Turn off hyperthreading on dual core Atom?
I'm building a few systems using dual core Atom processors, and have noted that when the system boots up it says it has four CPUs: 2 actual cores and 2 virtual ones. But performance is a bit unsteady, and I'm wondering if it's going to be better to turn hyperthreading off. With hyperthreading, the FreeBSD scheduler simply acts as if there are 4 CPUs. Each CPU gets clock interrupts (which add overhead), and the scheduler is naive about the fact that two of the CPUs are not separate chips and could be held up if its mate has a heavy load. I do not know if the supposed higher utilization of the resources on each chip (including executing one thread while the CPU waits for data for another) is worth it. What has your experience been? --Brett Glass ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to freebsd-questions-unsubscr...@freebsd.org
Re: Turn off hyperthreading on dual core Atom?
On Mon, 29 Aug 2011 12:24:08 -0500, Brett Glass br...@lariat.net wrote: With hyperthreading, the FreeBSD scheduler simply acts as if there are 4 CPUs. Each CPU gets clock interrupts (which add overhead), and the scheduler is naive about the fact that two of the CPUs are not separate chips and could be held up if its mate has a heavy load. I do not know if the supposed higher utilization of the resources on each chip (including executing one thread while the CPU waits for data for another) is worth it. What has your experience been? In my experience hyperthreading is useful in very few environments. I personally disabled it on my Atom machine which acts as a NAS among other things. I have noticed an improvement in performance and also in responsiveness. YMMV, etc etc etc :-) Regards, Mark ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to freebsd-questions-unsubscr...@freebsd.org
Re: Turn off hyperthreading on dual core Atom?
On 29/08/2011 18:24, Brett Glass wrote: With hyperthreading, the FreeBSD scheduler simply acts as if there are 4 CPUs. Each CPU gets clock interrupts (which add overhead), and the scheduler is naive about the fact that two of the CPUs are not separate chips and could be held up if its mate has a heavy load. I do not know if the supposed higher utilization of the resources on each chip (including executing one thread while the CPU waits for data for another) is worth it. What has your experience been? Actually, the ULE scheduler does know about HyperThreading and the topology of such CPUs. I don't know what it does with the information, but it probably works to optimize cache usage etc. -- Bruce Cran ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to freebsd-questions-unsubscr...@freebsd.org
Re: Turn off hyperthreading on dual core Atom?
At 01:55 PM 8/29/2011, Bruce Cran wrote: Actually, the ULE scheduler does know about HyperThreading and the topology of such CPUs. I don't know what it does with the information, but it probably works to optimize cache usage etc. Alas, during a recent kernel build, I used the -j2 command line option in make and watched as the scheduler repeatedly assigned two instances of cc (the most CPU-intensive program) to the same core. During that process, I also watched CPU utilization in top(1). The peak was 46% idle, which means that HTT appeared to be making at most a 4% difference. (If the peak were 50% idle, HTT would be doing nothing at all, because top(1) can't tell that there aren't really 4 CPUs.) --Brett Glass ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to freebsd-questions-unsubscr...@freebsd.org
Re: Turn off hyperthreading on dual core Atom?
On Mon, Aug 29, 2011 at 4:15 PM, Brett Glass br...@lariat.net wrote Alas, during a recent kernel build, I used the -j2 command line option in make and watched as the scheduler repeatedly assigned two instances of cc (the most CPU-intensive program) to the same core. During that process, I also watched CPU utilization in top(1). The peak was 46% idle, which means that HTT appeared to be making at most a 4% difference. (If the peak were 50% idle, HTT would be doing nothing at all, because top(1) can't tell that there aren't really 4 CPUs.) You can achieve definitive answers by timing several build runs of each setting and using ministat. -- Adam Vande More ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to freebsd-questions-unsubscr...@freebsd.org
Re: Turn off hyperthreading on dual core Atom?
On 29/08/2011 23:15, Brett Glass wrote: At 01:55 PM 8/29/2011, Bruce Cran wrote: Actually, the ULE scheduler does know about HyperThreading and the topology of such CPUs. I don't know what it does with the information, but it probably works to optimize cache usage etc. Alas, during a recent kernel build, I used the -j2 command line option in make and watched as the scheduler repeatedly assigned two instances of cc (the most CPU-intensive program) to the same core. I might be wrong, but that would be the result I expect if I were to pass -j2 to a dual core CPU. If I wanted the the compilation to run on both core I would use -j3. The good old number of cores+1. I think the last compilation slot is used to prepare the next compile/do trivial compile, so that the cores dedicated to compile can switch from one task to the next faster. The interesting test to do would be if you do -j3 would ULE assign the second compile to HT or to second core. During that process, I also watched CPU utilization in top(1). The peak was 46% idle, which means that HTT appeared to be making at most a 4% difference. (If the peak were 50% idle, HTT would be doing nothing at all, because top(1) can't tell that there aren't really 4 CPUs.) H.T varies greatly from one processor to the next, on the Pentium 4 1st gen it is a sad joke, on the Atom it is a small help for easy tasks, on i7 it is almost as good as a real core (Translation : if you use only the HT of a core you will get roughly the same perfs as if you use only the direct core) None the less H.T is just another entry point on the same core, so if HT is not used at all you will have 100% CPU power on direct, if direct is not used at all you will have 30%-95% cpu power on HT (depending on the processor). If both are used at he same time you will get between 45%+20% (P4 1st gen) to 50%+45% (i7 last gen). But since raw CPU power is not the only thing that matters in real world you can get up to +25% perfs on a I7 with HT enabled. --Brett Glass ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to freebsd-questions-unsubscr...@freebsd.org ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to freebsd-questions-unsubscr...@freebsd.org
Re: Turn off hyperthreading on dual core Atom?
On Mon, 29 Aug 2011 15:15:02 -0600 Brett Glass wrote: At 01:55 PM 8/29/2011, Bruce Cran wrote: Actually, the ULE scheduler does know about HyperThreading and the topology of such CPUs. I don't know what it does with the information, but it probably works to optimize cache usage etc. Alas, during a recent kernel build, I used the -j2 command line option in make and watched as the scheduler repeatedly assigned two instances of cc (the most CPU-intensive program) to the same core. Doesn't that make sense. If the scheduler can't put two threads from the same process on a core, it puts two processes running in the same binary. BTW I thought hyperthreading was off by default on security grounds. ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to freebsd-questions-unsubscr...@freebsd.org
