cvs commit: src/sys/i386/i386 local_apic.c src/sys/amd64/amd64 local_apic.c

[Maxim Sobolev]

John Baldwin wrote:
> On Tuesday 05 September 2006 13:15, Maxim Sobolev wrote:
>> sobomax     2006-09-05 17:15:25 UTC
>>
>>   FreeBSD src repository
>>
>>   Modified files:
>>     sys/i386/i386        local_apic.c 
>>     sys/amd64/amd64      local_apic.c 
>>   Log:
>>   The FreeBSD by default "disables" hyper-threading cores, by not scheduling
>>   any threads to them. However, it still counts those cores as "active but
>>   permanently idle" when calculating system-wide CPUs statistics. It is
>>   incorrect, since it skews statistics quite a bit and creates real problems
>>   for certain types of applications (monitoring applications for example),
>>   by making them believe that the system does have enough idle CPU 
> resources,
>>   while in fact it does not.
>>   
>>   Correct the problem by not calling performance counting routines 
> on "disabled"
>>   cores. The cleaner solution would be to just disable APIC timer interrupts 
> on
>>   those cores completely, but ENOTIME here and it is not clear if the
>>   additional complexity really worth minor performance gain.
> 
> Is this going to break various places dividing stats by hw.ncpu (in userland) 
> or mp_ncpus (in kernel)?

Well, I don't think so, since all those statistics tells is how many 
times system is found in particular state (idle, system, userland, 
interrupt),  it doesn't tell how exactly those states are distributed 
across available CPUs, so that dividing it by number of CPUs doesn't 
change the whole picture - you can only say that each CPU say is loaded 
by XX% when the whole system is loaded by XX%. Therefore, I can't 
imagine any situation when this change would create a problem.

>  (That is, are there any such places.   If so, you
> just broke them.)

No, I believe that I did not, unless you can provide example of the 
contrary.

-Maxim