Re: "git reflog expire --all" very slow

From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Mar 31, 2009, 05:38 UTC
Message-ID: <alpine.LFD.2.00.0903302231370.4093@localhost.localdomain>
In-Reply-To: <alpine.LFD.2.00.0903302154000.4093@localhost.localdomain>

On Mon, 30 Mar 2009, Linus Torvalds wrote:

Show 10 quoted lines

> 
> This if anything makes things just go slower.
> 
> Not much, but some. It went from 36.566s to 38.070s. That may be in the 
> noise, I've not done any sensitivity analysis.
> 
> I thought you perhaps had a missing "parse_commit()" making the 
> reachability thing not work (look_up_gently parses the object, but if it's 
> a tag deref_tag() will dereference it until it hits a commit, but never 
> parse the commit). But that wasn't it.

Ahhah.

I know why it makes things slower.

The slow case is already inside that whole:

	if (timestamp < cb->cmd->expire_unreachable) {

if-statement, so the thing that slows down is if we hit a commit that is _older_ than the expire limit.

But your whole "mark_reachable()" thing only marks things _younger_ than that reachable. So you mark exactly the wrong things reachable - you mark the ones that we don't even care about.

If I do

	mark_reachable(cb.ref_commit, 0);

instead (to traverse the _whole_ tree, with no regards to date), the time shrinks to 1.7s. But of course, that's also wrong.

Qutie frankly, I don't really understand why the logic isn't just

	if (timestamp < cb->cmd->expire_unreachable)
		goto prune;

why is that reachability so important?

			Linus