL, or a pointer to NULL.

qtperfdata_t* qtperf_iter_next(qtperf_iterator_t** iter)

This function advances the iterator one slot, and returns the qtperfdata_t it was pointing at before it advanced. You can use this in the termination condition of a loop as described at the top of this section.

qtperfdata_t* qtperf_iter_deref(qtperf_iterator_t * iter)

This function returns the qtperfdata_t that the iterator is currently pointing at. If the iterator is exhausted, it will return NULL.

qtperf_iterator_t* qtperf_iter_end()

This function returns a termination value that you can use to compare with your iterator to see if you’re done. Currently, this is just NULL, but in the future it may be a value that points to something else. The correct way to check to see if your iterator is done is with the comparison myiter == qtperf_iterator_end().

Teardown

Stopping the performance system and cleaning up is done with a single function call to qtperf_free_data(). It has no arguments and returns void, just call it at the end of your run to free up resources, particularly if your program will continue running after performance measurement is no longer needed.