checkout.c: enable fscache for checkout again

[atetubou]

This is retry of #1419.

I added flush_fscache macro to flush cached stats after disk writing
with tests for regression reported in #1438 and #1442.

git checkout checks each file path in sorted order, so cache flushing does not
make performance worse unless we have large number of modified files in
a directory containing many files.

Using chromium repository, I tested git checkout . performance when I
delete 10 files in different directories.
With this patch:
TotalSeconds: 4.307272
TotalSeconds: 4.4863595
TotalSeconds: 4.2975562
Avg: 4.36372923333333

Without this patch:
TotalSeconds: 20.9705431
TotalSeconds: 22.4867685
TotalSeconds: 18.8968292
Avg: 20.7847136

I confirmed this patch passed all tests in t/ with core_fscache=1.

Signed-off-by: Takuto Ikuta [email protected]

[atetubou]

Can we move forward on this PR? 20 seconds checkout is slow and give us less productivity.

git checkout for path runs like below (simplified very much)

for each path in git index
  if stat of path is different from index # This is slow if we call GetFileAttributes
    do some modification on the path
    update stat in git index # After this, stat of this file won't be used. So keeping fscache consistent with disk does not have much benefit.

If you don't hesitate to change git more largely, I can change git to take stat of all files before any file modification using fscache.

[dscho]

Can we move forward on this PR?

Sorry, I'd love to, but first I have to work on yet another Git for Windows version... there have been two stupid breakages in v2.16.1(2).

[atetubou]

I see. Please focus on the fix.

[atetubou]

Ping? How do you think the way like scoped fscache?

[dscho]

Ping? How do you think the way like scoped fscache?

Sorry for the long delay. I think you are right, this PR strikes the best balance between simplicity and "bang for the buck".