• via Digitalmars-d-bugs (62/62) Nov 06 2015 https://issues.dlang.org/show_bug.cgi?id=15299

via Digitalmars-d-bugs <digitalmars-d-bugs puremagic.com> writes:

https://issues.dlang.org/show_bug.cgi?id=15299

          Issue ID: 15299
           Summary: gibberish CPU/processor/thread information
           Product: D
           Version: D2
          Hardware: x86
                OS: Windows
            Status: NEW
          Severity: normal
          Priority: P1
         Component: phobos
          Assignee: nobody puremagic.com
          Reporter: 2krnk gmx.net

For parallel computations I like to know the number of physical cores the
computer system provides. For one, computation speed does not (significantly)
increase if using more parallel threads than there are cores. Also, for longer
running computations I like to keep one core free so that the system stays
responsive.

Unfortunately, the D libraries don't tell me want I need to know directly, but
only in an obfuscated way - or rather, by means of optimistic second guessing.
So there is an enhancement request hidden here. However, I filed it as a bug
because the information I do get is highly misleading, if not to say plain
wrong.

My computer system has (i7-2600):

1 processor
4 physical cores
8 logical cores ( i.e. hyperthreading feature available)

This is just using nomenclature most people can agree upon their meaning.

Now this is what the D library functions tell me about my system (windows):

std.parallelism.totalCPUs = 8
core.cpuid.coresPerCPU = 4
core.cpuid.threadsPerCPU = 8
core.cpuid.hyperThreading = true

Only core.cpuid.hyperThreading is unambiguously correct. The rest is ... the
horrors!

Naturally, it is debatable what should be understood under the term "CPU". Most
people and vendors mean what it used to mean before multi-processor
architectures came up: the whole (multi-)processor chip that sits in one
socket. AFAIK, technically speaking a CPU would refer to one physical core.

a) If CPU  == processor, the correct info would be

std.parallelism.totalCPUs = 1
core.cpuid.coresPerCPU = 4
core.cpuid.threadsPerCPU = 8

Apparently this is not what the library functions report, mainly due to
std.parallelism. Also: I have no way to test if the core.cpuid answer would be
still correct on a multi-socket/multi-processor system...


b) If CPU == physical cores, the correct info would be

std.parallelism.totalCPUs = 4
core.cpuid.coresPerCPU = 1
core.cpuid.threadsPerCPU = 2

Now this is at total mismatch to the actual answers from the D libraries. And,
of course, in this scenario "coresPerCPU" makes no sense as it will always be
1. However, the docs to std.parallelism indicate that this is the official
D-meaning of "CPU" (totalCPUs): "total number of CPU cores available".


Summary:

(1) Neither interpretation (a) nor (b) provides the correct information. Seems
like a simple inconsistency between std.parallelism and core.cpuid in
combination with bad documentation.

(2) This kind of information is irrelevant to the user anyway. Relevant
information would be: (a) total number of physical cores and (b) number of
logical cores or hyperthreading yes/no

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