[Sbcl-devel] [PATCH] Use MADV_DONTFORK for Lisp pages

[Stelian I. <sio...@cd...>]

Hi,

The attached patch ensures that on multi-threaded Linux, upon fork()
the kernel won't try to Copy-On-Write the mapping created by the SBCL
runtime.
This trick speeds up a fork() considerably, especially since the GC
often creates many small mappings, so a fork() becomes O(heap size).

Of course, this means that Lisp code cannot be executed any more after
a fork(), but that's unsafe anyway in a multithreaded environment.

Test methodology:

$ more /proc/cpuinfo
[...]
model name	: Intel(R) Xeon(R) CPU E3-1535M v5 @ 2.90GHz
$ uname -a
Linux vilya 4.1.20-11-default #1 SMP PREEMPT Fri Mar 18 14:42:07 UTC 2016 (0a392b2) x86_64 x86_64 x86_64 GNU/Linux

> (defparameter *a*
    (loop repeat (* 8 1024)
          collect (make-array (* 1024 1024)
                              :element-type '(unsigned-byte 8)
                              :initial-element 7))
*A*
> (time (dotimes (i 1000) (sb-ext:run-program "/usr/bin/ls" '("/dev/null") :wait t)))

Without MADV_DONTFORK:
Evaluation took:
  171.103 seconds of real time
  78.112000 seconds of total run time (0.108000 user, 78.004000 system)
  45.65% CPU
  496,886,717,858 processor cycles
  1,992,912 bytes consed

With MADV_DONTFORK:
Evaluation took:
  2.689 seconds of real time
  0.728000 seconds of total run time (0.096000 user, 0.632000 system)
  27.07% CPU
  7,809,738,979 processor cycles
  1,992,992 bytes consed

-- 
Stelian Ionescu a.k.a. fe[nl]ix
Quidquid latine dictum sit, altum videtur.