Re: [PATCH] strbuf_write: omit system call when length is zero

From: Junio C Hamano <gitster@pobox.com>
Date: Feb 25, 2016, 23:04 UTC
Message-ID: <xmqqk2lsmlwp.fsf@gitster.mtv.corp.google.com>
In-Reply-To: <1456439678-5433-1-git-send-email-sbeller@google.com>

Stefan Beller <sbeller@google.com> writes:

Show 13 quoted lines

> In case the length of the buffer is zero, we do not need to call the
> fwrite system call as a performance improvement.
>
> Signed-off-by: Stefan Beller <sbeller@google.com>
> ---
>
>  This applies on top of v17 for origin/sb/submodule-parallel-update.
>  
>  In case there are other reasons for origin/sb/submodule-parallel-update
>  to need a reroll I'll squash it in. But as this is a pure performance
>  optimization in a case we are not running into with that series and that
>  series is clashing with Davids refs backend series, I figure we may not
>  want to have a reroll for this fix alone.

Yeah, I tend to agree that this change by itself is probably not worth much.

While I agree with the idea of passing the output from children thru with as little modification as possible, I think that goal does not have to be incompatible with the idea you and Jonathan had to ensure that the output is safe in the presence of incomplete last line.

I think the output made from pp_collect_finished() for "other finished processes" is known to have the very last part from finished child, so it would be easy to make sure by calling strbuf_complete_line() that pp->children[i].err is terminated.

I haven't thought it through how the output from CP_WORKING process in pp_output() should be kept track of, though.

It might be just a matter of adding a bit "incomplete_last_line" to the parallel_process.children struct and maintain it every time pp_output() writes something out (we cannot add an extra newline in pp_output() because we don't know if the process will have further output, but we can keep track of the fact that the last output did not have the final newline). In pp_collect_finished(), we can use the bit and the contents of pp->children[i].err to decide if we need to call strbuf_complete_line() if we did so.

Or something like that ;-)