On Thu, Feb 9, 2012 at 2:53 PM, Mike Meyer <m...@mired.org> wrote: > For those of you not watching -ideas, or ignoring the "Python TIOBE > -3%" discussion, this would seem to be relevant to any discussion of > reworking the import mechanism: > > http://mail.scipy.org/pipermail/numpy-discussion/2012-January/059801.html > > Interesting. This gives me an idea for a way to cut stat calls per sys.path entry per import by roughly 4x, at the cost of a one-time directory read per sys.path entry.
That is, an importer created for a particular directory could, upon first use, cache a frozenset(listdir()), and the stat().st_mtime of the directory. All the filename checks could then be performed against the frozenset, and the st_mtime of the directory only checked once per import, to verify whether the frozenset() needed refreshing. Since a failed module lookup takes at least 5 stat checks (pyc, pyo, py, directory, and compiled extension (pyd/so)), this cuts it down to only 1, at the price of a listdir(). The big question is how long does a listdir() take, compared to a stat() or failed open()? That would tell us whether the tradeoff is worth making. I did some crude timeit tests on frozenset(listdir()) and trapping failed stat calls. It looks like, for a Windows directory the size of the 2.7 stdlib, you need about four *failed* import attempts to overcome the initial caching cost, or about 8 successful bytecode imports. (For Linux, you might need to double these numbers; my tests showed a different ratio there, perhaps due to the Linux stdib I tested having nearly twice as many directory entries as the directory I tested on Windows!) However, the numbers are much better for application directories than for the stdlib, since they are located earlier on sys.path. Every successful stdlib import in an application is equal to one failed import attempt for every preceding directory on sys.path, so as long as the average directory on sys.path isn't vastly larger than the stdlib, and the average application imports at least four modules from the stdlib (on Windows, or 8 on Linux), there would be a net performance gain for the application as a whole. (That is, there'd be an improved per-sys.path entry import time for stdlib modules, even if not for any application modules.) For smaller directories, the tradeoff actually gets better. A directory one seventh the size of the 2.7 Windows stdlib has a listdir() that's proportionately faster, but failed stats() in that directory are *not* proportionately faster; they're only somewhat faster. This means that it takes fewer failed module lookups to make caching a win - about 2 in this case, vs. 4 for the stdlib. Now, these numbers are with actual disk or network access abstracted away, because the data's in the operating system cache when I run the tests. It's possible that this strategy could backfire if you used, say, an NFS directory with ten thousand files in it as your first sys.path entry. Without knowing the timings for listdir/stat/failed stat in that setup, it's hard to say how many stdlib imports you need before you come out ahead. When I tried a directory about 7 times larger than the stdlib, creating the frozenset took 10 times as long, but the cost of a failed stat didn't go up by very much. This suggests that there's probably an optimal directory size cutoff for this trick; if only there were some way to check the size of a directory without reading it, we could turn off the caching for oversize directories, and get a major speed boost for everything else. On most platforms, the stat().st_size of the directory itself will give you some idea, but on Windows that's always zero. On Windows, we could work around that by using a lower-level API than listdir() and simply stop reading the directory if we hit the maximum number of entries we're willing to build a cache for, and then call it off. (Another possibility would be to explicitly enable caching by putting a flag file in the directory, or perhaps by putting a special prefix on the sys.path entry, setting the cutoff in an environment variable, etc.) In any case, this seems really worth a closer look: in non-pathological cases, it could make directory-based importing as fast as zip imports are. I'd be especially interested in knowing how the listdir/stat/failed stat ratios work on NFS - ISTM that they might be even *more* conducive to this approach, if setup latency dominates the cost of individual system calls. If this works out, it'd be a good example of why importlib is a good idea; i.e., allowing us to play with ideas like this. Brett, wouldn't you love to be able to say importlib is *faster* than the old C-based importing? ;-)
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