From: chris.m.thomasson.1@gmail.com   
      
   On 10/30/2025 5:27 PM, Waldek Hebisch wrote:   
   > Keith Thompson  wrote:   
   >> antispam@fricas.org (Waldek Hebisch) writes:   
   >> [...]   
   >>> Assuming that you have enough RAM you should try at least using   
   >>> 'make -j 3', that is allow make to use up to 3 jobs.  I wrote   
   >>> at least, because AFAIK cheapest PC CPU-s of reasonable age   
   >>> have at least 2 cores, so to fully utilize the machine you   
   >>> need at least 2 jobs.  3 is better, because some jobs may wait   
   >>> for I/O.   
   >>   
   >> I haven't been using make's "-j" option for most of my builds.   
   >> I'm going to start doing so now (updating my wrapper script).   
   >>   
   >> I initially tried replacing "make" by "make -j", with no numeric   
   >> argument.  The result was that my system nearly froze (the load   
   >> average went up to nearly 200).  It even invoked the infamous OOM   
   >> killer.  "make -j" tells make to use as many parallel processes   
   >> as possible.   
   >>   
   >> "make -j $(nproc)" is much better.  The "nproc" command reports the   
   >> number of available processing units.  Experiments with a fairly   
   >> large build show that arguments to "-j" larger than $(nproc) do   
   >> not speed things up (on a fairly old machine with nproc=4).  I had   
   >> speculated that "make -n 5" might be worthwhile of some processes   
   >> were I/O-bound, but that doesn't appear to be the case.   
   >   
   > I frequently build my project on a few different machines.  My   
   > machines typically are generously (compared to compiler need)   
   > equipped with RAM.  Measuring several builds '-j 3' gave me   
   > fastest build on 2 core machine (no hyperthreading), '-j 7'   
   > gave me fastest build on old 4 core machine with hyperthreading   
   > (so 'nproc' reported 8 cores).  In general, increasing number   
   > of jobs I see increasing total CPU time, but real time may go   
   > down because more jobs can use time where CPU(s) would be   
   > otherwise idle.  At some number of jobs I get best real time   
   > and with larger number of jobs overheads due to multiple jobs   
   > seem to dominate leading to increase in real time.  If number   
   > of jobs is too high I get slowdown due to lack of real memory.   
   >   
   > On 12 core machine (24 logical cores) I use '-j 20'.  Increasing   
   > number of jobs give sligtly faster build, but difference is   
   > small, so I prefer to have more cores availble for interactive   
   > use.   
   >   
   > Of course, that is balancing tradeoffs, your builds may have   
   > different characteristics than mine.  I just wanted to say   
   > that _sometimes_ going beyond number of cores is useful.   
   > IIUC what Bart wrote he got 3 times speedup using '-j 3'   
   > on two core machine, which is unusually good speedup.  IME   
   > normally 3 jobs on 2 core machine is neutral or gives small   
   > speedup.  OTOH with hyperthreading activationg logical core   
      
   Make sure to avoid false sharing when using hyperthreading... :^o   
      
      
   > my slow down its twin.  Consequently using less jobs than   
   > logical cores may be better.   
   >   
      
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