Timing differences in two tests: copy data from device to host

[DylanWangWQF]

Hi @jlwatson , When I test the timing results for copying data from device to host, I got the different results as shown:

1. First test to print uint64_t in DeviceData

   # util.cuh
   template<typename T, typename I>
   void printDeviceDataForInteger(DeviceData<T, I> &device_data)
   {
   std::vector<uint64_t> host_temp(device_data.size());
   thrust::copy(device_data.begin(), device_data.end(), host_temp.begin());
   
   for (int i = 0; i < host_temp.size(); i++) {
       printf("%lld ", host_temp[i]);
   }
   std::cout << std::endl;
   }
   # DeviceData.cu
   TYPED_TEST(DeviceDataTest, IntegerDeviceData) {
   using T = typename TestFixture::ParamType;
   DeviceData<T> d1 = {1, 2, 3};
   DeviceData<T> d2 = {1, 1, 1};
   d1 += d2;
   printDeviceDataForInteger(d1);
   }

   The result is around 32538ms

2. The second test is the original DRELU2 test in piranha:

   TYPED_TEST(FuncTest, DRELU2) {
   
   ....omit.....
   
   //Change to <uint8_t>
   DeviceData<uint64_t> super_result(result.size());
   reconstruct(result, super_result);
   
   printDeviceData(super_result, "actual", false);
   assertDeviceData(super_result, expected, false);
   }

   while the result is around 13ms as shown:

   [ RUN      ] FuncTest/1.DRELU2
   actual:
   0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 
   src/test/../mpc/../gpu/../util/util.cuh:343: Failure
   Expected: (fabs(host_result[i] - expected[i])) <= (epsilon), actual: 1 vs 0.001
   [  FAILED  ] FuncTest/1.DRELU2, where TypeParam = TPC<unsigned long, thrust::detail::normal_iterator<thrust::device_ptr<unsigned long> > > 
   (13 ms)

What's the reason for these two different timing results?

[DylanWangWQF]

The result is expected when I change the order of invoking each unit test. Maybe it's due to gtest.