Commits on Feb 15, 2021

1. unpack-trees:virtualfilesystem: Improve efficiency of clear_ce_flags …

   When the virtualfilesystem is enabled the previous implementation of
   clear_ce_flags would iterate all of the cache entries and query whether
   each one is in the virtual filesystem to determine whether to clear one
   of the SKIP_WORKTREE bits. For each cache entry, we would do a hash
   lookup for each parent directory in the is_included_in_virtualfilesystem
   function.
   
   The former approach is slow for a typical Windows OS enlistment with
   3 million files where only a small percentage is in the virtual
   filesystem. The cost is
   O(n_index_entries * n_chars_per_path * n_parent_directories_per_path).
   
   In this change, we use the same approach as apply_virtualfilesystem,
   which iterates the set of entries in the virtualfilesystem and searches
   in the cache for the corresponding entries in order to clear their
   flags. This approach has a cost of
   O(n_virtual_filesystem_entries * n_chars_per_path * log(n_index_entries)).
   
   The apply_virtualfilesystem code was refactored a bit and modified to
   clear flags for all names that 'alias' a given virtual filesystem name
   when ignore_case is set.
   
   n_virtual_filesystem_entries is typically much less than
   n_index_entries, in which case the new approach is much faster. We wind
   up building the name hash for the index, but this occurs quickly thanks
   to the multi-threading.
   
   Signed-off-by: Neeraj Singh <[email protected]>

   

   neerajsi-msft2 committed Feb 15, 2021
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