Re: [PATCH v6 00/14] Serialized Git Commit Graph

> On Fri, Mar 16, 2018 at 04:06:39PM -0400, Jeff King wrote:
>
>>> Furthermore, in order to look at an object it has to be zlib inflated
>>> first, and since commit objects tend to be much smaller than trees and
>>> especially blobs, there are a lot less bytes to inflate:
>>>
>>>    $ grep ^commit type-size |cut -d' ' -f2 |avg
>>>    34395730 / 53754 = 639
>>>    $ cat type-size |cut -d' ' -f2 |avg
>>>    3866685744 / 244723 = 15800
>>>
>>> So a simple revision walk inflates less than 1% of the bytes that the
>>> "enumerate objects packfiles" approach has to inflate.
>> I don't think this is quite accurate. It's true that we have to
>> _consider_ every object, but Git is smart enough not to inflate each one
>> to find its type. For loose objects we just inflate the header. For
>> packed objects, we either pick the type directly out of the packfile
>> header (for a non-delta) or can walk the delta chain (without actually
>> looking at the data bytes!) until we hit the base.
> Hmm, so that's a big part of the problem with this patch series. It
> actually _does_ unpack every object with --stdin-packs to get the type,
> which is just silly. With the patch below, my time for "commit-graph
> write --stdin-packs" on linux.git goes from over 5 minutes (I got bored
> and killed it) to 17 seconds.
>
> diff --git a/commit-graph.c b/commit-graph.c
> index 6348bab82b..cf1da2e8c1 100644
> --- a/commit-graph.c
> +++ b/commit-graph.c
> @@ -491,11 +491,12 @@ static int add_packed_commits(const struct object_id *oid,
>   {
>   	struct packed_oid_list *list = (struct packed_oid_list*)data;
>   	enum object_type type;
> -	unsigned long size;
> -	void *inner_data;
>   	off_t offset = nth_packed_object_offset(pack, pos);
> -	inner_data = unpack_entry(pack, offset, &type, &size);
> -	FREE_AND_NULL(inner_data);
> +	struct object_info oi = OBJECT_INFO_INIT;
> +
> +	oi.typep = &type;
> +	if (packed_object_info(pack, offset, &oi) < 0)
> +		die("unable to get type of object %s", oid_to_hex(oid));
>   
>   	if (type != OBJ_COMMIT)
>   		return 0;
>
> -Peff