[Python-Dev] Importing .pyc in -O mode and vice versa

["Martin v. Löwis"]

Greg Ewing schrieb:
>> That should never be better: the system will cache the directory
>> blocks, also, and it will do a better job than Python will.
> 
> If that's really the case, then why do discussions
> of how improve Python startup speeds seem to focus
> on the number of stat calls made?

A stat call will not only look at the directory entry, but also
look at the inode. This will require another disk access, as the
inode is at a different location of the disk.

> Also, cacheing isn't the only thing to consider.
> Last time I looked at the implementation of unix
> file systems, they mostly seemed to do directory
> lookups by linear search. Unless that's changed
> a lot, I have a hard time seeing how that's
> going to beat Python's highly-tuned dictionaries.

It depends on the file system you are using. An NTFS directory
lookup is a B-Tree search; NT has not been doing linear search
since its introduction 15 years ago. Linux only recently started
doing tree-based directories with the introduction of ext4.
However, Linux' in-memory directory cache (the dcache) doesn't
need to scan over the directory block structure; not sure whether
it uses linear search still.

For a small directory, the difference is likely negligible. For
a large directory, the cost of reading in the entire directory
might be higher than the savings gained from not having to
search it. Also, if we do our own directory caching, the question
is when to invalidate the cache.

Regards,
Martin