Configuration Information:
Machine: x86_64
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS: -O2 -g -pipe -Wall -Werror=format-security
-Wp,-D_FORTIFY_SOURCE=2 -Wp,-D_GLIBCXX_ASSERTIONS -fexceptions
-fstack-protector-strong -grecord-gcc-switches
-specs=/usr/lib/rpm/redhat/redhat-hardened-cc1
-specs=/usr/lib/rpm/redhat/redhat-annobin-cc1 -m64 -mtune=generic
-fasynchronous-unwind-tables -fstack-clash-protection -fcf-protection
-Wno-parentheses -Wno-format-security
uname output: Linux quietpc 5.6.13-200.fc31.x86_64 #1 SMP
Thu May 14 23:26:14 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux
Machine Type: x86_64-redhat-linux-gnu
Bash Version: 5.0
Patch Level: 11
Release Status: release
Description:
The 'while read' loop exhibits very different performance depending on
whether input is via redirection or pipeline (and increasingly diverging
performance with any increased data). Here are some results from my
machine (also observed in versions 3.0 and 4.0):
[tog@quietpc ~]$ ~/loop.bash
100,000 lines of length 4:
'<': real = 0.609, user = 0.522, sys = 0.087
'|': real = 1.000, user = 0.561, sys = 0.793
100,000 lines of length 8:
'<': real = 0.652, user = 0.567, sys = 0.084
'|': real = 1.788, user = 0.772, sys = 1.838
100,000 lines of length 16:
'<': real = 0.660, user = 0.541, sys = 0.118
'|': real = 2.900, user = 1.019, sys = 3.389
100,000 lines of length 32:
'<': real = 0.684, user = 0.576, sys = 0.107
'|': real = 5.962, user = 1.560, sys = 7.809
100,000 lines of length 64:
'<': real = 0.763, user = 0.677, sys = 0.086
'|': real = 11.602, user = 2.803, sys = 15.727
100,000 lines of length 128:
'<': real = 0.984, user = 0.874, sys = 0.109
'|': real = 22.837, user = 5.061, sys = 31.592
Repeat-By:
#!/bin/bash
count=${1:-10}
file=$(mktemp /tmp/loop.XXX)
string=""
TIMEFORMAT="real = %R, user = %U, sys = %S"
for loop in {1..6}
do
printf "\n%'d lines of length %d:\n" $count ${#string}
yes $string | sed ${count}q >$file
printf "'<': "; time while read line; do :; done <$file
printf "'|': "; time cat $file | while read line; do :; done
string+=$string
done
rm -f $file
