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

[Derrick Stolee <stolee@gmail.com>]

My v3 patch is incoming, but I wanted to respond directly to this message.

On 9/25/2017 7:42 PM, Stefan Beller wrote:
> 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?

Oops! I'll be more careful with my format-patch in the future.

>
>> ---
>>   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)
I renamed 'i' to 'pos' in my v3.

>
> 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.

You're right that sha1_to_hex_r is similar, in fact I based my work on 
it. There are a few reasons I didn't combine the two implementations:

* I wanted to be sure my patch was only touching the code for 
disambiguating short-shas. Modifying code in hex.c would touch many more 
code paths.

* I realize that the extra branch in my version is slower than the 
branchless loop body in sha1_to_hex_r, so either I would slow that 
method or make the method call more complicated by returning two chars 
at a time.

* I wanted to strongly hint that the method should be inlined, but I'm 
not sure how to guarantee that happens across a linker boundary without 
doing strange things in header files.

I'm happy to revisit this after my patch is complete, since I think 
there are interesting trade-offs to consider here. I'd prefer to keep 
this discussion separate from the focus on disambiguation.

Thanks,
-Stolee