Automagic GC

[yarikoptic]

I feel that we need to think about garbage collection now. /home on drogon ran out of space, and fscacher has contributed -- e.g. in my case it was 4GB for ~/.cache/fscacher.

• Ideally it should some kind of a LRU mechanism.
  Unfortunately I think we cannot rely on atime for deciding which cached files to be removed since filesystem might be mounted with relatime or noatime, so we might need another way to decide on what is recent and what is not.

• another idea could be just to help user to decide by creating lead directories for those extra tags (versions) we add into function signature (but they could become too long too fast :-/). But we could then may be make decision based on that level -- the oldest created/modified ones to go. If it was modified recently - means that the particular version is still in use

Any ideas @jwodder ?

[jwodder]

The only form of entry-deletion that joblib.Memory supports is deleting an entire cache. Here are various ideas I've come up with, in no particular order:

• We ask joblib to add LRU-based cache clearing.
• We ask joblib for the ability to iterate through a Memory cache & delete individual items and then implement LRU-based cache clearing ourselves via either:
  • Deleting items based on the file access time (as of the time the function was called & cached) in the file fingerprints
  • Asking joblib to add a "last accessed time" property to each cache item and using that to delete old items
• We ask diskcache to support use of flufl.lock for NFS-safe locking, and then we switch fscacher from joblib to diskcache.
• We write our own caching system in which each function's cache is a single SQLite3 database file with a column for last access time and with locking performed using flufl.lock.

[yarikoptic]

Overall -- I think it is worth pursuing on both (joblib and diskcache) fronts.

joblib -- I do not think it has any central DB across items to add such information, so might be a bit too much to ask but since would be generally useful, worth raising that question/issue and see what joblib authors think.

diskcache - worth raising an issue suggesting flufl.lock (I myself had no experience with it) for locking to provide support for NFS-mounted partitions. If gets implemented, we should look then indeed into diskcache as a replacement (but see below) for joblib.

our own -- I take my idea back on writing our own as a "good way forward" ;) but I wonder if we could/should provide abstraction to allow users to choose between joblib or diskcache for their specific use cases? I think joblib might be needed if arguments or return value would return numpy ndarrays -- not sure if diskcache would provide support for that. I do not think we have immediate usecases like that but I think they might arise.

[jwodder]

Issues filed:

• Add ability for joblib.Memory to delete entries not recently used joblib/joblib#1170
• NFS-safety via flufl.lock? grantjenks/python-diskcache#198

[yarikoptic]

note: another, more subtle need for GC or may be even an alternative caching mechanism -- we ran out of inodes on drogon, and fscacher's cache seems to be the major contributor :-/

[yarikoptic]

Followed up on joblib with joblib/joblib#1183