concurrency.go

package main

import (
	"fmt"
	"sync"
	"time"
)

const (
	maxConcurrency = 10
)

var (
	concurrency = 0
	mu          = new(sync.RWMutex)

	// Locker utility function
	lockTx = func(mu sync.Locker, fn func(sync.Locker)) {
		mu.Lock()
		defer mu.Unlock()
		fn(mu)
	}
	// Useful read only locker function
	readLockTx = func(mu *sync.RWMutex, fn func(sync.Locker)) {
		lockTx(mu.RLocker(), fn)
	}
)

func process(wg *sync.WaitGroup, i int) {
	lockTx(mu, func(locker sync.Locker) {
		concurrency++
	})
	defer func() {
		lockTx(mu, func(locker sync.Locker) {
			concurrency--
		})
		wg.Done()
	}()
	readLockTx(mu, func(locker sync.Locker) {
		fmt.Printf("%06d [%v] Concurrency %v\n", i, time.Now().Format(time.RFC3339), concurrency)
	})
	time.Sleep(1500 * time.Millisecond)
}

func main() {
	now := time.Now()
	defer func() {
		fmt.Printf("Result:\n  %f\n", float64((time.Now().UnixNano()-now.UnixNano()))/float64(time.Second))
	}()

	const n = 3000
	for i := 0; i < n; i += maxConcurrency {
		wg := new(sync.WaitGroup)
		for j := 0; j < maxConcurrency; j++ {
			wg.Add(1)
			go process(wg, i+j)
		}
		wg.Wait()
	}
}

Replace go process(wg, i+j) with process(wg, i+j) (just remove go!) if you confirm non-concurrent.