On Thu, 30 Mar 2023, Thomas Jahns wrote:
Hello Danny,
I spent some time thinking about improvements to autoconf configure scripts
(while waiting for builds to proceed). In my view, it is currently still easier
to seek small efficiency gains that, in sum, could still improve run-time
substantially than parallelizing the whole would be, because there is so much
often untapped potential:
* Use bash builtin shell commands to fork and especially exec less. In modern
systems with comparatively fast data paths compared to anything that affects
resource control, like changing memory mappings, dropping caches etc., syscalls
can be a substantial source of slow down.
* Use TMPDIR to prevent temporary files from hitting disk (use /dev/shm or
similar instead of /tmp)
* In the case of spack I've seen substantial improvements from running the
whole build in /dev/shm and think spack should at least mention the possibility
for systems with substantial amounts of RAM (and guess what kind of system many
sites using spack just happen to have?).
* The gcc option -pipe is similarly helpful to get build phases to start as
soon as possible.
The main problem with the above is that it is focused on one
particular shell, and operating system. While the combination is very
popular, Autoconf is still also a portability tool.
While continued optimization is important, I suggest that a good
strategy is to also spend more time on investigating useful caching
features. It is common that a given test permutation is executed more
than once. By implementing a smart shared cache, it should be
possible to identify similar permutations and cache the results so
that the next time they are encountered, the cached results will
simply be used. The cache could be at the user and/or whole system
level.
Autoconf already has good support for a local cache so that a
re-configure is much faster, and it has support for a system-wide
shared cache.
Speed-up due to caching can be explored by using the --config-cache
option, or using a config.cache file.
One would think that a "compiler" test should be cacheable given the
same compiler with similar options.
Bob
--
Bob Friesenhahn
bfrie...@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer, http://www.GraphicsMagick.org/
Public Key, http://www.simplesystems.org/users/bfriesen/public-key.txt
