Messages in this thread |  | | Date | Fri, 23 Aug 2002 09:10:33 +0100 (BST) | From | Hugh Dickins <> | Subject | Re: Hyperthreading |
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On 23 Aug 2002, Gilad Ben-Yossef wrote: > > hmm... isn't there an option to tell the kernel you are using a > HyperThreaded system, or is it detected on runtime? I mean, think about > a P4 Xeon 2 way SMP - unless told otherwise the kernel will 'see' it as > a 4 way SMP box *but* the proccessors are not equel!
There is the "noht" boot arg to tell the kernel not to use HT, and there is the X86_FEATURE_HT cpu capability flag, but I think the runtime indication you're looking for is smp_num_siblings: 1 without HT, 2 (ever more?) with HT.
> If for example, you have a task running and another task just woke up > and the scheduler needs to assign a CPU for it, choosing the other > 'instance' of the same CPU as the already running task to run it on as > opposed to choosing one of the 'instanaces' of the other seperate CPU > seems a mistake IMHO, but the scheduler won't be able to make the > judgment because it doesn't know it is running on a SMT box at all.
SMT I don't know. Your point is valid, and you'll find the 2.4 -aa and -ac trees (both using newer O1 scheduler) each have code in (or called out from) kernel/sched.c to deal with that.
The mainline 2.4 does not take that into consideration, and so far as I can see (please correct me), nor does 2.5 as yet - will probably get added from 2.4 -aa or -ac in due course. It's not an issue of correctness, just optimality.
Hugh
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