Performance issues with newFixedThreadPool vs newSingleThreadExecutor

I am trying to test client code. So I decided to write a multi-threaded program to benchmark my client code. I am trying to measure how much time (95 Percentile)

lower the method will take

attributes = deClient.getDEAttributes(columnsList);

So below is the multi-threaded code I wrote to benchmark with the above method. I see many variations in my two scenarios -

1) First, with multi-threaded code using 20 threads

and running for 15 minutes

. I am getting 95 percentile as 37ms

. And I am using -

ExecutorService service = Executors.newFixedThreadPool(20);

      

        
        
        
      

    

2) But if I use the same program for 15 minutes

using -

ExecutorService service = Executors.newSingleThreadExecutor();

instead

ExecutorService service = Executors.newFixedThreadPool(20);

I am getting 95 percentiles as 7ms

, which is less than the specified number when I run my code with newFixedThreadPool(20)

.

Can anyone tell me what could be causing such high performance problems -

newSingleThreadExecutor vs newFixedThreadPool(20)

And in both cases, I run my program for 15 minutes

.

Below is my code -

public static void main(String[] args) {

    try {

        // create thread pool with given size
        //ExecutorService service = Executors.newFixedThreadPool(20);
        ExecutorService service = Executors.newSingleThreadExecutor();

        long startTime = System.currentTimeMillis();
        long endTime = startTime + (15 * 60 * 1000);//Running for 15 minutes

        for (int i = 0; i < threads; i++) {
            service.submit(new ServiceTask(endTime, serviceList));
        }

        // wait for termination        
        service.shutdown();
        service.awaitTermination(Long.MAX_VALUE, TimeUnit.DAYS);
    } catch (InterruptedException e) {

    } catch (Exception e) {

    }
}

      

        
        
        
      

    

Following is the class that implements Runnable interface -

class ServiceTask implements Runnable {

    private static final Logger LOG = Logger.getLogger(ServiceTask.class.getName());
    private static Random random = new SecureRandom();

    public static volatile AtomicInteger countSize = new AtomicInteger();

    private final long endTime;
    private final LinkedHashMap<String, ServiceInfo> tableLists;

    public static ConcurrentHashMap<Long, Long> selectHistogram = new ConcurrentHashMap<Long, Long>();


    public ServiceTask(long endTime, LinkedHashMap<String, ServiceInfo> tableList) {
        this.endTime = endTime;
        this.tableLists = tableList;
    }

    @Override
    public void run() {

        try {

            while (System.currentTimeMillis() <= endTime) {

                double randomNumber = random.nextDouble() * 100.0;

                ServiceInfo service = selectRandomService(randomNumber);

                final String id = generateRandomId(random);
                final List<String> columnsList = getColumns(service.getColumns());

                List<DEAttribute<?>> attributes = null;

                DEKey bk = new DEKey(service.getKeys(), id);
                List<DEKey> list = new ArrayList<DEKey>();
                list.add(bk);

                Client deClient = new Client(list);

                final long start = System.nanoTime();

                attributes = deClient.getDEAttributes(columnsList);

                final long end = System.nanoTime() - start;
                final long key = end / 1000000L;
                boolean done = false;
                while(!done) {
                    Long oldValue = selectHistogram.putIfAbsent(key, 1L);
                    if(oldValue != null) {
                        done = selectHistogram.replace(key, oldValue, oldValue + 1);
                    } else {
                        done = true;
                    }
                }
                countSize.getAndAdd(attributes.size());

                handleDEAttribute(attributes);

                if (BEServiceLnP.sleepTime > 0L) {
                    Thread.sleep(BEServiceLnP.sleepTime);
                }
            }
        } catch (Exception e) {

        }
    }
}

      

        
        
        
      

    

Updated: -

Here is my processor spec - I am running my program from a linux machine with two processors defined as:

vendor_id       : GenuineIntel
cpu family      : 6
model           : 45
model name      : Intel(R) Xeon(R) CPU E5-2670 0 @ 2.60GHz
stepping        : 7
cpu MHz         : 2599.999
cache size      : 20480 KB
fpu             : yes
fpu_exception   : yes
cpuid level     : 13
wp              : yes
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss syscall nx rdtscp lm constant_tsc arch_perfmon pebs bts rep_good xtopology tsc_reliable nonstop_tsc aperfmperf pni pclmulqdq ssse3 cx16 sse4_1 sse4_2 popcnt aes hypervisor lahf_lm arat pln pts
bogomips        : 5199.99
clflush size    : 64
cache_alignment : 64
address sizes   : 40 bits physical, 48 bits virtual
power management: