Re: [PATCH] gc --aggressive: make it really aggressive

On Thu, 6 Dec 2007, Harvey Harrison wrote:

Heh. And this is why you want to do it exactly *once*, and then just export the end result for others ;)

But yeah, especially if you allow longer delta chains, the end result can be much smaller (and what makes the one-time repack more expensive is the window size, not the delta chain - you could make the delta chains longer with no cost overhead at packing time)

HOWEVER. 

The longer delta chains do make it potentially much more expensive to then use old history. So there's a trade-off. And quite frankly, a delta depth of 250 is likely going to cause overflows in the delta cache (which is only 256 entries in size *and* it's a hash, so it's going to start having hash conflicts long before hitting the 250 depth limit).

So when I said "--depth=250 --window=250", I chose those numbers more as an example of extremely aggressive packing, and I'm not at all sure that the end result is necessarily wonderfully usable. It's going to save disk space (and network bandwidth - the delta's will be re-used for the network protocol too!), but there are definitely downsides too, and using long delta chains may simply not be worth it in practice.

(And some of it might just want to have git tuning, ie if people think that long deltas are worth it, we could easily just expand on the delta hash, at the cost of some more memory used!)

That said, the good news is that working with *new* history will not be affected negatively, and if you want to be _really_ sneaky, there are ways to say "create a pack that contains the history up to a version one year ago, and be very aggressive about those old versions that we still want to have around, but do a separate pack for newer stuff using less aggressive parameters"

So this is something that can be tweaked, although we don't really have any really nice interfaces for stuff like that (ie the git delta cache size is hardcoded in the sources and cannot be set in the config file, and the "pack old history more aggressively" involves some manual scripting and knowing how "git pack-objects" works rather than any nice simple command line switch).

So the thing to take away from this is:
 - git is certainly flexible as hell
 - .. but to get the full power you may need to tweak things
 - .. happily you really only need to have one person to do the tweaking, 
   and the tweaked end results will be available to others that do not 
   need to know/care.

And whether the difference between 320MB and 500MB is worth any really involved tweaking (considering the potential downsides), I really don't know. Only testing will tell.

			Linus