name-rev --stdin is slow

name-rev --stdin is slow

[Lea Wiemann]

name-rev --stdin has some really high start-up time.  The example below 
is on my git.git branch.  Is this unavoidable to make name-rev fast for 
naming high numbers of revisions -- i.e. is it about amortization? 
Still, 0.7 seconds seems pretty excessive, and on linux-2.6 it even 
takes 6 seconds.

Anyone care to look into it?

$ time echo HEAD | git name-rev --stdin
HEAD

real	0m0.748s
user	0m0.588s
sys	0m0.080s
$ time git name-rev HEAD
HEAD master

real	0m0.041s
user	0m0.016s
sys	0m0.028s

Re: name-rev --stdin is slow

[Daniel Barkalow]

On Thu, 19 Jun 2008, Lea Wiemann wrote:

> name-rev --stdin has some really high start-up time.  The example below is on
> my git.git branch.  Is this unavoidable to make name-rev fast for naming high
> numbers of revisions -- i.e. is it about amortization? Still, 0.7 seconds
> seems pretty excessive, and on linux-2.6 it even takes 6 seconds.
> 
> Anyone care to look into it?

Actually, I think it's the reverse: without --stdin, name-rev can do a 
first pass that lets it eliminate a lot of data from consideration; with 
--stdin, it doesn't know if an item it hasn't seen is going to need some 
data, and so it's conservative and doesn't eliminate anything, which means 
that each request is slower, at least until it's cached everything that 
you turn out to care about.

	-Daniel
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Re: name-rev --stdin is slow

[Lea Wiemann]

Daniel Barkalow wrote:
> Actually, I think it's the reverse: without --stdin, name-rev can do a 
> first pass that lets it eliminate a lot of data from consideration; with 
> --stdin, it doesn't know if an item it hasn't seen is going to need some 
> data, and so it's conservative and doesn't eliminate anything,

Are you sure that's the case?  time git-name-rev --stdin < /dev/null 
gives the same high startup time without looking up anything.

Anyways, it would be great if someone could try to fix that, since a 
6-second startup time on a repository like linux-2.6 makes the --stdin 
option unusable for applications like gitweb (for which it would 
actually be quite useful to reduce the number of forks).

-- Lea

Re: name-rev --stdin is slow

[Daniel Barkalow]

On Fri, 20 Jun 2008, Lea Wiemann wrote:

> Daniel Barkalow wrote:
> > Actually, I think it's the reverse: without --stdin, name-rev can do a first
> > pass that lets it eliminate a lot of data from consideration; with --stdin,
> > it doesn't know if an item it hasn't seen is going to need some data, and so
> > it's conservative and doesn't eliminate anything,
> 
> Are you sure that's the case?  time git-name-rev --stdin < /dev/null gives the
> same high startup time without looking up anything.

Ah, okay. There's a startup pass that goes through all of the refs (tags 
in particular), and does stuff with them, but only if they aren't filtered 
out.

> Anyways, it would be great if someone could try to fix that, since a 6-second
> startup time on a repository like linux-2.6 makes the --stdin option unusable
> for applications like gitweb (for which it would actually be quite useful to
> reduce the number of forks).

It might be possible to collect all of the input, set up the filter, and 
do the (now cheaper) initial pass, but I don't think that would offer any 
real advantage over just collecting them in the caller and calling 
name-rev with a long command line.

	-Daniel
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