Discussion:
Multiple Forked Reader Processes
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'Stefan Braun' via pytables-users
2017-11-13 13:12:47 UTC
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Hi, I have an hdf5 file that I have to read with multiple processes forked
from a main process.

The target dataset has ~10k samples. A single sample is requested at a time
and needs further processing (hence the multi processing).

The main process creates a dataset class instance. The multiprocess fork is
done after the dataset class initialization. Two dataset classes are
available (see attached file):
(a) Open hdf5 during class initialization and pass the opened hdf5 object
to the forked processes. The get_data() method works directly with the
already opened hdf5 from the main process.
(b) Open hdf5 during the get_data method. Each forked process controls the
hdf5 state itself.

Both variants have disadvantages:

(a) Only works with 1 forked process. More processes throw errors as the
forked processes seem to disagree about the state of the opened hdf5 object
(b) Works with >1 forked processes, but ~10x slower as each forked process
has to reopen the file each time a new sample is requested.

Is there a way to use >1 processes while avoiding the slowdown by
reopening the hdf5 file for each sample? Perhaps I am missing something
obvious?

Thanks in advance

Best

Stefan
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