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new: Don't scan unchanged directories with no sub-directories

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This can substantially reduce the cost of notmuch new in some
situations, such as when the file system cache is cold or when the
Maildir is on NFS.
This commit is contained in:
Austin Clements 2013-10-24 17:38:59 -04:00 committed by David Bremner
parent 9f9a63c863
commit 516efb7807
1 changed files with 29 additions and 0 deletions
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29
notmuch-new.c
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@ -323,6 +323,35 @@ add_files (notmuch_database_t *notmuch,
}
db_mtime = directory ? notmuch_directory_get_mtime (directory) : 0;
/* If the directory is unchanged from our last scan and has no
* sub-directories, then return without scanning it at all. In
* some situations, skipping the scan can substantially reduce the
* cost of notmuch new, especially since the huge numbers of files
* in Maildirs make scans expensive, but all files live in leaf
* directories.
*
* To check for sub-directories, we borrow a trick from find,
* kpathsea, and many other UNIX tools: since a directory's link
* count is the number of sub-directories (specifically, their
* '..' entries) plus 2 (the link from the parent and the link for
* '.'). This check is safe even on weird file systems, since
* file systems that can't compute this will return 0 or 1. This
* is safe even on *really* weird file systems like HFS+ that
* mistakenly return the total number of directory entries, since
* that only inflates the count beyond 2.
*/
if (directory && fs_mtime == db_mtime && st.st_nlink == 2) {
/* There's one catch: pass 1 below considers symlinks to
* directories to be directories, but these don't increase the
* file system link count. So, only bail early if the
* database agrees that there are no sub-directories. */
db_subdirs = notmuch_directory_get_child_directories (directory);
if (!notmuch_filenames_valid (db_subdirs))
goto DONE;
notmuch_filenames_destroy (db_subdirs);
db_subdirs = NULL;
}
/* If the database knows about this directory, then we sort based
* on strcmp to match the database sorting. Otherwise, we can do
* inode-based sorting for faster filesystem operation. */