Re: [PATCH v2 2/4] csum-file.h: increase hashfile buffer size

[Jeff King <peff@peff.net>]

On Mon, May 17, 2021 at 12:24:50PM +0000, Derrick Stolee via GitGitGadget wrote:

> From: Derrick Stolee <dstolee@microsoft.com>
> 
> The hashfile API uses a hard-coded buffer size of 8KB and has ever since
> it was introduced in c38138c (git-pack-objects: write the pack files
> with a SHA1 csum, 2005-06-26). It performs a similar function to the
> hashing buffers in read-cache.c, but that code was updated from 8KB to
> 128KB in f279894 (read-cache: make the index write buffer size 128K,
> 2021-02-18). The justification there was that do_write_index() improves
> from 1.02s to 0.72s.
> 
> There is a buffer, check_buffer, that is used to verify the check_fd
> file descriptor. When this buffer increases to 128K to fit the data
> being flushed, it causes the stack to overflow the limits placed in the
> test suite. By moving this to a static buffer, we stop using stack data
> for this purpose, but we lose some thread-safety. This change makes it
> unsafe to write to multiple hashfiles across different threads.
> 
> By adding a new trace2 region in the chunk-format API, we can see that
> the writing portion of 'git multi-pack-index write' lowers from ~1.49s
> to ~1.47s on a Linux machine. These effects may be more pronounced or
> diminished on other filesystems. The end-to-end timing is too noisy to
> have a definitive change either way.

I think there is one thing missing from this commit message: why we want
to do this. You mentioned that read-cache got larger by using a bigger
buffer. But here we use a bigger buffer, and it produces no improvement
larger than the noise. And on top of it, you describe the static-buffer
downsides. So why not just skip it? :)

And the answer is in the larger series: we want to be able to make use
of the hashfile API in read-cache, but without regressing the
performance. One sentence at the end of the first paragraph would
clarify that quite a bit, I think.

> +static void verify_buffer_or_die(struct hashfile *f,
> +				 const void *buf,
> +				 unsigned int count)
> +{
> +	static unsigned char check_buffer[WRITE_BUFFER_SIZE];
> +	ssize_t ret = read_in_full(f->check_fd, check_buffer, count);
> +
> +	if (ret < 0)
> +		die_errno("%s: sha1 file read error", f->name);
> +	if (ret != count)
> +		die("%s: sha1 file truncated", f->name);
> +	if (memcmp(buf, check_buffer, count))
> +		die("sha1 file '%s' validation error", f->name);
> +}

Does this have to use the same-size buffer? We could read and check
smaller chunks, like:

  while (count > 0) {
	static unsigned char chunk[1024];
	unsigned int chunk_len = sizeof(chunk) < count ? sizeof(chunk) : count;
	ssize_t ret = read_in_full(f->check_fd, chunk, chunk_len);

        if (ret < 0)
	   ...
	if (ret != count)
	   ...
	if (memcmp(buf, chunk, chunk_len))
	   ...
	buf += chunk_len;
	count -= chunk_len;
  }

We may prefer to use the larger buffer size for performance, but I think
this "check" mode is only used for "index-pack --verify" and similar.
The performance may matter a lot less to us there than for more
frequently used code paths like index writing.

I don't have a strong preference either way, but it's nice to avoid
introducing non-reentrancy to a function (Junio's heap suggestion is
also quite reasonable).

-Peff