APUE Note

apue.io.about_io_efficiency.sh

#! /bin/bash

set -euo pipefail

CODE=$(cat <<EOF
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

#define BUFFER_SIZE BUFFER_SIZE_T

void
main(void)
{
    int n;
    char buffer[BUFFER_SIZE];

    while (n = read(STDIN_FILENO, buffer, BUFFER_SIZE)) {
        if (write(STDOUT_FILENO, buffer, n) != n) {
            printf("failed to write\n");
            exit(1);
        }
    }

    if (n < 0) {
        printf("failed to read\n");
        exit(1);
    }

    exit(0);
}
EOF
)

code() {
  local size=${1:-4096}
  local file_name="bufsize_${size}"

  echo "${CODE}" | sed -e "s|BUFFER_SIZE_T|${size}|g" > ${file_name}.c
  gcc ${file_name}.c -o ${file_name}

  echo ${file_name}
}

random_string() {
  len=${1:-4096}
  cat /dev/urandom | tr -dc 'a-zA-Z0-9' | head -c ${len}
}
export -f random_string

run_code() {
  local size=${1:-4096}
  local len=${2:-4096}
  local binary=$(code ${size})

  result="$(sh -c "time random_string ${len} | ./${binary} &> /dev/null" 2>&1)"
  echo "${result}"
}

clear_code() {
  rm -f bufsize_*
}
trap clear_code EXIT

echo "BufSize Real  User  System"
TOTAL_BYTES=4096
for i in $(seq 0 1 19); do
  bufsize=$(echo "2 ^ ${i}" | bc)

  run_code ${bufsize} ${TOTAL_BYTES} | awk '
{
    for (i=1; i<=NF; i++)  {
        a[NR,i] = $i
    }
}
NF>p { p = NF }
END {
    for(j=1; j<=p; j++) {
        str=a[1,j]
        for(i=2; i<=NR; i++){
            str=str" "a[i,j];
        }
        print str
    }
}' | awk -v var=${bufsize} '{if (NR!=1) {print var,$0}}'
done