What is the runtime api to provide number of logical processors? - multithreading

Question

Below UNIX command:
$ lscpu
Architecture: x86_64
CPU op-mode(s): 32-bit, 64-bit
Byte Order: Little Endian
CPU(s): 4
On-line CPU(s) list: 0-3
Thread(s) per core: 2
Core(s) per socket: 2
Socket(s): 1
shows,
CPU(s): 4
Thread(s) per core: 2
which is 4 X 2 = 8 logical processors. Correct me.
Below is another Linux command:
$ cat /proc/cpuinfo
processor : 0
....
cpu cores : 2
.....
processor : 1
.....
cpu cores : 2
.....
processor : 2
.....
cpu cores : 2
.....
processor : 3
.....
cpu cores : 2
.....
$
But the below program shows only 4 logical processors:
package main
import (
"fmt"
"runtime"
)
func main() {
fmt.Println(runtime.GOMAXPROCS(0)) // gives 4
fmt.Println(runtime.NumCPU()) // gives 4
}
Output:
$ go install github.com/myhub/cs61a
$ bin/cs61a
4
4
code$
More details:
$ go version
go version go1.14.1 linux/amd64
$ uname -a
Linux mohet01-ubuntu 4.15.0-99-generic #100-Ubuntu SMP Wed Apr 22 20:32:56 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux
Documentation says,
NumCPU returns the number of logical CPUs usable by the current process.
My understanding is,
Go scheduler creates OS threads(M) which will be equivalent to number of logical processors.
Why runtime api does not give value as 8?

Answer 1

According to all of your listings above, you have:
two cores per socket
and one socket
which means you have only two CPU cores. However, you also have:
two threads per core
which means your two CPUs can run up to four threads simultaneously (with some potential drawbacks, but at least in most cases this should offer significantly more computing power than using two threads total).
Since that's the actual hardware limit, the fact that Go computes the number of threads to use as 4 seems correct.
(I think the reason you are counting to eight is that you are assuming that each of the "cpus" that Linux reports supports two threads. That is not the case: there are only two physical cores, but each supports two threads, so Linux reports this as four "cpus".)