Stats: igt-gpu-tools Reference Manual

## Stats Stats — Tools for statistical analysis |

## Functions

void | igt_stats_init ()
void | igt_stats_init_with_size ()
void | igt_stats_fini ()
bool | igt_stats_is_population ()
void | igt_stats_set_population ()
void | igt_stats_push ()
void | igt_stats_push_float ()
void | igt_stats_push_array ()
uint64_t | igt_stats_get_min ()
uint64_t | igt_stats_get_max ()
uint64_t | igt_stats_get_range ()
void | igt_stats_get_quartiles ()
double | igt_stats_get_iqr ()
double | igt_stats_get_iqm ()
double | igt_stats_get_mean ()
double | igt_stats_get_trimean ()
double | igt_stats_get_median ()
double | igt_stats_get_variance ()
double | igt_stats_get_std_deviation ()
double | igt_stats_get_std_error ()
void | igt_mean_init ()
void | igt_mean_add ()
double | igt_mean_get ()
double | igt_mean_get_variance ()

## Types and Values

| igt_stats_t
struct | igt_mean

## Includes

    #include <igt.h>

## Description

Various tools to make sense of data.

igt_stats_t is a container of data samples. `igt_stats_push()` is used to add new samples and various results (mean, variance, standard deviation, ...) can then be retrieved.

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    igt_stats_t stats;

    igt_stats_init(&stats, 8);

    igt_stats_push(&stats, 2);
    igt_stats_push(&stats, 4);
    igt_stats_push(&stats, 4);
    igt_stats_push(&stats, 4);
    igt_stats_push(&stats, 5);
    igt_stats_push(&stats, 5);
    igt_stats_push(&stats, 7);
    igt_stats_push(&stats, 9);

    printf("Mean: %lf\n", igt_stats_get_mean(&stats));

    igt_stats_fini(&stats);

## Functions

### igt_stats_init ()

    void
    igt_stats_init (igt_stats_t *stats);

Initializes an igt_stats_t instance. `igt_stats_fini()` must be called once finished with `stats` .

#### Parameters

stats | An igt_stats_t instance |

### igt_stats_init_with_size ()

    void
    igt_stats_init_with_size (igt_stats_t *stats,
                              unsigned int capacity);

Like `igt_stats_init()` but with a size to avoid reallocating the underlying array(s) when pushing new values. Useful if we have a good idea of the number of data points we want `stats` to hold.

igt_stats_fini() must be called once finished with `stats` .

#### Parameters

stats | An igt_stats_t instance |
capacity | Number of data samples `stats` can contain |

### igt_stats_fini ()

    void
    igt_stats_fini (igt_stats_t *stats);

Frees resources allocated in `igt_stats_init()`.

#### Parameters

stats | An igt_stats_t instance |

### igt_stats_is_population ()

    bool
    igt_stats_is_population (igt_stats_t *stats);

#### Parameters

stats | An igt_stats_t instance |

#### Returns

true if `stats` represents a population, false if only a sample.

See `igt_stats_set_population()` for more details.

### igt_stats_set_population ()

    void
    igt_stats_set_population (igt_stats_t *stats,
                              bool full_population);

In statistics, we usually deal with a subset of the full data (which may be a continuous or infinite set). Data analysis is then done on a sample of this population.

This has some importance as only having a sample of the data leads to biased estimators. We currently used the information given by this method to apply Bessel's correction to the variance.

Note that even if we manage to have an unbiased variance by multiplying a sample variance by the Bessel's correction, n/(n - 1), the standard deviation derived from the unbiased variance isn't itself unbiased. Statisticians talk about a "corrected" standard deviation.

When giving true to this function, the data set in `stats` is considered a full population. It's considered a sample of a bigger population otherwise.

When newly created, `stats` defaults to holding sample data.

#### Parameters

stats | An igt_stats_t instance |
full_population | Whether we're dealing with sample data or a full population |

### igt_stats_push ()

    void
    igt_stats_push (igt_stats_t *stats,
                    uint64_t value);

Adds a new value to the `stats` dataset and converts the igt_stats from an integer collection to a floating point one.

#### Parameters

stats | An igt_stats_t instance |
value | An floating point |

### igt_stats_push_float ()

    void
    igt_stats_push_float (igt_stats_t *stats,
                          double value);

### igt_stats_push_array ()

    void
    igt_stats_push_array (igt_stats_t *stats,
                          const uint64_t *values,
                          unsigned int n_values);

Adds an array of values to the `stats` dataset.

#### Parameters

stats | An igt_stats_t instance |
values | A pointer to an array of data points. | [array length=n_values]
n_values | The number of data points to add |

### igt_stats_get_min ()

    uint64_t
    igt_stats_get_min (igt_stats_t *stats);

Retrieves the minimal value in `stats`

#### Parameters

stats | An igt_stats_t instance |

### igt_stats_get_max ()

    uint64_t
    igt_stats_get_max (igt_stats_t *stats);

Retrieves the maximum value in `stats`

#### Parameters

stats | An igt_stats_t instance |

### igt_stats_get_range ()

    uint64_t
    igt_stats_get_range (igt_stats_t *stats);

Retrieves the range of the values in `stats` . The range is the difference between the highest and the lowest value.

The range can be a deceiving characterization of the values, because there can be extreme minimal and maximum values that are just anomalies. Prefer the interquatile range (see `igt_stats_get_iqr()`) or an histogram.

#### Parameters

stats | An igt_stats_t instance |

### igt_stats_get_quartiles ()

    void
    igt_stats_get_quartiles (igt_stats_t *stats,
                             double *q1,
                             double *q2,
                             double *q3);

Retrieves the quartiles of the `stats` dataset.

#### Parameters

stats | An igt_stats_t instance |
q1 | lower or 25th quartile. | [out]
q2 | median or 50th quartile. | [out]
q3 | upper or 75th quartile. | [out]

### igt_stats_get_iqr ()

    double
    igt_stats_get_iqr (igt_stats_t *stats);

Retrieves the interquartile range (IQR) of the `stats` dataset.

#### Parameters

stats | An igt_stats_t instance |

### igt_stats_get_iqm ()

    double
    igt_stats_get_iqm (igt_stats_t *stats);

Retrieves the interquartile mean (IQM) of the `stats` dataset.

The interquartile mean is a "statistical measure of central tendency". It is a truncated mean that discards the lowest and highest 25% of values, and calculates the mean value of the remaining central values.

It's useful to hide outliers in measurements (due to cold cache etc).

#### Parameters

stats | An igt_stats_t instance |

### igt_stats_get_mean ()

    double
    igt_stats_get_mean (igt_stats_t *stats);

Retrieves the mean of the `stats` dataset.

#### Parameters

stats | An igt_stats_t instance |

### igt_stats_get_trimean ()

    double
    igt_stats_get_trimean (igt_stats_t *stats);

Retrieves the trimean of the `stats` dataset.

The trimean is a the most efficient 3-point L-estimator, even more robust than the median at estimating the average of a sample population.

#### Parameters

stats | An igt_stats_t instance |

### igt_stats_get_median ()

    double
    igt_stats_get_median (igt_stats_t *stats);

Retrieves the median of the `stats` dataset.

#### Parameters

stats | An igt_stats_t instance |

### igt_stats_get_variance ()

    double
    igt_stats_get_variance (igt_stats_t *stats);

Retrieves the variance of the `stats` dataset.

#### Parameters

stats | An igt_stats_t instance |

### igt_stats_get_std_deviation ()

    double
    igt_stats_get_std_deviation (igt_stats_t *stats);

Retrieves the standard deviation of the `stats` dataset.

#### Parameters

stats | An igt_stats_t instance |

### igt_stats_get_std_error ()

    double
    igt_stats_get_std_error (igt_stats_t *stats);

Retrieves the standard error of the mean from the `stats` dataset.

#### Parameters

stats | An igt_stats_t instance |

### igt_mean_init ()

    void
    igt_mean_init (struct igt_mean *m);

Initializes or resets `m` .

#### Parameters

m | tracking structure |

### igt_mean_add ()

    void
    igt_mean_add (struct igt_mean *m,
                  double v);

Adds a new value `v` to `m` .

#### Parameters

m | tracking structure |
v | value |

### igt_mean_get ()

    double
    igt_mean_get (struct igt_mean *m);

Computes the current mean of the samples tracked in `m` .

#### Parameters

m | tracking structure |

### igt_mean_get_variance ()

    double
    igt_mean_get_variance (struct igt_mean *m);

Computes the current variance of the samples tracked in `m` .

#### Parameters

m | tracking structure |

## Types and Values

### igt_stats_t

    typedef struct {
    	unsigned int n_values;
    	unsigned int is_float : 1;
    	union {
    		uint64_t *values_u64;
    		double *values_f;
    	};
    } igt_stats_t;

#### Members

unsigned int `n_values`; | The number of pushed values |
unsigned int `is_float` : 1; | Whether `values_f` or `values_u64` is valid |
uint64_t *`values_u64`; | An array containing pushed integer values |
double *`values_f`; | An array containing pushed float values |

### struct igt_mean

    struct igt_mean {
    };

Structure to compute running statistical numbers. Needs to be initialized with `igt_mean_init()`. Read out data using `igt_mean_get()` and `igt_mean_get_variance()`.