Maxpool2d edge value is negative at output

Arun_Vishwanathan · January 29, 2020, 7:06pm

I have a simple network consisting of a conv2d following by maxpool2d.

My maxpool2d has padding of (2,2) and stride is (1,1) and kernel_size is (5,5) and as per the documentation If :attr:`padding` is non-zero, then the input is implicitly zero-padded on both sides

Now when I print my input to pool this is how it looks (2,4,7,7 is the input size)

          [ 0.0489,  1.6420,  0.2287, -1.0550, -1.1927, -0.8047,  0.0784],
          [ 3.1435, -1.5051, -1.0993, -1.2291,  1.9592,  0.9374,  1.0278],
          [ 4.1902, -1.6815, -1.4005,  0.3315,  0.4801,  0.6345,  2.0060],
          [-0.3247, -2.6899, -1.1491,  3.6877,  3.4306,  2.6198,  0.1040],
          [-0.5162,  0.6600,  2.8160,  5.4908,  3.5650,  2.4575,  1.6125],
          [-5.6328, -0.9014,  1.5804,  2.9726,  2.2904,  0.5041,  3.1346]],

         [[-0.1914, -2.8677,  0.0729,  2.4558,  4.0144,  1.8283,  0.5931],
          [ 0.1440,  0.3167, -0.2808,  0.9678,  3.0057,  0.9184,  0.6368],
          [ 3.9118, -1.1746,  0.4501, -1.8484,  0.5270,  2.2912,  1.9351],
          [ 2.6868,  0.3502,  1.0592,  1.5951, -0.4269,  0.0184,  1.6321],
          [-0.4935, -0.2281,  1.2582,  5.1018,  2.0337,  4.3594,  1.0276],
          [ 2.2973,  0.4393,  1.2802,  2.7683,  3.5030,  0.4795,  3.1873],
          [-1.6443,  0.5248,  1.1553,  2.5217,  1.1599,  5.2611, -0.7326]],

         [[-3.9278, -3.7870, -1.1932,  1.3942,  4.4397,  1.5615, -0.1242],
          [-1.4697, -2.1559, -0.3979, -0.3652,  1.0954,  0.1458, -0.3445],
          [ 3.2035, -3.2459, -1.2674, -1.8574,  0.8088, -0.2128,  1.6267],
          [ 3.3607, -1.1613, -1.5472,  0.2017,  0.0222,  0.2070,  2.4390],
          [ 0.2863, -1.8395, -1.5054,  2.4502,  3.5512,  3.7417, -0.4721],
          [-0.5731,  0.1661,  1.1135,  4.8935, -0.7547,  1.6938,  1.6246],
          [-3.2584,  0.2666,  2.7086,  2.9518,  2.2872,  2.5348,  0.3620]],

         [[-4.0493, -4.8643, -0.7076,  2.3171,  5.8105,  2.9847, -2.6179],
          [-2.4079,  0.6828, -0.6392, -0.5281,  1.1452,  1.7984, -0.5071],
          [ 1.6854, -1.5713, -2.0388, -2.2313,  1.1435,  2.6450,  1.4343],
          [ 3.2826, -0.0465, -0.6639,  0.1978, -0.4772, -0.7204,  1.9651],
          [-0.2371, -2.9784, -1.3908,  2.1289,  4.9562,  4.0141, -0.4528],
          [ 1.6566, -1.1490,  0.1557,  3.4228,  4.1824,  1.4596,  2.2735],
          [-4.5200, -0.5847,  1.4765,  2.0553,  3.2786,  2.6815, -0.7931]]],


        [[[ 2.1042,  3.3742,  0.5247,  0.8450,  0.2137, -0.1025,  1.3134],
          [ 1.0647,  0.8145,  2.5763,  1.5895,  0.5657, -2.8018, -1.0151],
          [-1.0696,  1.5710,  0.5461,  0.1239,  1.1746,  0.8900,  0.8370],
          [ 3.2110,  3.5812,  0.6116, -0.2446,  0.8808, -0.7343, -2.4390],
          [ 1.7637,  3.0690,  4.1770,  1.4025,  0.7210, -3.2826, -3.2330],
          [ 5.7765,  6.8915,  4.2196,  2.6918, -0.9390, -2.1477, -2.2693],
          [ 3.8918,  0.8171,  4.3219,  1.7632, -3.7690, -4.8244, -4.2342]],

         [[ 2.2697,  2.8911,  3.0222, -0.1710,  1.7580,  0.7359,  2.5586],
          [-0.8067,  1.7193,  0.8415,  0.9246,  0.9927,  0.9173, -0.7195],
          [ 0.8351,  1.7259,  2.4022,  2.6045,  2.1432,  0.5825,  1.1809],
          [ 2.3973,  2.6272,  1.1421,  1.1446,  2.9422, -0.9794, -1.4130],
          [ 1.9513,  3.1972,  3.0604,  0.2871, -0.1983, -0.5607, -1.0176],
          [ 3.8872,  3.6584,  3.3600,  2.3792,  0.1604, -2.4171, -0.4698],
          [ 2.5361,  4.2509,  3.3117,  3.2684, -0.5432, -2.7799, -1.8438]],

         [[ 0.5719,  3.5698,  4.4450,  0.6703, -0.7591, -0.3706,  0.6389],
          [-1.2735,  1.0070,  0.0426,  1.1367,  0.1103, -1.4961, -2.4907],
          [ 2.8407,  3.4860,  0.5163,  0.9328,  1.9300,  1.5186, -1.0641],
          [ 1.7709,  1.3212,  1.1954,  1.3916,  0.5054, -1.4010, -1.6475],
          [ 3.0165,  2.8974,  2.8397, -0.0489,  0.5612, -3.9587, -2.1370],
          [ 4.8533,  6.0433,  4.3979,  2.2355,  0.6366, -2.7814, -1.8583],
          [ 2.5606,  3.4099,  4.3756,  4.0021, -2.2347, -2.7669, -3.4700]],

         [[ 2.5396,  3.8363,  2.3450,  0.6624, -0.2863,  0.3438,  1.9313],
          [ 0.1720,  1.0229,  2.3096,  0.9799, -1.3438, -2.1497, -2.2450],
          [ 0.2471,  1.9793,  1.8274,  0.7245,  0.3894,  0.7552,  1.4527],
          [ 3.4289,  2.2729, -0.5627,  0.5417,  1.3856, -1.4114, -1.7625],
          [ 2.4366,  4.1636,  2.5470,  0.7687, -0.3675, -2.0235, -2.7498],
          [ 6.1177,  6.5956,  3.6907,  2.4760, -0.5046, -2.4810, -1.7314],
          [ 3.7165,  3.0131,  4.2347,  2.3551, -1.5218, -3.7080, -4.3321]]]],

and the output of pool is also (2,4,7,7) :

          [ 4.1902,  4.1902,  5.4995,  5.4995,  5.4995,  5.4995,  5.4995],
          [ 4.1902,  4.1902,  5.4995,  5.4995,  5.4995,  5.4995,  5.4995],
          [ 4.1902,  5.4908,  5.4908,  5.4908,  5.4908,  5.4908,  3.5650],
          [ 4.1902,  5.4908,  5.4908,  5.4908,  5.4908,  5.4908,  3.5650],
          [ 4.1902,  5.4908,  5.4908,  5.4908,  5.4908,  5.4908,  3.5650],
          [ 2.8160,  5.4908,  5.4908,  5.4908,  5.4908,  5.4908,  3.5650]],

         [[ 3.9118,  3.9118,  4.0144,  4.0144,  4.0144,  4.0144,  4.0144],
          [ 3.9118,  3.9118,  4.0144,  4.0144,  4.0144,  4.0144,  4.0144],
          [ 3.9118,  5.1018,  5.1018,  5.1018,  5.1018,  5.1018,  4.3594],
          [ 3.9118,  5.1018,  5.1018,  5.1018,  5.1018,  5.1018,  4.3594],
          [ 3.9118,  5.1018,  5.1018,  5.2611,  5.2611,  5.2611,  5.2611],
          [ 2.6868,  5.1018,  5.1018,  5.2611,  5.2611,  5.2611,  5.2611],
          [ 2.2973,  5.1018,  5.1018,  5.2611,  5.2611,  5.2611,  5.2611]],

         [[ 3.2035,  3.2035,  4.4397,  4.4397,  4.4397,  4.4397,  4.4397],
          [ 3.3607,  3.3607,  4.4397,  4.4397,  4.4397,  4.4397,  4.4397],
          [ 3.3607,  3.3607,  4.4397,  4.4397,  4.4397,  4.4397,  4.4397],
          [ 3.3607,  4.8935,  4.8935,  4.8935,  4.8935,  4.8935,  3.7417],
          [ 3.3607,  4.8935,  4.8935,  4.8935,  4.8935,  4.8935,  3.7417],
          [ 3.3607,  4.8935,  4.8935,  4.8935,  4.8935,  4.8935,  3.7417],
          [ 2.7086,  4.8935,  4.8935,  4.8935,  4.8935,  4.8935,  3.7417]],

         [[ 1.6854,  2.3171,  5.8105,  5.8105,  5.8105,  5.8105,  5.8105],
          [ 3.2826,  3.2826,  5.8105,  5.8105,  5.8105,  5.8105,  5.8105],
          [ 3.2826,  3.2826,  5.8105,  5.8105,  5.8105,  5.8105,  5.8105],
          [ 3.2826,  3.4228,  4.9562,  4.9562,  4.9562,  4.9562,  4.9562],
          [ 3.2826,  3.4228,  4.9562,  4.9562,  4.9562,  4.9562,  4.9562],
          [ 3.2826,  3.4228,  4.9562,  4.9562,  4.9562,  4.9562,  4.9562],
          [ 1.6566,  3.4228,  4.9562,  4.9562,  4.9562,  4.9562,  4.9562]]],


        [[[ 3.3742,  3.3742,  3.3742,  3.3742,  2.5763,  1.5895,  1.3134],
          [ 3.5812,  3.5812,  3.5812,  3.5812,  2.5763,  1.5895,  1.3134],
          [ 4.1770,  4.1770,  4.1770,  4.1770,  4.1770,  1.5895,  1.3134],
          [ 6.8915,  6.8915,  6.8915,  6.8915,  4.2196,  2.6918,  1.1746],
          [ 6.8915,  6.8915,  6.8915,  6.8915,  4.3219,  2.6918,  1.1746],
          [ 6.8915,  6.8915,  6.8915,  6.8915,  4.3219,  2.6918,  0.8808],
          [ 6.8915,  6.8915,  6.8915,  6.8915,  4.3219,  2.6918,  0.7210]],

         [[ 3.0222,  3.0222,  3.0222,  3.0222,  3.0222,  2.6045,  2.5586],
          [ 3.0222,  3.0222,  3.0222,  3.0222,  3.0222,  2.9422,  2.9422],
          [ 3.1972,  3.1972,  3.1972,  3.1972,  3.0604,  2.9422,  2.9422],
          [ 3.8872,  3.8872,  3.8872,  3.6584,  3.3600,  2.9422,  2.9422],
          [ 4.2509,  4.2509,  4.2509,  4.2509,  3.3600,  3.2684,  2.9422],
          [ 4.2509,  4.2509,  4.2509,  4.2509,  3.3600,  3.2684,  2.9422],
          [ 4.2509,  4.2509,  4.2509,  4.2509,  3.3600,  3.2684,  0.1604]],

         [[ 4.4450,  4.4450,  4.4450,  4.4450,  4.4450,  1.9300,  1.9300],
          [ 4.4450,  4.4450,  4.4450,  4.4450,  4.4450,  1.9300,  1.9300],
          [ 4.4450,  4.4450,  4.4450,  4.4450,  4.4450,  1.9300,  1.9300],
          [ 6.0433,  6.0433,  6.0433,  6.0433,  4.3979,  2.2355,  1.9300],
          [ 6.0433,  6.0433,  6.0433,  6.0433,  4.3979,  4.0021,  1.9300],
          [ 6.0433,  6.0433,  6.0433,  6.0433,  4.3979,  4.0021,  0.6366],
          [ 6.0433,  6.0433,  6.0433,  6.0433,  4.3979,  4.0021,  0.6366]],

         [[ 3.8363,  3.8363,  3.8363,  3.8363,  2.3450,  1.9313,  1.9313],
          [ 3.8363,  3.8363,  3.8363,  3.8363,  2.3450,  1.9313,  1.9313],
          [ 4.1636,  4.1636,  4.1636,  4.1636,  2.5470,  1.9313,  1.9313],
          [ 6.5956,  6.5956,  6.5956,  6.5956,  3.6907,  2.4760,  1.4527],
          [ 6.5956,  6.5956,  6.5956,  6.5956,  4.2347,  2.4760,  1.4527],
          [ 6.5956,  6.5956,  6.5956,  6.5956,  4.2347,  2.4760,  1.3856],
          [ 6.5956,  6.5956,  6.5956,  6.5956,  4.2347,  2.4760, -0.3675]]]],

Could someone please explain to me why the last value is negative the -0.3675?? Should it not be 0 as the max as it is zero padded? The last block contains all negative values with zero padding in the pool kernel

albanD · January 30, 2020, 1:11am

What is the padding size you use? Could you give a small code sample that reproduces this?

Arun_Vishwanathan · January 30, 2020, 7:11pm

I don’t have a reproduction code to share unfortunately. Padding size is (2,2) and kernel size is (5,5) , rest are all defaults. Input coming in is (2,4,7,7). Maybe this can be reproduced if you initialize all the input values to pool to be negative at the bottom right corner that contains the kernel for pooling with zero padding as you see in my example above.

albanD · January 30, 2020, 7:22pm

In the cpu implementation we have, they are actually ignored. here is the implementation. But possibly, the cuda code does something different.

Could you open an issue on github stating that in the best case the doc is incorrect and in the worst case our implementations across devices are not consistent.
A simple repro to put there:

import torch
inp = -torch.ones(2, 4, 7, 7)

print(torch.nn.MaxPool2d(5, padding=2)(inp))

Arun_Vishwanathan · January 30, 2020, 7:46pm

Thanks so much for confirming this! I have not checked the cuda code yet. To clarify, there is no option today to configure at the user API level how to use the padding, correct?

albanD · January 30, 2020, 7:52pm

No there is no configuration for how the padding is used.