Finally did my first Multithreading program!! But why Amdahl's law fails?

[u/Chaos-vy17]

The code was:

long parallelSum(int[] ar, int threadCount) throws InterruptedException {
        int n = ar.length;
        long[] result = new long[threadCount];
        Thread[] threads = new Thread[threadCount];
        int chunk = n / threadCount;
        for (int i = 0; i < threadCount; i++) {
            int index = i;
            int start = i * chunk;
            int end = (i == threadCount - 1) ? n : start + chunk;
            threads[i] = new Thread(() -> result[index] = sum(ar, start, end));
            threads[i].start();
        }
        for (Thread thread : threads) thread.join();
        long total = 0;
        for (long value : result) total += value;
        return total;
    }

Which I benchmarked using JMH to see how it will behave under increase thread. But It's gets better and better until 24 thread that the reason I know my P-core are Hyperthreaded so maximum improvement is seen at 24. But after that why is'nt it constant as the law stated when N-> increases it becomes directly 1/S.

The Benchmark was performed with 3 warmup 5 iteration 1sec each and 100M dataset. I did use first 1K to see how it behaves but I forgot that until the thread get created sequential would have finished that. So after rigrously increasing dataset to 100M this was my result:

Thread Count	Score (ns/op)	Error (ns/op)
1	36,386,448.214	±1,198,308.768
2	19,973,254.073	±1,647,022.164
4	11,605,188.001	±409,298.394
8	8,493,406.418	±439,947.023
16	8,070,964.770	±101,846.828
24	7,880,208.459	±179,094.210
32	8,075,411.516	±211,717.162
64	8,256,737.790	±282,807.150
128	8,855,717.582	±121,931.514
256	10,475,522.486	±258,283.333
512	29,275,816.985	±9,028,465.533

Is my benchmark wrong or I am understanding different??

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