Re: 1.3.0 creating bigger packs than 1.2.3

[Nicolas Pitre <nico@cam.org>]

On Thu, 20 Apr 2006, Shawn Pearce wrote:

> Based on Linus' comment I changed your patch to just the following.
> It still produced the 46M pack file, so the first hunk apears to
> not have had much of an affect with this data.
> 
> From a running time perspective it appears as though this patch is
> making things slightly better, not worse.  I ran it a few times
> for each case always using the 46M pack as input for
>  "git-repack -a -d -f".
> 
>   'next'       137.13 real        95.82 user        25.24 sys
>   'next'+patch 131.62 real        89.35 user        28.56 sys
> 
> but even if the running time was an extra 6 seconds I'd still rather
> spend 4% more running time to use 1/2 the storage space.
> 
> 
> diff --git a/pack-objects.c b/pack-objects.c
> index 09f4f2c..f7d6217 100644
> --- a/pack-objects.c
> +++ b/pack-objects.c
> @@ -1052,7 +1052,7 @@ static int try_delta(struct unpacked *cu
>         if (cur_entry->delta)
>                 max_size = cur_entry->delta_size-1;
>         if (sizediff >= max_size)
> -               return -1;
> +               return 0;
>         delta_buf = diff_delta(old->data, oldsize,
>                                cur->data, size, &delta_size, max_size);
>         if (!delta_buf)

I can confirm this is indeed the best fix so far.  Any "smarter" 
solution I could think of did increase the size of the final pack quite 
spectacularly and rather unexpectedly with Shawn's repository.

Of course removing the if (sizediff >= max_size) entirely does produce a 
smaller pack (39MB) but it takes about twice the CPU.

With the patch above the Linux kernel pack is 0.3% smaller with 1% more 
CPU usage.  But like for the diff-delta hash list limiting code this 
small overhead is certainly a good compromize to avoid big degradations 
in some other cases.


Nicolas