Re: Two identical systems, radically different performance

On 10/11/2012 04:19 PM, Claudio Freire wrote:
> On Thu, Oct 11, 2012 at 11:14 AM, Andrea Suisani <sickpig@opinioni.net> wrote:
>> sorry to come late to the party, but being in a similar condition
>> I've googled a bit and I've found a way to disable hyperthreading without
>> the need to reboot the system and entering the bios:
>>
>> echo 0 >/sys/devices/system/node/node0/cpuX/online
>>
>> where X belongs to 1..(#cores * 2) if hyperthreading is enabled
>> (cpu0 can't be switched off).
>>
>> didn't try myself on live system, but I definitely will
>> as soon as I have a new machine to test.
>
> Question is... will that remove the performance penalty of HyperThreading?

So I've added to my todo list to perform a test to verify this claim :)

> I don't think so, because a big one is the register file split (half
> the hardware registers go to a CPU, half to the other). If that action
> doesn't tell the CPU to "unsplit", some shared components may become
> unbogged, like the decode stage probably, but I'm not sure it's the
> same as disabling it from the BIOS.

Although I think that you're probably right to assume that disabling HT
through the syfs interface won't remove the performance penalty for real.

thanks

Andrea