---
title: Understanding task resource metrics
description: Interpret CPU, memory, and I/O resource usage metrics in Nextflow.
---

# Understanding task resource metrics

This tutorial explains how resource usage metrics are computed from execution reports. See [Execution report][execution-report] for more information about how to enable them.

## CPU Usage

CPU Usage plots report how CPU resources are used by each process.

![CPU usage](../_static/report-resource-cpu-noheader.png)

**Raw Usage** tabs are expected to show 100% core usage if processes perform one task of pure computation. If tasks are distributed over, 2, 3, or 4 CPUs, the raw usage will be 200%, 300%, or 400%, respectively. **% Allocated** tabs rescale raw usage values relative to the number of CPUs that are set with the `cpus` directive. If the `cpus` directive is not set, CPUs are set to `1` and **% Allocated** tabs will show the same values **Raw Usage** tabs.

For example, using the [stress](https://people.seas.harvard.edu/~apw/stress/) program, the following script would report 100% CPU usage in the **Raw Usage** tab and 50% CPU usage in the **% Allocated** tab as the process requested double the number of CPUs that are required:

```nextflow
process cpu_usage_ex1 {
    cpus 2

    script:
    """
    stress -c 1 -t 10 # compute square-root of random numbers during 10s using 1 CPU
    """
}

workflow{
    cpu_usage_ex1() // Stress using 1 CPU
}
```

:::tip
See [Linux stress command with examples](https://www.geeksforgeeks.org/linux-stress-command-with-examples/) for more information about the stress command.
:::

CPU usage decreases if processes spend some time performing pure computation and some time waiting for CPUs. For example, using the `stress` and `sleep` commands, the following script would report 75% CPU usage in the **Raw Usage** tab:

```nextflow
process cpu_usage_ex2 {
    cpus 1

    script:
    """
    stress -c 1 -t 10 # compute square-root of random numbers during 10s using 1 CPU
    stress -c 1 -t 5 # compute square-root of random numbers during 5s using 1 CPU
    sleep 5 # use no CPU during 5s
    """
}

workflow{
    cpu_usage_ex2() // Stress using 1 CPU and sleep
}
```

In the above example, CPU usage is a weighted average that accounts for the percentage of the CPU used and duration of each individual program over the job duration:

$$
\frac{ 100\% \times 10s + 100\% \times 5s + 0\% \times 5s }{10s+5s+5s} = 75\%
$$

CPU usage increases if a single step is forked on multiple CPUs:

```nextflow
process cpu_usage_ex3 {
    cpus 2

    script:
    """
    stress -c 2 -t 10 # compute square-root of random numbers during 10 s using 2 CPUs
    sleep 10 # use no CPU during 10s
    """
}

workflow{
    cpu_usage_ex3() // Stress using 2 CPUs and sleep
}
```

In the above example, the **Raw Usage** tab would report 100%:

$$
\frac{ 200\% \times 10s + 0\% \times 10s }{10s+10s} = 100\%
$$

However, the **% Allocated** tab would report 50%. It would not be relevant to change the `cpus` directive from `2` to `1` as the process uses 2 CPUs at it peak load.

## Memory Usage

Memory Usage plots report how memory was used by each process. It has three tabs, **Physical (RAM)**, **Virtual (RAM + Disk swap)**, and **% RAM Allocated**, showing the usage of the physical memory (RAM), the virtual memory (vmem), and the percentage of RAM used by the process relative to the memory that the `memory` directive set, respectively.

Peak usage during process executions is reported for both physical and virtual memories. The total amount of memory used by a process is the `virtual memory (vmem)`. The `vmem` contains all memory areas, including in the physical memory (RAM), in the swap space, on the disk, or shared with other processes. The `resident set size (RSS)` is the amount of `physical memory (RAM)` held by a process.

The relationship is:


$$
vmem \geq RSS + Swap
$$

The behavior of **Memory Usage** plots can be examined using two programs written in C. The first program allocates a variable of 1 GiB:

```c
#include <stdio.h>
#include <stdlib.h>
#include <sys/resource.h>
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>
#include <time.h>

/* Get vmem and rss usage from /proc/<pid>/statm */
static int mem_used(pid_t pid, unsigned long* vmem, unsigned long* rss) {
    FILE* file;
    char path[40];
    unsigned int page_size;

    snprintf(path, 40, "/proc/%ld/statm", (long) pid);
    file = fopen(path, "r");
    // vmem and rss are the first values in the file
    fscanf(file, "%lu %lu", vmem, rss);
    // values in statm are in pages so to get bytes we need to know page size
    page_size = (unsigned) getpagesize();
    *vmem = *vmem * page_size;
    *rss = *rss * page_size;

    fclose(file);
    return 0;
}

int main(int argc, char **argv) {
    unsigned char *address;
    char input;
    size_t size = 1024*1024*1024;  // 1 GiB
    unsigned long i;
    unsigned long vmem = 0;
    unsigned long rss = 0;
    pid_t pid;

    pid = getpid();
    printf("Pid: %ld\n", (long) pid);

    mem_used(pid, &vmem, &rss);
    printf("VMEM: %lu RSS: %lu\n", vmem, rss);

    address = malloc(size);
    printf("Allocated %d Bytes of memory\n", (int) size);

    mem_used(pid, &vmem, &rss);
    printf("VMEM: %lu RSS: %lu\n", vmem, rss);

    // Leave time for nextflow to get information
    sleep(15);

    free(address);
    return 0;
}
```

The second program allocates a variable of 1 GiB and fills it with data:

```c
#include <stdio.h>
#include <stdlib.h>
#include <sys/resource.h>
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>
#include <time.h>

/* Get vmem and rss usage from /proc/<pid>/statm */
static int mem_used(pid_t pid, unsigned long* vmem, unsigned long* rss) {
    FILE* file;
    char path[40];
    unsigned int page_size;

    snprintf(path, 40, "/proc/%ld/statm", (long) pid);
    file = fopen(path, "r");
    // vmem and rss are the first values in the file
    fscanf(file, "%lu %lu", vmem, rss);
    // values in statm are in pages so to get bytes we need to know page size
    page_size = (unsigned) getpagesize();
    *vmem = *vmem * page_size;
    *rss = *rss * page_size;

    fclose(file);
    return 0;
}

int main(int argc, char **argv) {
    unsigned char *address;
    char input;
    size_t size = 1024*1024*1024;  // 1 GiB
    unsigned long i;
    unsigned long vmem = 0;
    unsigned long rss = 0;
    pid_t pid;

    pid = getpid();
    printf("Pid: %ld\n", (long) pid);

    mem_used(pid, &vmem, &rss);
    printf("VMEM: %lu RSS: %lu\n", vmem, rss);

    address = malloc(size);
    printf("Allocated %d Bytes of memory\n", (int) size);

    mem_used(pid, &vmem, &rss);
    printf("VMEM: %lu RSS: %lu\n", vmem, rss);

    printf("Filling memory with data...");
    fflush(stdout);
    for (i = 0; i < size; i++) {
        *(address + i) = 123;
    }

    mem_used(pid, &vmem, &rss);
    printf("\nVMEM: %lu RSS: %lu\n", vmem, rss);

    // Leave time for nextflow to get information
    sleep(15);

    free(address);
    return 0;
}
```

The first and second programs are executed as `malloc` and `malloc_fill`, respectively, in the following script:

```nextflow
process malloc {
    memory '1.5 GB'

    script:
    """
    memory_vmem_1GiB_ram_0Gib
    """
}

process malloc_fill {
    memory '1.5 GB'

    script:
    """
    memory_vmem_1GiB_ram_1Gib
    """
}

workflow{
    malloc() // Allocates a variable of 1 GiB
    malloc_fill() // Allocates a variable of 1 GiB and fills it with data
}
```

The **Virtual (RAM + Disk swap)** tab shows that both `malloc` and `malloc_fill` use the same amount of virtual memory (~1 GiB):

![Memory vmem](../_static/report-resource-memory-vmem.png)

However, the **Physical (RAM)** tab shows that `malloc_fill` uses ~1 GiB of RAM while `malloc` uses ~0 GiB of RAM:

![Memory ram](../_static/report-resource-memory-ram.png)

The **% RAM Allocated** tab shows that `malloc` and `malloc_fill` used 0% and 67% of resources set in the `memory` directive, respectively:

![Memory pctram](../_static/report-resource-memory-pctram.png)

:::warning
Memory and storage metrics are reported in bytes. For example, 1 KB = 1024 bytes, 1 MB = 1024² bytes, and 1 GB = 1024³ bytes.
:::

## Job Duration

**Job Duration** plots report how long each process took to run. It has two tabs. The **Raw Usage** tab shows the job duration and the **% Allocated** tab shows the time that was requested relative to what was requested using the `time` directive. Job duration is sometimes known as elapsed real time, real time or wall time.

![Job duration](../_static/report-resource-job-duration.png)

## I/O Usage

I/O Usage plots show how much data was read and written by processes. The amount of data that was read by a process (`rchar` in trace files) is the number of bytes the process read using read-like system calls. The amount of data that was written by a process (`wchar` in trace files) is the number of bytes the process wrote using write-like system call. Read and write data is read from the file `/proc/$pid/io`.

**Read** tabs shows how much data was read and **Write** tabs shows how much data was written by each process. For example, the following script reads and writes different data volumes:

```nextflow
process io_read_write_1G {
    script:
    """
    dd if=/dev/zero of=/dev/null bs=1G count=1
    """
}

process io_read_write_256M {
    script:
    """
    dd if=/dev/zero of=/dev/null bs=256M count=1
    """
}

workflow{
    io_read_write_1G()   // Read and write 1 GiB
    io_read_write_256M() // Read and write 256 Mb
}
```

The **Read** tab shows that ~1 Gib and ~256 Mb are read:

![Resources io read](../_static/report-resource-io-read.png)

The **Write** tab shows that ~1 Gib and ~256 Mb are written:

![Resources io write](../_static/report-resource-io-write.png)

[execution-report]: ../reports#execution-report
