From: Jeff King <peff@peff.net>
To: Michael Haggerty <mhagger@alum.mit.edu>
Cc: "Junio C Hamano" <gitster@pobox.com>,
"Stefan Beller" <sbeller@google.com>,
"Johannes Schindelin" <Johannes.Schindelin@gmx.de>,
"Nguyễn Thái Ngọc Duy" <pclouds@gmail.com>,
"Ævar Arnfjörð Bjarmason" <avarab@gmail.com>,
"Brandon Williams" <bmwill@google.com>,
git@vger.kernel.org
Subject: Re: [PATCH v2 13/21] packed_ref_cache: keep the `packed-refs` file mmapped if possible
Date: Wed, 20 Sep 2017 14:40:58 -0400 [thread overview]
Message-ID: <20170920184058.w3tipgsz54ig7dm5@sigill.intra.peff.net> (raw)
In-Reply-To: <b32234e07a1bd1e60442a13d97d7c4e51edf3336.1505799700.git.mhagger@alum.mit.edu>
On Tue, Sep 19, 2017 at 08:22:21AM +0200, Michael Haggerty wrote:
> Keep a copy of the `packed-refs` file contents in memory for as long
> as a `packed_ref_cache` object is in use:
>
> * If the system allows it, keep the `packed-refs` file mmapped.
>
> * If not (either because the system doesn't support `mmap()` at all,
> or because a file that is currently mmapped cannot be replaced via
> `rename()`), then make a copy of the file's contents in
> heap-allocated space, and keep that around instead.
>
> We base the choice of behavior on a new build-time switch,
> `MMAP_PREVENTS_DELETE`. By default, this switch is set for Windows
> variants.
>
> This whole change is still pointless, because we only read the
> `packed-refs` file contents immediately after instantiating the
> `packed_ref_cache`. But that will soon change.
The overall strategy for this compile-time knob makes sense, but one
thing confused me:
> +ifdef MMAP_PREVENTS_DELETE
> + BASIC_CFLAGS += -DMMAP_PREVENTS_DELETE
> +else
> + ifdef USE_WIN32_MMAP
> + BASIC_CFLAGS += -DMMAP_PREVENTS_DELETE
> + endif
> +endif
So setting the knob does what you'd expect. But if you don't set it,
then we still auto-tweak it based on the USE_WIN32_MMAP knob. Do we need
that? It seems like we set our new knob in config.mak.uname any time
we'd set USE_WIN32_MMAP. So this only has an effect in two cases:
1. You aren't on Windows, but you set USE_WIN32_MMAP yourself.
2. You are on Windows, but you manually unset MMAP_PREVENTS_DELETE.
I expect both cases are rare (and would probably involve somebody
actively debugging these knobs). Probably it's a minor convenience in
case 1, but in case 2 it would be actively confusing, I'd think.
> +enum mmap_strategy {
> + /*
> + * Don't use mmap() at all for reading `packed-refs`.
> + */
> + MMAP_NONE,
> +
> + /*
> + * Can use mmap() for reading `packed-refs`, but the file must
> + * not remain mmapped. This is the usual option on Windows,
> + * where you cannot rename a new version of a file onto a file
> + * that is currently mmapped.
> + */
> + MMAP_TEMPORARY,
I suspect you originally distinguished these cases so that NO_MMAP does
not read into a fake-mmap buffer, followed by us copying it into another
buffer. But AFAICT we handle the "NONE" and "TEMPORARY" cases exactly
the same (by just doing a read_in_full() into our own buffer). Do we
actually need separate strategies?
-Peff
next prev parent reply other threads:[~2017-09-20 18:41 UTC|newest]
Thread overview: 40+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-09-19 6:22 [PATCH v2 00/21] Read `packed-refs` using mmap() Michael Haggerty
2017-09-19 6:22 ` [PATCH v2 01/21] ref_iterator: keep track of whether the iterator output is ordered Michael Haggerty
2017-09-19 6:22 ` [PATCH v2 02/21] prefix_ref_iterator: break when we leave the prefix Michael Haggerty
2017-09-20 20:25 ` Stefan Beller
2017-09-21 4:59 ` Jeff King
2017-09-21 17:29 ` Stefan Beller
2017-09-21 7:42 ` Michael Haggerty
2017-09-19 6:22 ` [PATCH v2 03/21] packed_ref_cache: add a backlink to the associated `packed_ref_store` Michael Haggerty
2017-09-19 6:22 ` [PATCH v2 04/21] die_unterminated_line(), die_invalid_line(): new functions Michael Haggerty
2017-09-19 6:22 ` [PATCH v2 05/21] read_packed_refs(): use mmap to read the `packed-refs` file Michael Haggerty
2017-09-19 6:22 ` [PATCH v2 06/21] read_packed_refs(): only check for a header at the top of the file Michael Haggerty
2017-09-19 6:22 ` [PATCH v2 07/21] read_packed_refs(): make parsing of the header line more robust Michael Haggerty
2017-09-19 6:22 ` [PATCH v2 08/21] read_packed_refs(): read references with minimal copying Michael Haggerty
2017-09-20 18:27 ` Jeff King
2017-09-21 7:34 ` Michael Haggerty
2017-09-19 6:22 ` [PATCH v2 09/21] packed_ref_cache: remember the file-wide peeling state Michael Haggerty
2017-09-19 6:22 ` [PATCH v2 10/21] mmapped_ref_iterator: add iterator over a packed-refs file Michael Haggerty
2017-09-19 6:22 ` [PATCH v2 11/21] mmapped_ref_iterator_advance(): no peeled value for broken refs Michael Haggerty
2017-09-20 18:29 ` Jeff King
2017-09-19 6:22 ` [PATCH v2 12/21] packed-backend.c: reorder some definitions Michael Haggerty
2017-09-19 6:22 ` [PATCH v2 13/21] packed_ref_cache: keep the `packed-refs` file mmapped if possible Michael Haggerty
2017-09-19 12:44 ` Michael Haggerty
2017-09-24 6:56 ` Junio C Hamano
2017-09-20 18:40 ` Jeff King [this message]
2017-09-20 18:51 ` Jeff King
2017-09-21 8:04 ` Michael Haggerty
2017-09-19 6:22 ` [PATCH v2 14/21] read_packed_refs(): ensure that references are ordered when read Michael Haggerty
2017-09-20 18:50 ` Jeff King
2017-09-21 8:27 ` Michael Haggerty
2017-09-25 15:44 ` Johannes Schindelin
2017-09-19 6:22 ` [PATCH v2 15/21] packed_ref_iterator_begin(): iterate using `mmapped_ref_iterator` Michael Haggerty
2017-09-19 6:22 ` [PATCH v2 16/21] packed_read_raw_ref(): read the reference from the mmapped buffer Michael Haggerty
2017-09-19 6:22 ` [PATCH v2 17/21] ref_store: implement `refs_peel_ref()` generically Michael Haggerty
2017-09-19 6:22 ` [PATCH v2 18/21] packed_ref_store: get rid of the `ref_cache` entirely Michael Haggerty
2017-09-19 6:22 ` [PATCH v2 19/21] ref_cache: remove support for storing peeled values Michael Haggerty
2017-09-19 6:22 ` [PATCH v2 20/21] mmapped_ref_iterator: inline into `packed_ref_iterator` Michael Haggerty
2017-09-19 6:22 ` [PATCH v2 21/21] packed-backend.c: rename a bunch of things and update comments Michael Haggerty
2017-09-19 19:53 ` [PATCH v2 00/21] Read `packed-refs` using mmap() Johannes Schindelin
2017-09-20 18:57 ` Jeff King
2017-09-25 15:55 ` Johannes Schindelin
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