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

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@xxxxxxxxx> 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.


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  |