Intermittent OutOfMemoryError in Cupy - cupy

Question

I'm making progress with Cupy and have been able to accelerate my iterative image reconstruction that previously used numpy and C++ by about a factor of 3. Now I have an intermittent issue that points out that I seem to miss something important.
I'm running this on a Mac Pro Late 2013, OSX 10.13.6 using a NVIDIA GeForce GTX 1080 Ti.
I'm limited by memory and keep loading data to the GPU using cupy.asarray() and then removing it with setting the variables to None. This works but intermittently I get:
cupy.cuda.memory.OutOfMemoryError
I run the following loop within the iteration:
# iterations
for i in range(nr_iterations):
[...]
# loop within iterations
for idx in range(dim2):
print("GPU memory info - iteration: " + str(i) + " - used: " + str(mempool.used_bytes()) + ", total: " + str(mempool.total_bytes()) + " pinned :" + str(pinned_mempool.n_free_blocks()) + " in loop with idx = " + str(idx) )
cupy_array = cp.asarray( cpp_function(numpy_array[:,flow_idx:flow_idx+1,...]) )
# do all the work
[...]
# last line in for loop
cupy_array = None
The print out shows stable memory usage, until suddenly it throws an error at the line with cupy_array = cp.asarray(). Here an excerpt of the logging statements:
GPU memory info - iteration: 0 - used: 3477175296, total: 8529552896 pinned :2 in loop with idx = 0
GPU memory info - iteration: 0 - used: 3477175296, total: 8529552896 pinned :2 in loop with idx = 1
GPU memory info - iteration: 1 - used: 3477175296, total: 8529552896 pinned :2 in loop with idx = 0
GPU memory info - iteration: 1 - used: 3477175296, total: 8529552896 pinned :2 in loop with idx = 1
GPU memory info - iteration: 2 - used: 3477175296, total: 8529552896 pinned :2 in loop with idx = 0
GPU memory info - iteration: 2 - used: 3477175296, total: 8529552896 pinned :2 in loop with idx = 1
GPU memory info - iteration: 3 - used: 3477175296, total: 8529552896 pinned :2 in loop with idx = 0
GPU memory info - iteration: 3 - used: 3477175296, total: 8529552896 pinned :2 in loop with idx = 1
GPU memory info - iteration: 4 - used: 3477175296, total: 8529552896 pinned :2 in loop with idx = 0
File "cupy/core/core.pyx", line 1712, in cupy.core.core.array
File "cupy/core/core.pyx", line 1751, in cupy.core.core.array
File "cupy/core/core.pyx", line 134, in cupy.core.core.ndarray.__init__
File "cupy/cuda/memory.pyx", line 518, in cupy.cuda.memory.alloc
File "cupy/cuda/memory.pyx", line 1085, in cupy.cuda.memory.MemoryPool.malloc
File "cupy/cuda/memory.pyx", line 1106, in cupy.cuda.memory.MemoryPool.malloc
File "cupy/cuda/memory.pyx", line 934, in cupy.cuda.memory.SingleDeviceMemoryPool.malloc
File "cupy/cuda/memory.pyx", line 949, in cupy.cuda.memory.SingleDeviceMemoryPool._malloc
File "cupy/cuda/memory.pyx", line 697, in cupy.cuda.memory._try_malloc
cupy.cuda.memory.OutOfMemoryError: out of memory to allocate 2158119936 bytes (total 10687672832 bytes)
I am specifically surprised that in the instance of the error, the system seem to require 10687672832 bytes, where the logging suggests that 3477175296 bytes are used, while allocating 2158119936 bytes.
While I realized that I need ~twice the memory available to load an array, in this instance it actually requires more than 3 times the size of the array being loaded.
Anything obvious that I'm missing? Thank you for looking at this.