Re: [PATCH v3] cat-file: skip expanding default format

[John Cai <johncai86@gmail.com>]

Hi Taylor,

On 8 Mar 2022, at 17:30, Taylor Blau wrote:

> On Tue, Mar 08, 2022 at 10:08:46PM +0000, John Cai via GitGitGadget wrote:
>> diff --git a/builtin/cat-file.c b/builtin/cat-file.c
>> index 7b3f42950ec..e2edba70b41 100644
>> --- a/builtin/cat-file.c
>> +++ b/builtin/cat-file.c
>> @@ -351,6 +351,13 @@ static void print_object_or_die(struct batch_options *opt, struct expand_data *d
>>  	}
>>  }
>>
>> +static int print_default_format(char *buf, int len, struct expand_data *data)
>> +{
>> +	return xsnprintf(buf, len, "%s %s %"PRIuMAX"\n", oid_to_hex(&data->oid),
>> +			 type_name(data->type),
>> +			 (uintmax_t)*data->info.sizep);
>> +}
>
> Two small nits here. It looks like the indentation on the second and
> third lines is off a little bit, since we'd typically expect those to be
> indented to the same margin as the first argument to xsnprintf().

Thanks for bringing this up. I did have a question about indentation in this
case. for the second line, I did try to indent it to align with buf. I attempted
to do the same with the third line, but it's the ( that lines up with buf so
optically it looks a little off.

>
> The other is that you're reading data->info.sizep by dereferencing it,
> but we know that it points to data->size. So I think there it makes
> sense to just read the value directly out of data->size, though note
> that you'll still need the cast to uintmax_t since you're formatting it
> with PRIuMAX.

good point, I'll adjust this in the next version.

>
>> +
>>  /*
>>   * If "pack" is non-NULL, then "offset" is the byte offset within the pack from
>>   * which the object may be accessed (though note that we may also rely on
>> @@ -381,10 +388,16 @@ static void batch_object_write(const char *obj_name,
>>  		}
>>  	}
>>
>> -	strbuf_reset(scratch);
>> -	strbuf_expand(scratch, opt->format, expand_format, data);
>> -	strbuf_addch(scratch, '\n');
>> -	batch_write(opt, scratch->buf, scratch->len);
>> +	if (!opt->format) {
>> +		char buf[1024];
>> +		int len = print_default_format(buf, 1024, data);
>> +		batch_write(opt, buf, len);
>
> Just curious (and apologies if this was discussed earlier and I missed
> it), but: is there a reason that we have to use a scratch buffer here
> that is separate from the strbuf we already have allocated?
>
> That would avoid a large-ish stack variable, but it means that the two
> paths are a little more similar, and can share the batch_write call
> outside of the if/else statement.

This was holdover code from before. Looks like the scratch buffer gets passed
in. Do you mean we don't need to allocate char buf[1024] and instead we can just
use scratch and pass it into print_default_format?

>
> The rest of the changes in this file all look good to me.
>
>> diff --git a/t/perf/p1006-cat-file.sh b/t/perf/p1006-cat-file.sh
>> new file mode 100755
>> index 00000000000..e463623f5a3
>> --- /dev/null
>> +++ b/t/perf/p1006-cat-file.sh
>> @@ -0,0 +1,16 @@
>> +#!/bin/sh
>> +
>> +test_description='Basic sort performance tests'
>
> Is this description a hold-over from p0071? If so, it may be worth
> updating here.
>
>> +test_expect_success 'setup' '
>> +	git rev-list --all >rla
>> +'
>> +
>> +test_perf 'cat-file --batch-check' '
>> +	git cat-file --batch-check <rla
>> +'
>
> We could probably get away with dropping the setup test and using
> `--batch-all-objects` here. Note that right now you're only printing
> commit objects, so there would be a slight behavior change from the way
> the patch is currently written, but it should demonstrate the same
> performance improvement.

This sounds good to me!

>
> Thanks,
> Taylor