'setup_work_tree()' considered harmful

[ Dscho cc'd because I think he is the primary culprits for this thing, I 
  think. Commit e90fdc39b6903502192b2dd11e5503cea721a1ad in particular, 
  methinks. ]

So because I was looking at system call traces for object creation (due to those "rename to the final resting place" patches I did that got merged recently), I've noticed that some commands have *much* worse system call patterns than others.

In particular, doing a "git add ." will use absolute pathnames for all git files, while a "git diff" will not. And this is quite noticeable - the absolute pathnames are not just longer, they have more path components in them. Making them a lot slower to look up and use.

Of course, "a lot" depends on things a bit, but it really is noticeable.

To test, I created a kernel tree (no .git), and did a "git init" followed by a "git add .". Here's the timings for current git head:

	[torvalds@woody kernel]$ time git add .

	real    0m8.377s
	user    0m6.556s
	sys     0m1.656s

After that, I fixed "write_loose_object()" to not unnecessarily try to open a git object file, because every single caller has already done a "has_sha1_file(sha1)" or "has_loose_object(sha1)" check before calling that function, so trying to open it again is just pointless.

As a result, git add sped up a tiny bit:

	[torvalds@woody kernel]$ time ~/git/git-add .

	real    0m8.341s
	user    0m6.588s
	sys     0m1.548s

but I'll admit that it's not exactly noticeable. Half a percentage point is not a big deal.

Then I created a hack that just made "setup_work_tree()" a no-op (because it does all that crazy stuff that forces GIT_DIR to be an absolute path etc). As a result I got:

	[torvalds@woody kernel]$ time ~/git/git-add .

	real    0m7.849s
	user    0m6.420s
	sys     0m1.296s

ie now we're talking about a 5%+ performance difference.

Of course, this is all for the hot-cache case, and it wouldn't be noticeable for a cold-cache case, but it really can be a real performance issue. And no, it's not that "setup_work_tree()" itself is expensive, but depending on how GIT_DIR is set up, you get very different system call patterns.

Here's what current git does for one file (sound/usb/usbmixer_maps.c):

	lstat("sound/usb/usbmixer_maps.c", {st_mode=S_IFREG|0664, st_size=10230, ...}) = 0
	open("sound/usb/usbmixer_maps.c", O_RDONLY) = 4
	mmap(NULL, 10230, PROT_READ, MAP_PRIVATE, 4, 0) = 0x7fa247228000
	close(4)                                = 0
	access("/home/torvalds/kernel/.git/objects/d7/55be0ad8115da5c1296211678f81a7774277d5", F_OK) = -1 ENOENT (No such file or directory)
	open("/home/torvalds/kernel/.git/objects/d7/55be0ad8115da5c1296211678f81a7774277d5", O_RDONLY) = -1 ENOENT (No such file or directory)
	open("/home/torvalds/kernel/.git/objects/d7/tmp_obj_55be0ad8115da5c12962116_vDqXRl", O_RDWR|O_CREAT|O_EXCL, 0600) = 4
	write(4, "x\1\315ZmS\333H\22\276\257\370W\364\361\t\210\1K\206$\300.\273\6\314\306\273@(l\'"..., 3402) = 3402
	fchmod(4, 0444)                         = 0
	close(4)                                = 0
	link("/home/torvalds/kernel/.git/objects/d7/tmp_obj_55be0ad8115da5c12962116_vDqXRl", "/home/torvalds/kernel/.git/objects/d7/55be0ad8115da5c1296211678f81a7774277d5") = 0
	unlink("/home/torvalds/kernel/.git/objects/d7/tmp_obj_55be0ad8115da5c12962116_vDqXRl") = 0
	munmap(0x7fa247228000, 10230)           = 0

and here is the fixed version:

	lstat("sound/usb/usbmixer_maps.c", {st_mode=S_IFREG|0664, st_size=10230, ...}) = 0
	open("sound/usb/usbmixer_maps.c", O_RDONLY) = 4
	mmap(NULL, 10230, PROT_READ, MAP_PRIVATE, 4, 0) = 0x7f54c231a000
	close(4)                                = 0
	access(".git/objects/d7/55be0ad8115da5c1296211678f81a7774277d5", F_OK) = -1 ENOENT (No such file or directory)
	open(".git/objects/d7/tmp_obj_55be0ad8115da5c12962116_fKy6px", O_RDWR|O_CREAT|O_EXCL, 0600) = 4
	write(4, "x\1\315ZmS\333H\22\276\257\370W\364\361\t\210\1K\206$\300.\273\6\314\306\273@(l\'"..., 3402) = 3402
	fchmod(4, 0444)                         = 0
	close(4)                                = 0
	link(".git/objects/d7/tmp_obj_55be0ad8115da5c12962116_fKy6px", ".git/objects/d7/55be0ad8115da5c1296211678f81a7774277d5") = 0
	unlink(".git/objects/d7/tmp_obj_55be0ad8115da5c12962116_fKy6px") = 0
	munmap(0x7f54c231a000, 10230)           = 0

ie note how it does one unnecessary "open()" less, but more importantly, notice the difference between "/home/torvalds/kernel/.git/*" and ".git/*", and realize that that second difference was the much more noticeable one.

And no, obviously the right fix is not to just comment out all of "setup_work_tree()" (it will break stuff that depends on GIT_WORKTREE), but I did that as a minimal example of showing what the bad effect of that function is.

In general, I think we've gone in the wrong direction with a lot of the "make_absolute_path" stuff. See above. 5% performance loss is not good.

		Linus