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

[Taylor Blau <me@ttaylorr.com>]

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().

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.

> +
>  /*
>   * 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.

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.

Thanks,
Taylor