Re: Delta compression not so effective

[Marius Storm-Olsen <mstormo@gmail.com>]

On 3/5/2017 19:14, Linus Torvalds wrote:
> On Sat, Mar 4, 2017 at 12:27 AM, Marius Storm-Olsen <mstormo@gmail.com> wrote:
> I guess you could do the printout a bit earlier (on the
> "to_pack.objects[]" array - to_pack.nr_objects is the count there).
> That should show all of them. But the small objects shouldn't matter.
>
> But if you have a file like
>
>    extern/win/FlammableV3/x64/lib/FlameProxyLibD.lib
>
> I would have assumed that it has a size that is > 50. Unless those
> "extern" things are placeholders?

No placeholders, the FlameProxyLibD.lib is a debug lib, and probably the 
largest in the whole repo (with a replace count > 5).


> I do wonder if your dll data just simply is absolutely horrible for
> xdelta. We've also limited the delta finding a bit, simply because it
> had some O(m*n) behavior that gets very expensive on some patterns.
> Maybe your blobs trigger some of those case.

Ok, but given that the SVN delta compression, which forward-linear only, 
is ~45% better, perhaps that particular search could be done fairly 
cheap? Although, I bet time(stamps) are out of the loop at that point, 
so it's not a factor anymore. Even if it where, I'm not sure it would 
solve anything, if there's other factors also limiting deltafication.


> The diff-delta work all goes back to 2005 and 2006, so it's a long time ago.
>
> What I'd ask you to do is try to find if you could make a reposity of
> just one of the bigger DLL's with its history, particularly if you can
> find some that you don't think is _that_ sensitive.
>
> Looking at it, for example, I see that you have that file
>
>    extern/redhat-5/FlammableV3/x64/plugins/libFlameCUDA-3.0.703.so
>
> that seems to have changed several times, and is a largish blob. Could
> you try creating a repository with git fast-import that *only*
> contains that file (or pick another one), and see if that delta's
> well?

I'll filter-branch to extern/ only, however the whole FlammableV3 needs 
to go too, I'm afaid (extern for that project, but internal to $WORK).
I'll do some rewrites and see what comes up.

> And if you find some case that doesn't xdelta well, and that you feel
> you could make available outside, we could have a test-case...

I'll try with this repo first, if not, I'll see if I can construct one.

Thanks!


-- 
.marius