How can I use 100% of machine power when i use [Parallel.ForEach]?

Question

        long lngSum = 0;


        Parallel.ForEach(Partitioner.Create(0L, lngMax + 1), range =>
        {
            long local = 0;
            //4cores : 9 ranges
            //64cores : 97 ranges
            for (long i = range.Item1; i <= range.Item2 - 1; i++)
            {
                if (i % 2 == 0)
                {
                    local += i;
                }
                else
                {
                    local -= i;
                }
            }
            Interlocked.Add(ref lngSum,  local);
        });

When I run this C# code in 4 cores machine, total CPU usage was 100%.

but

When I run this C# code in 64 cores machine, total CPU usage was not 100%, just only 50~60%

How can I use 100% of machine power?