How can I disable hyperthreads, but NOT smp ?
FreeBSD 6.4-RELEASE system with two physical CPUs, each of which are HT capable. From dmesg: cpu0 (BSP): APIC ID: 0 cpu1 (AP): APIC ID: 1 cpu2 (AP): APIC ID: 6 cpu3 (AP): APIC ID: 7 I also see this: machdep.hyperthreading_allowed: 0 The problem is, I have reason to believe that my workload benefits greatly from SMP, but is not playing nicely with hyperthreading. That is, it is trying to farm out 4 things at a time to a cpu setup that really is only able to do two things at a time. How can I disable HT completely, but still retain ALL the benefits of SMP ? Thanks. ___ 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: How can I disable hyperthreads, but NOT smp ?
On 20/09/2011 21:34, Jason Usher wrote: FreeBSD 6.4-RELEASE system with two physical CPUs, each of which are HT capable. From dmesg: cpu0 (BSP): APIC ID: 0 cpu1 (AP): APIC ID: 1 cpu2 (AP): APIC ID: 6 cpu3 (AP): APIC ID: 7 I also see this: machdep.hyperthreading_allowed: 0 The problem is, I have reason to believe that my workload benefits greatly from SMP, but is not playing nicely with hyperthreading. That is, it is trying to farm out 4 things at a time to a cpu setup that really is only able to do two things at a time. How can I disable HT completely, but still retain ALL the benefits of SMP ? Thanks. It's usually a setting in the BIOS to disable Hyperthreading then reconfigure the kernel to only use 2 cores (which it might do automatically). Hyperthreading is outside the world of FreeBSD. As far as it knows there is still 4 logical processors. Disabling hyperthreading it should see 2 LPs. Richard -- Richard Collyer Blue Apex Ltd 07900 997 380 Registered as a company in England and Wales (Reg Number: 05982514). Registered Address: 42 King Richards Hill, Earl Shilton, Leicester, LE9 7EY __ Information from ESET NOD32 Antivirus, version of virus signature database 6480 (20110920) __ The message was checked by ESET NOD32 Antivirus. http://www.eset.com ___ 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: How can I disable hyperthreads, but NOT smp ?
On Tue, 20 Sep 2011 22:28:18 +0100 Richard Collyer wrote: On 20/09/2011 21:34, Jason Usher wrote: FreeBSD 6.4-RELEASE system with two physical CPUs, each of which are HT capable. From dmesg: cpu0 (BSP): APIC ID: 0 cpu1 (AP): APIC ID: 1 cpu2 (AP): APIC ID: 6 cpu3 (AP): APIC ID: 7 I also see this: machdep.hyperthreading_allowed: 0 How can I disable HT completely, but still retain ALL the benefits of SMP ? Thanks. It's usually a setting in the BIOS to disable Hyperthreading then reconfigure the kernel to only use 2 cores (which it might do automatically). Hyperthreading is outside the world of FreeBSD. As far as it knows there is still 4 logical processors. It's not completely outside; an OS should still be able to determine which logical CPUs are on the same core. And, presumably machdep.hyperthreading_allowed=0 tells the scheduler not to use the additional logical CPUs. I'm wondering if hyperthreading is unused, but the full set of logical CPU is still displayed, or perhaps 6.4 is failing to recognise the hardware. ___ 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: How can I disable hyperthreads, but NOT smp ?
On 20/09/2011 21:34, Jason Usher wrote: FreeBSD 6.4-RELEASE system with two physical CPUs, each of which are HT capable. From dmesg: cpu0 (BSP): APIC ID: 0 cpu1 (AP): APIC ID: 1 cpu2 (AP): APIC ID: 6 cpu3 (AP): APIC ID: 7 I also see this: machdep.hyperthreading_allowed: 0 The problem is, I have reason to believe that my workload benefits greatly from SMP, but is not playing nicely with hyperthreading. That is, it is trying to farm out 4 things at a time to a cpu setup that really is only able to do two things at a time. How can I disable HT completely, but still retain ALL the benefits of SMP ? If you are sure about your machine's hardware, and that machdep.hyperthreading_allowed is really set to 0 in loader.conf(5), then you may want to glance at: http://www.freebsd.org/cgi/cvsweb.cgi/src/sys/i386/i386/mp_machdep.c?annotate=1.252.2.16 to see where the enforcement of this tunable is failing, and file a PR. Although I have not tried to use them, there are supposed to be alternative mechanisms to disable cpus, or at least to ignore them in certain circumstances: --on recent versions of FreeBSD ( 6) you could use cpuset(1) to ignore some of them in certain circumstances; --on older versions of FreeBSD ( 9) you could use the sysctl machdep.hlt_logical_cpus=1 (see smp(4)); --on older versions of FreeBSD ( 9) you could use the sysctl machdep.hlt_cpus to add a bitmap of the logical cpus that you want to disable (see smp(4)); --you can use the loader hint hint.lapic.X.disable=1 to disable a cpu with APIC ID X. But note the warnings in the Andriy Gapon's commit message and UPDATING entry for: http://svnweb.FreeBSD.org/base?view=revisionrevision=222853 It's usually a setting in the BIOS to disable Hyperthreading then reconfigure the kernel to only use 2 cores (which it might do automatically). Hyperthreading is outside the world of FreeBSD. As far as it knows there is still 4 logical processors. Disabling hyperthreading it should see 2 LPs. This isn't completely true. Yes, many systems have a BIOS setting to toggle hyperthreading on and off. The loader tunable can be used for those machines that don't have such a setting, or for remote machines for which it is not convenient to change the setting. But FreeBSD attempts to determine if any of the logical cpus are hyperthreads, so that the loader tunable machdep.hyperthreading_allowed can be enforced (it was introduced a long time ago when a security hole was discovered with some early hyperthread implementations), and also because hyperthreads are treated differently from physical cpus with regard to interrupt handling, scheduling (with ULE), ACPI, PMC, etc. So FreeBSD is supposed to be hyperthread-aware. b. ___ 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
