[Numpy-discussion] Changing FFT cache to a bounded LRU cache

[Sebastian Berg]

On Fr, 2016-05-27 at 22:51 +0200, Lion Krischer wrote:
> Hi all,
> 
> I was told to take this to the mailing list. Relevant pull request:
> https://github.com/numpy/numpy/pull/7686
> 
> 
> NumPy's FFT implementation caches some form of execution plan for
> each
> encountered input data length. This is currently implemented as a
> simple
> dictionary which can grow without bounds. Calculating lots of
> different
> FFTs thus cause a memory leak from the users' perspective. We
> encountered a real world situation where this is an issue.
> 
> The PR linked above proposes to replace the simple dictionary with an
> LRU (least recently used) cache. It will remove the least recently
> used
> pieces of data if it grows beyond a specified size (currently an
> arbitrary limit of 100 MB per cache). Thus almost all users will
> still
> benefit from the caches but their total memory size is now limited.
> 
> 
> Things to consider:
> 
> * This adds quite some additional complexity but I could not find a
> simple way to achieve the same result.
> * What is a good limit on cache size? I used 100 MB because it works
> for
> my uses cases.
> 

I am +1 in principle, since I don't like that the cache might just grow
forever and I see that as a bug right now personally. One that rarely
hits maybe, but a bug.

I guess if you have the time, the perfect thing would be if you could
time how big the cache difference is anyway, etc.?
The cache mostly gets a working array I think, so does it even help for
large arrays or is the time spend for the allocation negligible anway
then?
We also have a small array cache in numpy anyway nowadays (not sure how
small small is here). Maybe this already achieves everything that the
fftcache was designed for and we could even just disable it as default?

The complexity addition is a bit annoying I must admit, on python 3
functools.lru_cache could be another option, but only there.

- Sebastian


> 
> Cheers!
> 
> Lion
> _______________________________________________
> NumPy-Discussion mailing list
> NumPy-Discussion at scipy.org
> https://mail.scipy.org/mailman/listinfo/numpy-discussion
> 
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 819 bytes
Desc: This is a digitally signed message part
URL: <http://mail.python.org/pipermail/numpy-discussion/attachments/20160528/cceea7d9/attachment.sig>