From: Michael Haggerty <mhagger@alum.mit.edu>
To: Jeff King <peff@peff.net>
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: Thu, 21 Sep 2017 10:04:01 +0200 [thread overview]
Message-ID: <b092c424-987a-fe00-43d4-17062000dfc0@alum.mit.edu> (raw)
In-Reply-To: <20170920185119.upnyomtlxobxwvlw@sigill.intra.peff.net>
On 09/20/2017 08:40 PM, Jeff King wrote:
> [...]
> 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.
Makes sense. I'll delete the `else` clause from the above hunk.
On 09/20/2017 08:51 PM, Jeff King wrote:
> On Wed, Sep 20, 2017 at 02:40:58PM -0400, Jeff King wrote:
>
>>> +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?
>
> In case you are reading these sequentially, I think I talked myself into
> the utility of this during the next patch. ;)
Sorry about that. I'll add a forward breadcrumb to the log message of
this commit.
Michael
next prev parent reply other threads:[~2017-09-21 8:04 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
2017-09-20 18:51 ` Jeff King
2017-09-21 8:04 ` Michael Haggerty [this message]
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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