Am 05.02.20 um 04:19 schrieb Derrick Stolee:
> On 2/4/2020 4:24 PM, René Scharfe wrote:
>> We can calculate the size of new name easily and precisely. Open-code
>> the xstrfmt() calls and grow the buffers as needed before filling them.
>> This provides a surprisingly large benefit when working with the
>> Chromium repository; here are the numbers measured using hyperfine
>> before:
>>
>> Benchmark #1: ./git -C ../chromium/src name-rev --all
>> Time (mean ± σ): 5.822 s ± 0.013 s [User: 5.304 s, System: 0.516 s]
>> Range (min … max): 5.803 s … 5.837 s 10 runs
>>
>> ... and with this patch:
>>
>> Benchmark #1: ./git -C ../chromium/src name-rev --all
>> Time (mean ± σ): 1.527 s ± 0.003 s [User: 1.015 s, System: 0.511 s]
>> Range (min … max): 1.524 s … 1.535 s 10 runs
>
> Nice!
>
>> + if (name->generation > 0) {
>> + strbuf_grow(&sb, len +
>> + 1 + decimal_width(name->generation) +
>> + 1 + decimal_width(parent_number));
>
> Just curious: these strbuf_grow() calls are what _really_ improve the
> performance, right? If you dropped them, then can we expect the performance
> to be similar to the old code?
The replaced xstrfmt() is defined in strbuf.c and trivially wraps
xstrvfmt(), which in turn does:
struct strbuf buf = STRBUF_INIT;
strbuf_vaddf(&buf, fmt, ap);
return strbuf_detach(&buf, NULL);
And strbuf_addf() trivially wraps strbuf_vaddf(). So indeed, all
I did was to inline xstrfmt() and add the strbuf_grow() calls.
Their high impact shocked me a bit. vsnprintf(3) really doesn't seem
to like working with a buffer that is too small (when it's supposed to
just calculate how many bytes it would have needed), at least not in
the one from glibc 2.29.
>> + strbuf_addf(&sb, "%.*s~%d^%d", (int)len, name->tip_name,
>> + name->generation, parent_number);
>> + } else {
>> + strbuf_grow(&sb, len +
>> + 1 + decimal_width(parent_number));
>> + strbuf_addf(&sb, "%.*s^%d", (int)len, name->tip_name,
>> + parent_number);
>> + }
>> + return strbuf_detach(&sb, NULL);
>> }
>>
>> static void name_rev(struct commit *start_commit,
>> --
>> 2.25.0
>>
>
