ULE and Prescott question
ivoras at freebsd.org
Thu Jul 23 20:23:08 UTC 2009
> On Wed, 22 Jul 2009 22:33:46 +0200
> Ivan Voras <ivoras at freebsd.org> wrote:
>> Scott Bennett wrote:
>>> This is a curiousity question. I'm running 7.2-STABLE at
>>> present on an old Inspiron XPS, which has a 3.4 GHz P4 Prescott
>>> CPU. I have hyperthreading enabled in the kernel. The question
>>> is: is there any appreciable performance difference to be expected
>>> with this hardware setup between the ULE scheduler and the 4BSD
>>> scheduler? Or does the fact that there is only one core eliminate
>>> any difference in performance characteristics?
>> I'd guess the second thing. It's not like there's cache to be shared
>> between cores, etc.
> But with hyperthreading enabled, don't you have virtual CPUs sharing
> L1 cache
> rather that cores sharing L2 cache, making the case for ULE
> even stronger?
If you're thinking about ULEs "soft-pinning" of processes to CPUs then I
don't think so for two reasons: it's not like 4BSD forces processes
ping-ponging everywhere - for 2 logical CPUs it's not that there's much
choice of where to schedule a process - and thread switches between HTT
logical CPUs is supposed to be cheap - I think since the L1 is shared,
HTT cores have access to cached data from "the other" core for no cost.
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