[GitHub] sxjscience opened a new issue #10167: HybridBlock can be slower than Block

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sxjscience opened a new issue #10167: HybridBlock can be slower than Block
URL: https://github.com/apache/incubator-mxnet/issues/10167
 
 
   Here's an interesting example that HybridBlock can sometimes be much slower 
than Block. We define an Identity block using two strategies and compare the 
running time.
   
   ```python
   import mxnet as mx
   import time
   from mxnet.gluon import Block, HybridBlock
   
   
   class HybridIdentity(HybridBlock):
       def hybrid_forward(self, F, x):
           return x
   
   class NoHybridIdentity(Block):
       def forward(self, x):
           return x
   
   
   class LoopIdentityBlock(Block):
       def __init__(self, use_hybrid=False, prefix=None, params=None):
           super(LoopIdentityBlock, self).__init__(prefix=prefix, params=params)
           if use_hybrid:
               self.identity = HybridIdentity()
           else:
               self.identity = NoHybridIdentity()
   
       def forward(self, x):
           for i in range(100000):
               x = self.identity(x)
           return x
   
   
   for _ in range(5):
       _ = mx.nd.ones((100, 100)).sum().asscalar()
   
   hybrid_loop_block = LoopIdentityBlock(use_hybrid=True)
   loop_block = LoopIdentityBlock(use_hybrid=False)
   x = mx.nd.ones((1024, 1024, 512))
   start = time.time()
   for _ in range(5):
       loop_block(x)
       mx.nd.waitall()
   end = time.time()
   print('Identity W/o Hybrid:', end - start)
   
   start = time.time()
   for _ in range(5):
       hybrid_loop_block(x)
       mx.nd.waitall()
   end = time.time()
   print('Identity With Hybrid:', end - start)
   
   
   hybrid_loop_block.hybridize()
   start = time.time()
   for _ in range(5):
       hybrid_loop_block(x)
       mx.nd.waitall()
   end = time.time()
   print('Identity With Hybrid + hybridize:', end - start)
   ```
   
   The result is:
   ```log
   Identity W/o Hybrid: 0.3092458248138428
   Identity With Hybrid: 3.6667261123657227
   Identity With Hybrid + hybridize: 16.659943342208862
   ```
   We can find that in this case, using Block is a better choice than using 
HybridBlock.

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