Re: [PATCH v2 4/5] sha1_name: Parse less while finding common prefix

[Stefan Beller <sbeller@google.com>]

On Mon, Sep 25, 2017 at 2:54 AM, Derrick Stolee <dstolee@microsoft.com> wrote:
> Create get_hex_char_from_oid() to parse oids one hex character at a
> time. This prevents unnecessary copying of hex characters in
> extend_abbrev_len() when finding the length of a common prefix.
>
> p0008.1: find_unique_abbrev() for existing objects
> --------------------------------------------------
>
> For 10 repeated tests, each checking 100,000 known objects, we find the
> following results when running in a Linux VM:
>
> |       | Pack  | Packed  | Loose  | Base   | New    |        |
> | Repo  | Files | Objects | Objects| Time   | Time   | Rel%   |
> |-------|-------|---------|--------|--------|--------|--------|
> | Git   |     1 |  230078 |      0 | 0.08 s | 0.08 s |   0.0% |
> | Git   |     5 |  230162 |      0 | 0.17 s | 0.16 s | - 5.9% |
> | Git   |     4 |  154310 |  75852 | 0.14 s | 0.12 s | -14.3% |
> | Linux |     1 | 5606645 |      0 | 0.50 s | 0.25 s | -50.0% |
> | Linux |    24 | 5606645 |      0 | 2.41 s | 2.08 s | -13.7% |
> | Linux |    23 | 5283204 | 323441 | 1.99 s | 1.69 s | -15.1% |
> | VSTS  |     1 | 4355923 |      0 | 0.40 s | 0.22 s | -45.0% |
> | VSTS  |    32 | 4355923 |      0 | 2.09 s | 1.99 s | - 4.8% |
> | VSTS  |    31 | 4276829 |  79094 | 3.60 s | 3.20 s | -11.1% |
>
> For the Windows repo running in Windows Subsystem for Linux:
>
>     Pack Files: 50
> Packed Objects: 22,385,898
>  Loose Objects: 492
>      Base Time: 4.61 s
>       New Time: 4.61 s
>          Rel %: 0.0%
>
> p0008.2: find_unique_abbrev() for missing objects
> -------------------------------------------------
>
> For 10 repeated tests, each checking 100,000 missing objects, we find
> the following results when running in a Linux VM:
>
> |       | Pack  | Packed  | Loose  | Base   | New    |        |
> | Repo  | Files | Objects | Objects| Time   | Time   | Rel%   |
> |-------|-------|---------|--------|--------|--------|--------|
> | Git   |     1 |  230078 |      0 | 0.06 s | 0.05 s | -16.7% |
> | Git   |     5 |  230162 |      0 | 0.14 s | 0.15 s | + 7.1% |
> | Git   |     4 |  154310 |  75852 | 0.12 s | 0.12 s |   0.0% |
> | Linux |     1 | 5606645 |      0 | 0.40 s | 0.17 s | -57.5% |
> | Linux |    24 | 5606645 |      0 | 1.59 s | 1.30 s | -18.2% |
> | Linux |    23 | 5283204 | 323441 | 1.23 s | 1.10 s | -10.6% |
> | VSTS  |     1 | 4355923 |      0 | 0.25 s | 0.12 s | -52.0% |
> | VSTS  |    32 | 4355923 |      0 | 1.45 s | 1.34 s | - 7.6% |
> | VSTS  |    31 | 4276829 |  79094 | 1.59 s | 1.34 s | -15.7% |
>
> For the Windows repo running in Windows Subsystem for Linux:
>
>     Pack Files: 50
> Packed Objects: 22,385,898
>  Loose Objects: 492
>      Base Time: 3.91 s
>       New Time: 3.08 s
>          Rel %: -21.1%
>

These number look pretty cool!

> Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
>
> Signed-off-by: Derrick Stolee <dstolee@microsoft.com>

double signoff?

> ---
>  sha1_name.c | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/sha1_name.c b/sha1_name.c
> index f2a1ebe49..bb47b6702 100644
> --- a/sha1_name.c
> +++ b/sha1_name.c
> @@ -480,13 +480,22 @@ struct min_abbrev_data {
>         char *hex;
>  };
>
> +static inline char get_hex_char_from_oid(const struct object_id *oid, int i)

'i' is not very descriptive, maybe add a comment?
(I realize it is just walking through the char*s one by one)

Maybe this function (together with the change in the while below)
could go into hex.c as "int progressively_cmp_oids" that returns
the position at which the given oids differ?

> +{
> +       static const char hex[] = "0123456789abcdef";
> +
> +       if ((i & 1) == 0)
> +               return hex[oid->hash[i >> 1] >> 4];
> +       else
> +               return hex[oid->hash[i >> 1] & 0xf];
> +}

sha1_to_hex_r has very similar code, though it iterates less
and covers both cases in the loop body.

That is the actual reason I propose moving this function
(or a variant thereof) to hex.c as there we can share code.