I have just started to clone some interesting Linux git trees to watch
the development more closely, and therefore also started to use git. I
noticed that 'git checkout' takes some time, and especially that the
'git checkout' command does lots and lots of lstat() calls.
After some more investigation and thinking, I have made 4 patches and
been able to optimise away over 42% of all lstat() calls in some cases
for the 'git checkout' command. I have not tested other git porcelain
commands for reduced lstat() calls, but I would guess that the more
effective 'lstat_cache()' compared to 'has_leading_symlink_cache()',
should also give better numbers in other cases.
All the 4 patches is against git master, and the git 'make test'
test suite still passes after each patch.
To document the improvement, below is some numbers, which compares
before and after all 4 patches. To reproduce the numbers:
- git clone the Linux git tree to be able to get the Linux tags
'v2.6.25' and 'v2.6.27'.
- git checkout -b my-v2.6.27 v2.6.27
- git checkout -b my-v2.6.25 v2.6.25
Then, when the current branch is 'my-v2.6.25' do:
strace -o strace_to27 -T git checkout -q my-v2.6.27
And then you pretty print and collect stats from the 'strace_to27'
file. If someone wants a copy of the strace_stat.pl script, which I
made/used to do the pretty printing, then give me a hint.
Below is the stats/numbers from the current git version (before the 4
patches). Notice that we do an lstat() call on the "arch" directory
over 6000 times!
TOTAL 185151 100.000% OK:165544 NOT: 19607 11.136001 sec 60 usec/call
lstat64 120954 65.327% OK:107013 NOT: 13941 5.388727 sec 45 usec/call
strings 120954 tot 30163 uniq 4.010 /uniq 5.388727 sec 45 usec/call
files 61491 tot 28712 uniq 2.142 /uniq 2.740520 sec 45 usec/call
dirs 45522 tot 1436 uniq 31.701 /uniq 1.994448 sec 44 usec/call
errors 13941 tot 5189 uniq 2.687 /uniq 0.653759 sec 47 usec/call
6297 5.206% OK: 6297 NOT: 0 "arch"
4544 3.757% OK: 4544 NOT: 0 "drivers"
1816 1.501% OK: 1816 NOT: 0 "arch/arm"
1499 1.239% OK: 1499 NOT: 0 "include"
912 0.754% OK: 912 NOT: 0 "arch/powerpc"
764 0.632% OK: 764 NOT: 0 "fs"
746 0.617% OK: 746 NOT: 0 "drivers/net"
662 0.547% OK: 662 NOT: 0 "net"
652 0.539% OK: 325 NOT: 327 "arch/sparc/include"
636 0.526% OK: 636 NOT: 0 "drivers/media"
606 0.501% OK: 606 NOT: 0 "include/linux"
533 0.441% OK: 533 NOT: 0 "arch/sh"
522 0.432% OK: 260 NOT: 262 "arch/powerpc/include"
488 0.403% OK: 243 NOT: 245 "arch/sh/include"
413 0.341% OK: 413 NOT: 0 "arch/sparc"
390 0.322% OK: 390 NOT: 0 "arch/x86"
383 0.317% OK: 383 NOT: 0 "Documentation"
370 0.306% OK: 184 NOT: 186 "arch/ia64/include"
366 0.303% OK: 366 NOT: 0 "drivers/media/video"
348 0.288% OK: 173 NOT: 175 "arch/arm/include"
Here is the stats/numbers after applying the 4 patches. Notice how
nice the top 20 entries list now looks!
TOTAL 133655 100.000% OK:121615 NOT: 12040 10.429999 sec 78 usec/call
lstat64 69603 52.077% OK: 63218 NOT: 6385 3.419920 sec 49 usec/call
strings 69603 tot 30163 uniq 2.308 /uniq 3.419920 sec 49 usec/call
files 61491 tot 28712 uniq 2.142 /uniq 3.034869 sec 49 usec/call
dirs 1727 tot 1164 uniq 1.484 /uniq 0.075681 sec 44 usec/call
errors 6385 tot 5189 uniq 1.230 /uniq 0.309370 sec 48 usec/call
4 0.006% OK: 4 NOT: 0 ".gitignore"
4 0.006% OK: 4 NOT: 0 ".mailmap"
4 0.006% OK: 4 NOT: 0 "CREDITS"
4 0.006% OK: 4 NOT: 0 "Documentation/00-INDEX"
4 0.006% OK: 4 NOT: 0 "Documentation/ABI/testing/sysfs-block"
4 0.006% OK: 4 NOT: 0 "Documentation/ABI/testing/sysfs-firmware-acpi"
4 0.006% OK: 4 NOT: 0 "Documentation/CodingStyle"
4 0.006% OK: 4 NOT: 0 "Documentation/DMA-API.txt"
4 0.006% OK: 4 NOT: 0 "Documentation/DMA-mapping.txt"
4 0.006% OK: 4 NOT: 0 "Documentation/DocBook/Makefile"
4 0.006% OK: 4 NOT: 0 "Documentation/DocBook/gadget.tmpl"
4 0.006% OK: 4 NOT: 0 "Documentation/DocBook/kernel-api.tmpl"
4 0.006% OK: 4 NOT: 0 "Documentation/DocBook/kernel-locking.tmpl"
4 0.006% OK: 4 NOT: 0 "Documentation/DocBook/procfs-guide.tmpl"
4 0.006% OK: 4 NOT: 0 "Documentation/DocBook/procfs_example.c"
4 0.006% OK: 4 NOT: 0 "Documentation/DocBook/rapidio.tmpl"
4 0.006% OK: 4 NOT: 0 "Documentation/DocBook/s390-drivers.tmpl"
4 0.006% OK: 4 NOT: 0 "Documentation/DocBook/uio-howto.tmpl"
4 0.006% OK: 4 NOT: 0 "Documentation/DocBook/videobook.tmpl"
4 0.006% OK: 4 NOT: 0 "Documentation/DocBook/writing_usb_driver.tmpl"
Note that the overall used system time as recorded from 'strace -T',
does not drop so much that the reduced lstat() time should indicate
for _this_ particular test run. This is because now each unlink()
call takes much more time, at least for me on an slow ide disk (using
ext3) on a laptop.
A simple test gives me an overall improvement of 2.937 seconds: real
time drops from 28.195s (best of 5 runs with 'time git ...'), to
25.381s (best of 5 runs).
Comments?
Kjetil Barvik (4):
Optimised, faster, more effective symlink/directory detection
Use 'lstat_cache()' instead of 'has_symlink_leading_path()'
create_directories() inside entry.c: only check each directory once!
remove the old 'has_symlink_leading_path()' function
Makefile | 2 +-
builtin-add.c | 5 ++-
builtin-apply.c | 5 ++-
builtin-update-index.c | 5 ++-
cache.h | 17 +++++++-
diff-lib.c | 5 ++-
dir.c | 4 +-
entry.c | 86 +++++++++++++++++++++++++++++++--------
lstat_cache.c | 105 ++++++++++++++++++++++++++++++++++++++++++++++++
symlinks.c | 64 -----------------------------
unpack-trees.c | 10 ++++-
11 files changed, 217 insertions(+), 91 deletions(-)
create mode 100644 lstat_cache.c
delete mode 100644 symlinks.c
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