Re: [PATCH] git-pack-objects: cache small deltas between big objects

> Creating deltas between big blobs is a CPU and memory intensive task.
> In the writing phase, all (not reused) deltas are redone.
>
> This patch adds support for caching deltas from the deltifing phase, so
> that that the writing phase is faster.
>
> The caching is limited to small deltas to avoid increasing memory usage very much.
> The implemented limit is (memory needed to create the delta)/1024.
>
> Signed-off-by: Martin Koegler <mkoegler@auto.tuwien.ac.at>
> ---
>  builtin-pack-objects.c |   35 +++++++++++++++++++++++++----------
>  1 files changed, 25 insertions(+), 10 deletions(-)

> diff --git a/builtin-pack-objects.c b/builtin-pack-objects.c
> index d165f10..13429d0 100644
> --- a/builtin-pack-objects.c
> +++ b/builtin-pack-objects.c
> ...
> @@ -1294,10 +1302,17 @@ static int try_delta(struct unpacked *trg, struct unpacked *src,
>  	if (!delta_buf)
>  		return 0;
>  
> +	if (trg_entry->delta_data)
> +		free (trg_entry->delta_data);
> +	trg_entry->delta_data = 0;
>  	trg_entry->delta = src_entry;
>  	trg_entry->delta_size = delta_size;
>  	trg_entry->depth = src_entry->depth + 1;
> -	free(delta_buf);
> +	/* cache delta, if objects are large enough compared to delta size */
> +	if ((src_size >> 20) + (trg_size >> 21) > (delta_size >> 10))
> +		trg_entry->delta_data = delta_buf;
> +	else
> +		free(delta_buf);
>  	return 1;
>  }