imports and python locking
mbp at sourcefrog.net
Thu Jul 21 13:37:36 BST 2005
Some benchmarking on bzr's hashcache showed a downside to our current
policy of doing imports in inner scopes.
Every import statement results in a builtin_import call, which needs to
take and release a lock protecting the module table. This is done by
semaphores (sem_post, etc) by default. On Linux, for some reason I
don't understand yet, this always causes a system call to release the
futex, even though I would think it's not necessary. In any case for
'bzr st' this can take up to 40% of the time.
This is compiled out on python built --without-threads, and it would be
kind of neat to run that for applications that don't use threads, but
that's not really in our scope here.
Practically, I think we should change the practice to generally doing
imports at the top level of the module, except that the top-level code
should only import modules when they're needed for the selected command.
Another approach is to use something like the attached hook to defer
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