Re: [PATCH] cache-tree: remove use of strbuf_addf in update_one

[Kevin Willford <kcwillford@gmail.com>]

On 8/10/2017 3:03 PM, Jeff King wrote:
> On Thu, Aug 10, 2017 at 11:58:34AM -0700, Stefan Beller wrote:
>
>> On Thu, Aug 10, 2017 at 11:47 AM, Kevin Willford <kcwillford@gmail.com> wrote:
>>> String formatting can be a performance issue when there are
>>> hundreds of thousands of trees.
>> When changing this for the sake of performance, could you give
>> an example (which kind of repository you need for this to become
>> a bottleneck? I presume the large Windows repo? Or can I
>> reproduce it with a small repo such as linux.git or even git.git?)
>> and some numbers how this improves the performance?
> I was about to say the same thing. Normally I don't mind a small
> optimization without numbers if the result is obviously an improvement.
>
> But in this case the result is a lot less readable, and it's not
> entirely clear to me that it would always be an improvement (we now
> always run 3 strbuf calls instead of one, and have to check the length
> for each one).
>
> What I'm wondering specifically is if vsnprintf() on Kevin's platform
> (which I'll assume is Windows) is slow, and we would do better to
> replace it with a faster compat/ routine.
>
> -Peff

The strbuf_add call is essentially only having to do a memcpy whereas
the strbuf_addf will have to parse the string, determine the types,
convert the data, and then get it in the buffer.  That could be made
faster with a better compat/ routine but I fear still far from
the length check and memcpy.

void strbuf_add(struct strbuf *sb, const void *data, size_t len)
{
	strbuf_grow(sb, len);
	memcpy(sb->buf + sb->len, data, len);
	strbuf_setlen(sb, sb->len + len);
}

Here are some of the performance numbers from the windows repo.
I will work on writing a perf test for this change so that we
have a better idea on smaller repo what the impact of this change
is on them.

              | w/o     | with fix |
-----------------------------------
git checkout | 36.08 s | 33.34 s  |
-----------------------------------
git checkout | 32.54 s | 28.26 s  |
-----------------------------------
git checkout | 44.10 s | 38.13 s  |
-----------------------------------
git merge    | 32.90 s | 30.56 s  |
-----------------------------------
git rebase   | 46.14 s | 42.18 s  |