Re: Performance issue of 'git branch'

["Carlos R. Mafra" <crmafra2@gmail.com>]

Sorry for the delay and missing the "strace -ttT" request,
but today was a "Physics" day and took me longer to 
notice your email.

On Fri 24.Jul'09 at 14:21:20 -0700, Linus Torvalds wrote:
> 
> What a huge difference!
> 
> And the NO_CURL version really does load a lot faster in cold-cache. We're 
> not talking small differences:

With NO_CURL=1 the strace log contained 242 lines (vs 404), but
the time difference was not as great as you got. But it was
better:

0.55 +- 0.06 (for 8 runs)

So I repeated the tests with curl enabled and this time
I got:

0.77 +- 0.03 (for 6 runs)

(yesterday I got 0.61 +- 0.08, so there is lot of noise)

So it is better, but not by the same factor as you saw.
But I may have an explanation for this.

After I clear the cache I wait a few seconds to stabilize,
and I do the 'time git branch' test when I see that
there is no activity in the disk by looking at
the 'btrace' output in another xterm. 

I noticed that after dropping the cache and before
I do the test there is lot of activity of something
called 'preload', with lines which look like these:

8,0  0  42881   495.067655112 17777  Q   R 51244367 + 552 [preload]
8,0  0  42882   495.067659931 17777  G   R 51244367 + 552 [preload]
8,0  0  42883   495.067664401 17777  I   R 51244367 + 552 [preload]

I hadn't noticed this before and now I checked that,

"preload is an adaptive readahead daemon that prefetches files mapped by
applications from the disk to reduce application startup time."

So I guess that my tests here for your NO_CURL=1 idea is inconclusive,
as I am not sure what preload is prefetching.