Improve performance of index cleanup: use readdir(3), not access(2) #2819
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This change makes index cleanup ~4x faster by changing how we determine whether a file mentioned by the database still exists on disk. Previously, we'd call access(2) for each file the database mentioned. Doing so produced a lot of system call overhead. Now, we read the directory entries of the directories containing the files whose existence we're checking, build a hash table from what we find, then do the existence check against this hash table instead of entering the kernel. The semantics of the cleanup check do change subtly, however. Previously, we checked whether the mentioned file was *readable*. Now we check merely that it exists. Extant but unreadable files in maildirs should be rare. BEFORE: $ time mu index --lazy-check lazily indexing maildir /home/dancol/Mail -> store /home/dancol/.cache/mu/xapian / indexing messages; checked: 0; updated/new: 0; cleaned-up: 0 real 0m19.310s user 0m1.803s sys 0m12.999s AFTER: $ time mu --debug index --lazy-check lazily indexing maildir /home/dancol/Mail -> store /home/dancol/.cache/mu/xapian - indexing messages; checked: 0; updated/new: 0; cleaned-up: 0 real 0m4.584s user 0m2.433s sys 0m2.133s
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Thanks! Merged locally / pushed. |
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This change makes index cleanup ~4x faster by changing how we determine whether a file mentioned by the database still exists on disk. Previously, we'd call access(2) for each file the database mentioned. Doing so produced a lot of system call overhead. Now, we read the directory entries of the directories containing the files whose existence we're checking, build a hash table from what we find, then do the existence check against this hash table instead of entering the kernel.
The semantics of the cleanup check do change subtly, however. Previously, we checked whether the mentioned file was readable. Now we check merely that it exists. Extant but unreadable files in maildirs should be rare.
BEFORE:
$ time mu index --lazy-check
lazily indexing maildir /home/dancol/Mail -> store /home/dancol/.cache/mu/xapian / indexing messages; checked: 0; updated/new: 0; cleaned-up: 0
real 0m19.310s
user 0m1.803s
sys 0m12.999s
AFTER:
$ time mu --debug index --lazy-check
lazily indexing maildir /home/dancol/Mail -> store /home/dancol/.cache/mu/xapian
real 0m4.584s
user 0m2.433s
sys 0m2.133s