Re: Performance issue of 'git branch'

From: Carlos R. Mafra <crmafra2@gmail.com>
Date: Jul 24, 2009, 23:46 UTC
Message-ID: <20090724234648.GA4616@Pilar.aei.mpg.de>
In-Reply-To: <alpine.LFD.2.01.0907241349390.3960@localhost.localdomain>

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:

Show 5 quoted lines

> 
> 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.