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fscache: add not-found directory cache to fscache #994

Merged
merged 2 commits into from
Jan 25, 2017
Merged

fscache: add not-found directory cache to fscache #994

merged 2 commits into from
Jan 25, 2017

Commits on Jan 24, 2017

  1. fscache: add key for GIT_TRACE_FSCACHE 

    Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
    @jeffhostetler
    jeffhostetler committed Jan 24, 2017
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  2. fscache: remember not-found directories 

    Teach FSCACHE to remember "not found" directories.

This is a performance optimization.

FSCACHE is a performance optimization available for Windows.  It
intercepts Posix-style lstat() calls into an in-memory directory
using FindFirst/FindNext.  It improves performance on Windows by
catching the first lstat() call in a directory, using FindFirst/
FindNext to read the list of files (and attribute data) for the
entire directory into the cache, and short-cut subsequent lstat()
calls in the same directory.  This gives a major performance
boost on Windows.

However, it does not remember "not found" directories.  When STATUS
runs and there are missing directories, the lstat() interception
fails to find the parent directory and simply return ENOENT for the
file -- it does not remember that the FindFirst on the directory
failed. Thus subsequent lstat() calls in the same directory, each
re-attempt the FindFirst.  This completely defeats any performance
gains.

This can be seen by doing a sparse-checkout on a large repo and
then doing a read-tree to reset the skip-worktree bits and then
running status.

This change reduced status times for my very large repo by 60%.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
    @jeffhostetler
    jeffhostetler committed Jan 24, 2017
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