8 Replies Latest reply on Mar 6, 2015 8:51 AM by Amit Katz

    flow simulation runs faster with fewer cores?

    Dan Hofstetter

      Hi all,


      I have two PCs that I routinely use for Flow Simulation (running version 2015 SP0.0 currently).  One of them is an older six-core i7-3960X clocked at 4.5GHz, and the other is a newer six core i7-4960x clocked at 4.0GHz.  I have noticed lately that the 4960x simulations seem to be taking too long to run.  To test this, I set up the same simulation on each machine side-by-side and let them go.  Sure enough, the 3960x @ 4.5GHz was averaging 5 seconds per iteration, and the 4960x @ 4.0GHz was closer to 30s per iteration.  I tried running the simulation on the 4960x @ 4GHz again, but set the solver to only use 10 cores (instead of the maximum number which is 12 by default), and the simulation is outperforming the faster machine now at ~ 4 seconds per iteration.


      Has anyone else seen such behavior?  I'm solving using all 12 cores on the 3960x @ 4.5GHz and it works just fine.  Maybe there is an issue with parallel processing with the newer processor chips (or motherboard implementation).


      - Dan

        • Re: flow simulation runs faster with fewer cores?
          Josiah Lund

          I had this same question and have a hypothesis, but cannot confirm that I am correct. I am thinking that by using all of the processing power to compute, but computer gets hung up in the read/write stages. by leaving one or two cores free, I decreased the simulation time and also found myself dealing with a lot fewer crashes.

            • Re: flow simulation runs faster with fewer cores?
              Dan Hofstetter

              I'm not sure about that, since writing from memory to hard disk should be limited mainly by the speed of the hard disk interface and the read/write speed of your drive.  I can say that if your working folder is on an external (USB) hard drive and you are saving results periodically, the write speed for large .fld files will really slow things down (30MB/sec write speed versus 100MB/sec or better for a run-of-the-mill internal drive).


              I ran the same simulation again on the 4960x, but forgot to limit the number of cores, and it ran just as fast with all 12 as it had the run before with only 10 enabled.  Weird.  I think it's just a glitch in the solver programming.

            • Re: flow simulation runs faster with fewer cores?
              Jared Conway

              a tonne of variables here

              is hyperthreading enabled?

              are these systems on the same ram/HDD setup?

              do either of them have turboboost?


              my testing shows if you have hyperthreading disabled and everything else stays the same, core speed will win, potentially even with newer processors that are at a lower clockspeed but should be superior from benchmarks.


              it sounds like what you found was a couple of point cases. I'd guess you had something else going on with the system.

              • Re: flow simulation runs faster with fewer cores?
                Amit Katz

                We did a little investigation of this at our office, on two similar computers. One had 4 HT cores (8 virtual cores) and the other had 8 HT cores (16 virtual cores). We tested with and without HT, and what we saw is that 4 threads is about the limit of usefulmultitasking with this software. However, we didn't see any adverse effects to running it on a machine with more than 4 threads.