GalSim-developers / GalSim-developers/GalSim

random number generators perform unnecessary copy

Open
#1,351 1 comment 0 reactions 0 assignees View on GitHub

Nobody has claimed this yet.

optimization/performance
Dominant language
Python
Stars
272
Forks
121
PR merge metrics
No merged PRs in 30d

Description

Taking generate as an example:
https://github.com/GalSim-developers/GalSim/blob/ee177aa50424dbbc5cec4a71cadd6297109c2aa1/galsim/random.py#L265-L271

The result of ascontinguousarray can share memory with the input array yet the if array_1d.data != array.data: check can fail resulting in an unnecessary copy.

The issue can be illustrated in the following code:

import numpy as np
array = np.empty((10, 20, 30), dtype=float)
array_1d = np.ascontiguousarray(array.ravel(),dtype=float)

# data is not the same
assert array.data != array_1d.data

# because array_1d is a view of array
assert array.data == array_1d.base.data  # note the use of base here

# arrays share memory
assert np.shares_memory(array, array_1d)

# double check
array_1d[:] = 2
assert np.all(array == 2)

Replacing if array_1d.data != array.data: with if not np.shares_memory(array, array_1d): should fix the issue.

Contributor guide

No contributing guide indexed for this repository

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start in galsim/random.py at generate, especially the linked lines 265-271, and reproduce the NumPy example from the issue to confirm that the flattened array can share memory with the input. Verify that the completed change avoids an unnecessary copy when memory is shared and preserves the expected array contents.

Written by the indexing model from the issue text.

Assessment

Tech stack
numpy, python
Domain
backend
Issue type
Bug
Difficulty
2/5
Estimated time
1-3 hours
Activity status
Stale
Clarity
Clearly specified
Newbie friendliness
45/100

Get new issues in your inbox

A short digest of beginner-friendly GitHub issues.