git@vger.kernel.org list mirror (unofficial, one of many)
 help / color / mirror / code / Atom feed
From: Duy Nguyen <pclouds@gmail.com>
To: Jeff King <peff@peff.net>
Cc: Junio C Hamano <gitster@pobox.com>,
	Johannes Schindelin via GitGitGadget <gitgitgadget@gmail.com>,
	Git Mailing List <git@vger.kernel.org>,
	Johannes Schindelin <johannes.schindelin@gmx.de>
Subject: Re: [PATCH 4/3] untracked-cache: use FLEX_ALLOC to create internal structs
Date: Fri, 19 Apr 2019 16:18:25 +0700	[thread overview]
Message-ID: <CACsJy8BHwM5dUqa-FOjr3smxJT6kW4z7vLUJvsHuhjxwUjvkBw@mail.gmail.com> (raw)
In-Reply-To: <20190418212405.GA18623@sigill.intra.peff.net>

On Fri, Apr 19, 2019 at 4:24 AM Jeff King <peff@peff.net> wrote:
>
> On Thu, Apr 18, 2019 at 05:14:08PM -0400, Jeff King wrote:
>
> > Just so we don't forget about it, I wrote this fix up as a patch. And in
> > fact it led to a few other cleanups. I think the first one is definitely
> > worth doing now, even if there are other similar cases lurking in the
> > rest of the index code.
> >
> > The other two are optional, though I think they are worth it (and not
> > too hard to verify that they are doing the right thing).
> >
> > These are on top of js/untracked-cache-allocfix (though they could
> > easily be ported to a separate topic if we want).
> >
> >   [1/3]: untracked-cache: be defensive about missing NULs in index
> >   [2/3]: untracked-cache: simplify parsing by dropping "next"
> >   [3/3]: untracked-cache: simplify parsing by dropping "len"
>
> I also wondered if we could just accept the cost of calloc() here and
> use FLEX_ALLOC to simplify things. That resulted in the patch below, but
> I didn't include it with the initial 3, because I think it's too
> subtle/gross for my tastes.

It's probably ok. If I remember correctly, reading UNTR extension (on
a huge repo) took the longest time after Ben added support for reading
the index with multiple threads. So performance is a concern, but I
don't think calloc() would be the problem. malloc() itself without the
memory pool could probably slow down more when we have lots and lots
of directories.

> -- >8 --
> Subject: untracked-cache: use FLEX_ALLOC to create internal structs
>
> The untracked_cache_dir struct has a FLEX_ARRAY in it. Let's use
> FLEX_ALLOC_MEM to allocate it, which saves us having to compute the
> length ourselves.
>
> In theory this could be slightly slower, since the FLEX_ALLOC macros use
> calloc (and we just memcpy over most of the contents anyway). But in
> practice this distinction is not generally measurable.
>
> Note that because we then fill in the pre-flex elements of the struct
> using a memcpy, we need to take care to use the exact size of that
> space and _not_ "sizeof(ud)", since the latter may include padding (or
> even an extra byte on systems where FLEX_ARRAY is 1).
>
> Signed-off-by: Jeff King <peff@peff.net>
> ---
> If we wanted to go this route, I think it would make sense to provide a
> FLEX_ALLOC macro that takes a "template" set of bytes as a ptr/len pair,
> and writes it before we fill in the flex portion.
>
> Then we could do something like:
>
>   FLEX_ALLOC_COPY(untracked, &ud, sizeof(ud), name, data, eos - data);
>
> If this is the only such case, it's probably not worth it (I didn't
> really look around for more, though).
>
>  dir.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/dir.c b/dir.c
> index 60438b2cdc..7cd4eec198 100644
> --- a/dir.c
> +++ b/dir.c
> @@ -2757,9 +2757,9 @@ static int read_one_dir(struct untracked_cache_dir **untracked_,
>         if (!eos || eos == end)
>                 return -1;
>
> -       *untracked_ = untracked = xmalloc(st_add3(sizeof(*untracked), eos - data, 1));
> -       memcpy(untracked, &ud, sizeof(ud));
> -       memcpy(untracked->name, data, eos - data + 1);
> +       FLEX_ALLOC_MEM(untracked, name, data, eos - data);
> +       memcpy(untracked, &ud, offsetof(struct untracked_cache_dir, name));
> +       *untracked_ = untracked;
>         data = eos + 1;
>
>         for (i = 0; i < untracked->untracked_nr; i++) {
> --
> 2.21.0.1092.g8b0302e9c4
>


-- 
Duy

  reply	other threads:[~2019-04-19 18:27 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-04-10 12:56 [PATCH 0/1] Fix an off-by-one bug in the untracked cache code Johannes Schindelin via GitGitGadget
2019-04-10 12:56 ` [PATCH 1/1] untracked cache: fix off-by-one Johannes Schindelin via GitGitGadget
2019-04-10 16:20   ` Jeff King
2019-04-12  1:48     ` Junio C Hamano
2019-04-18 21:14       ` [PATCH 0/3] untracked cache parsing fixups Jeff King
2019-04-18 21:17         ` [PATCH 1/3] untracked-cache: be defensive about missing NULs in index Jeff King
2019-04-19  5:29           ` Junio C Hamano
2019-04-18 21:17         ` [PATCH 2/3] untracked-cache: simplify parsing by dropping "next" Jeff King
2019-04-19  5:33           ` Junio C Hamano
2019-04-18 21:18         ` [PATCH 3/3] untracked-cache: simplify parsing by dropping "len" Jeff King
2019-04-18 21:24         ` [PATCH 4/3] untracked-cache: use FLEX_ALLOC to create internal structs Jeff King
2019-04-19  9:18           ` Duy Nguyen [this message]
2019-04-19 19:43             ` Jeff King

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

  List information: http://vger.kernel.org/majordomo-info.html

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=CACsJy8BHwM5dUqa-FOjr3smxJT6kW4z7vLUJvsHuhjxwUjvkBw@mail.gmail.com \
    --to=pclouds@gmail.com \
    --cc=git@vger.kernel.org \
    --cc=gitgitgadget@gmail.com \
    --cc=gitster@pobox.com \
    --cc=johannes.schindelin@gmx.de \
    --cc=peff@peff.net \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
Code repositories for project(s) associated with this public inbox

	https://80x24.org/mirrors/git.git

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).