Faster forking of large processes on Linux?

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On Linux, you can use posix_spawn(2) with the POSIX_SPAWN_USEVFORK flag to avoid the overhead of copying page tables when forking from a large process.

See Minimizing Memory Usage for Creating Application Subprocesses for a good summary of posix_spawn(2), its advantages and some examples.

To take advantage of vfork(2), make sure you #define _GNU_SOURCE before #include <spawn.h> and then simply posix_spawnattr_setflags(&attr, POSIX_SPAWN_USEVFORK)

I can confirm that this works on Debian Lenny, and provides a massive speed-up when forking from a large process.

benchmarking the various spawns over 1000 runs at 100M RSS
                            user     system      total        real
fspawn (fork/exec):     0.100000  15.460000  40.570000 ( 41.366389)
pspawn (posix_spawn):   0.010000   0.010000   0.540000 (  0.970577)

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