Re: virtual memory, was Re: Largest size array in Gnat 2005 for the PC?

[Ludovic Brenta <ludovic@ludovic-brenta.org>]

tmoran@acm.org writes:
>>How much virtual memory does your computer have?
>   I wonder how many programmers today have any experience with the
> judicious use of virtual memory.  PC users almost always buy enough
> physical memory to satisfy all their apps, and with 32 bit machines
> that was cheap and easy.
>   There will probably also be a learning curve as todays' programmers
> find out what "judicious use" means w.r.t. multiple CPUs.

My laptop has two cores and two gigs of memory.  I routinely build GCC
4.2 in a 1280-MiB tmpfs in memory.  This is large enough for all
temporary files; at the end of the build, just under 990 MiB are still
in use.  The build uses 3 threads (i.e. one more than the number of
cores) and the system uses approximately 400 Mb of physical swap, but
that's because I keep X, emacs and galeon running all the time :) No
thrashing takes place and the system is very responsive even during a
build.

In my experience, building in a tmpfs is approximately 3 times as fast
as on a regulsr (XFS) filesystem, even though the kernel has lots of
physical memory to use as a disk cache.

I suggested this solution to Xavier Grave, who builds GCC 4.2 on a
dual-core, dual-thread POWER5 machine with 8 GiB of physical RAM.  He
also reported a three-fold improvement (roughly) even though each of
the cores has a 36 MiB level 3 cache that can probably hold all of
GCC :)

Note that we didn't even bother to measure the improvement precisely,
so it is very unscientific.

-- 
Ludovic Brenta.