On Sat, 25 Mar 2006, Alex Riesen wrote:
>
> Even more impressive on Cygwin (>50x!):
>
> .../git-win$ time git --exec-path=$(pwd) diff initial.. > /dev/null
> real 0m1.485s
> user 0m0.567s
> sys 0m0.840s
>
> ../git-win$ time git diff initial.. >/dev/null
> real 1m20.781s
> user 0m31.806s
> sys 0m20.717s
Yeah. That's the difference between "unusable" and "retty damn good".
Now, if we didn't even bother to write temporary files (and just did the
object entirely in memory) I'd be even happier. I suspect it would help
cygwin more too.
I've done a "strace" on "git-diff-tree" doing the 5-second diff of the
kernel tree, and almost all of it looks like this:
..
open("/tmp/.diff_WgWi1X", O_RDWR|O_CREAT|O_EXCL, 0600) = 3
write(3, "/*\n * Driver for Digigram pcxhr "..., 6121) = 6121
close(3) = 0
open("/tmp/.diff_hCzrFe", O_RDWR|O_CREAT|O_EXCL, 0600) = 3
write(3, "/*\n * Driver for Digigram pcxhr "..., 6138) = 6138
close(3) = 0
rt_sigaction(SIGINT, {0x1000f650, [INT], SA_RESTART}, {0x1000f650, [INT], SA_RESTART}, 8) = 0
open("/tmp/.diff_WgWi1X", O_RDONLY) = 3
fstat64(3, {st_mode=S_IFREG|0600, st_size=6121, ...}) = 0
read(3, "/*\n * Driver for Digigram pcxhr "..., 6121) = 6121
close(3) = 0
open("/tmp/.diff_hCzrFe", O_RDONLY) = 3
fstat64(3, {st_mode=S_IFREG|0600, st_size=6138, ...}) = 0
read(3, "/*\n * Driver for Digigram pcxhr "..., 6138) = 6138
close(3) = 0
unlink("/tmp/.diff_WgWi1X") = 0
unlink("/tmp/.diff_hCzrFe") = 0
..
which is just ridiculous. Those are _literally_ the only system calls we
do any more after the conversion, if you ignore a few "brk()" calls here
and there to allocate/free memory and obviously a number of "write(1,..."
calls to actually write out the result!
(This is with a fully packed tree, so we just set up the object store with
a single mmap at the beginning, which is why there are no reads to read
the actual source contents).
Now, Linux is good at temp-files, but still: it adds nothing but overhead
to first write out and then read back in over three _thousand_ filepairs
(only to delete them immediately after reading), when the new code
actually just wants to do the diff in memory anyway.
Linus
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