Buffer your IO on CMake!

November 05, 2014

A small CMake optimisation win that’s probably worth sharing.

Calling

file (APPEND

can massively impact your configure time performance. Especially on Windows, where it appears that filesystem writes are unbuffered (at least this is the case for CMake on Windows or Windows generally. I’m not sure).

BufferedIO.cmake

set (MY_FILE_CONTENTS)
foreach (VAR RANGE 500)
    list (APPEND MY_FILE_CONTENTS "${VAR}\n")
endforeach ()
file (WRITE output ${MY_FILE_CONTENTS})

AppendEverything.cmake

set (MY_FILE_CONTENTS)
foreach (VAR RANGE 500)
    file (APPEND output "${VAR}\n")
endforeach ()

$ time cmake -P BufferedIO.cmake

real 0m.032s
user 0m0.000s
sys 0m0.000s

$ time cmake -P AppendEverything.cmake

real 0m.657s
user 0m0.000s
sys 0m0.000s

Ouch.