Subject: Re: [PATCH v5 3/3] push: teach --recurse-submodules the on-demand option

From: Heiko Voigt <[email-masked]>
Date: Mon, 26 Mar 2012 21:33:34 +0200

On Wed, Feb 15, 2012 at 11:28:02PM +0100, Jens Lehmann wrote:
> Am 14.02.2012 04:34, schrieb Junio C Hamano:
> > Heiko Voigt <[email-masked]> writes:
> > 
> >> diff --git a/submodule.c b/submodule.c
> >> index 3c714c2..ff0cfd8 100644
> >> --- a/submodule.c
> >> +++ b/submodule.c
> >> @@ -411,6 +411,54 @@ int check_submodule_needs_pushing(unsigned char new_sha1[20],
> >>  	return needs_pushing->nr;
> >>  }
> >>  
> >> +static int push_submodule(const char *path)
> >> +{
> >> +	if (add_submodule_odb(path))
> >> +		return 1;
> >> +
> >> +	if (for_each_remote_ref_submodule(path, has_remote, NULL) > 0) {
> >> +		struct child_process cp;
> >> +		const char *argv[] = {"push", NULL};
> >> +
> >> +		memset(&cp, 0, sizeof(cp));
> >> +		cp.argv = argv;
> >> +		cp.env = local_repo_env;
> >> +		cp.git_cmd = 1;
> >> +		cp.no_stdin = 1;
> >> +		cp.dir = path;
> >> +		if (run_command(&cp))
> >> +			return 0;
> >> +		close(cp.out);
> >> +	}
> >> +
> >> +	return 1;
> >> +}
> > 
> > Hmm, this makes me wonder if we fire subprocesses and have them run in
> > parallel (to a reasonably limited parallelism), it might make the overall
> > user experience more pleasant, and if we did the same on the fetching
> > side, it would be even nicer.
> 
> Yeah, I had the same idea and did some experiments when working on
> fetch some time ago.
> 
> > We would need to keep track of children and after firing a handful of them
> > we would need to start waiting for some to finish and collect their exit
> > status before firing more, and at the end we would need to wait for the
> > remaining ones and find how each one of them did before returning from
> > push_unpushed_submodules().  If we were to do so, what are the missing
> > support we would need from the run_command() subsystem?
> 
> We would not only have to collect the exit status but also the output
> lines. You don't want to see the output of multiple fetches or pushes
> mixed together, so it makes sense to just defer that until the command
> exited and then print everything at once. The interesting part I couldn't
> come up with an easy solution for is to preserve the output order between
> the stdout and stdin lines, as they contain different parts of the
> progress which would look strange when shuffled around.
> 
> And I saw that sometimes parallel fetches took way longer than doing them
> sequentially (in my case because of strange DNS behavior of my DSL router),
> so we would definitely want a config option for that (maybe setting the
> maximum number of simultaneous threads to be used).
> 
> But don't get me wrong, I'm all for having that feature! :-)

Me too, but I would suggest that we first implement the basic recursive
push functionality and then start to do optimizations like this. I think
we will definitely be interested in parallel pushing once we start using it.

Cheers Heiko