On Thu, 12 Jul 2007, Brian Downing wrote:
> On Thu, Jul 12, 2007 at 02:38:30AM -0400, Nicolas Pitre wrote:
> > This apparently makes BRian's patological case worse (although better
> > than before his same-size-shallower patch), but I think that the
> > improvement in the general case is worth it. Even Brian's pack gets
> > smaller so...
>
> I've found why this makes my case worse, and I think it's correctable
> and will benefit everything when fixed:
>
> Let's say we've currently got a delta match of 11 bytes at depth 5.
> So trg_entry->delta_size = 11 and trg_entry->depth = 5. max_depth is
> 100.
>
> Now let's say the next object we're comparing against is at depth 2
> (src_entry->depth = 2). Even if we can find a delta of the same size
> we should take it.
>
> Now, with Nico's new patch:
>
> max_size = trg_entry->delta_size * max_depth /
> (max_depth - trg_entry->depth + 1);
>
> max_size is now 11. So far so good.
>
> Now, however, the other bias happens:
>
> max_size = max_size * (max_depth - src_entry->depth) / max_depth;
>
> max_size = 11 * (100 - 2) / 100;
> max_size = 1078 / 100;
> max_size = 10;
Hmmm... Integer truncation errors.
In theory, the allowed max_size should be slightly higher than what we
got in the depth 5 case because this case is less deep. So...
max_size = trg_entry->delta_size * max_depth /
(max_depth - trg_entry->depth + 1);
max_size = 11 * 100 / (100 - 5 + 1) = 11.4583
max_size = max_size * (max_depth - src_entry->depth) / max_depth;
max_size = 11.4583 * (100 - 2) / 100 = 11.2292
So the max_size, because the depth is less, is slightly higher.
> This was okay when max_size was always (trg_size/2 - 20) here, but now
> it's cutting it off too much. max_size is now 10, and we can't make
> a better depth match of the same size anymore.
>
> I think the second bias equation should be scaled so as not to take
> effect unless (src_entry->depth [+ 1?] > trg_entry->depth).
Better yet, the integer truncation error should be compensated for, with
this:
max_size =
(trg_entry->delta_size * max_depth + max_depth - trg_entry->depth) /
(max_depth - trg_entry->depth + 1);
Nicolas
-
To unsubscribe from this list: send the line "unsubscribe git" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
