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From: "Jakub Narębski" <jnareb@gmail.com>
To: git@vger.kernel.org
Cc: "Git Mailing List" <git@vger.kernel.org>,
	"Torsten Bögershausen" <tboegi@web.de>,
	mlbright@gmail.com,
	"Remi Galan Alfonso" <remi.galan-alfonso@ensimag.grenoble-inp.fr>,
	"Nguyen Thai Ngoc Duy" <pclouds@gmail.com>,
	"Eric Wong" <e@80x24.org>,
	"Ramsay Jones" <ramsay@ramsayjones.plus.com>,
	"Jeff King" <peff@peff.net>,
	"Johannes Schindelin" <johannes.schindelin@gmx.de>
Subject: Re: [PATCH v2 5/5] convert: add filter.<driver>.process option
Date: Sat, 30 Jul 2016 01:11:24 +0200	[thread overview]
Message-ID: <1a009e19-8830-7dea-2811-d475cf482ea3@gmail.com> (raw)
In-Reply-To: <xmqqshusny86.fsf@gitster.mtv.corp.google.com>

W dniu 2016-07-29 o 19:35, Junio C Hamano pisze:
> Lars Schneider <larsxschneider@gmail.com> writes:
> 
>> I think sending it upfront is nice for buffer allocations of big files
>> and it doesn't cost us anything to do it.
> 
> While I do NOT think "total size upfront" MUST BE avoided at all costs,
> I do not think the above statement to justify it makes ANY sense.
> 
> Big files are by definition something you cannot afford to hold its
> entirety in core, so you do not want to be told that you'd be fed 40GB
> and ask xmalloc to allocate that much.

I don't know much how filter driver work internally, but in some cases
Git reads or writes from file (file descriptor), in other cases it reads
or writes from str+len pair (it probably predates strbuf) - I think in
those cases file needs to fit in memory (in size_t).  So in some cases
Git reads file into memory.  Whether it uses xmalloc or mmap, I don't
know.

> 
> It allows the reader to be lazy for buffer allocations as long as
> you know the file fits in-core, at the cost of forcing the writer to
> somehow come up with the total number of bytes even before sending a
> single byte (in other words, if the writer cannot produce and hold
> the data in-core, it may even have to spool the data in a temporary
> file only to count, and then play it back after showing the total
> size).

For some types of filters you can know the size upfront:
 - for filters such as rot13, with 1-to-1 transformation, you know
   that the output size is the same as the input size
 - for block encodings, and for constant-width to constant-width
   encoding conversion, filter can calculate output size from the
   input size (e.g. <output size> = 2*<input size>)
 - filter may have get size from somewhere, for example LFS filter
   stub is constant size, and files are stored in artifactory with
   their length 

> 
> It is good that you allow both mode of operations and the size of
> the data can either be given upfront (which allows a single fixed
> allocation upfront without realloc, as long as the data fits in
> core), or be left "(atend)".

I think the protocol should be either: <size> + <contents>, or
<size unknown> + <contents> + <flush>, that is do not use flush
packet if size is known upfront -- it would be a second point
of truth (SPOT principle).
 
> I just don't want to see it oversold as a "feature" that the size
> has to come before data.  That is a limitation, not a feature.
> 
> Thanks.
> 



  reply	other threads:[~2016-07-29 23:11 UTC|newest]

Thread overview: 77+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-07-22 15:48 [PATCH v1 0/3] Git filter protocol larsxschneider
2016-07-22 15:48 ` [PATCH v1 1/3] convert: quote filter names in error messages larsxschneider
2016-07-22 15:48 ` [PATCH v1 2/3] convert: modernize tests larsxschneider
2016-07-26 15:18   ` Remi Galan Alfonso
2016-07-26 20:40     ` Junio C Hamano
2016-07-22 15:49 ` [PATCH v1 3/3] convert: add filter.<driver>.useProtocol option larsxschneider
2016-07-22 22:32   ` Torsten Bögershausen
2016-07-24 12:09     ` Lars Schneider
2016-07-22 23:19   ` Ramsay Jones
2016-07-22 23:28     ` Ramsay Jones
2016-07-24 17:16     ` Lars Schneider
2016-07-24 22:36       ` Ramsay Jones
2016-07-24 23:22         ` Jakub Narębski
2016-07-25 20:32           ` Lars Schneider
2016-07-26 10:58             ` Jakub Narębski
2016-07-25 20:24         ` Lars Schneider
2016-07-23  0:11   ` Jakub Narębski
2016-07-23  7:27     ` Eric Wong
2016-07-26 20:00       ` Jeff King
2016-07-24 18:36     ` Lars Schneider
2016-07-24 20:14       ` Jakub Narębski
2016-07-24 21:30         ` Jakub Narębski
2016-07-25 20:16           ` Lars Schneider
2016-07-26 12:24             ` Jakub Narębski
2016-07-25 20:09         ` Lars Schneider
2016-07-26 14:18           ` Jakub Narębski
2016-07-23  8:14   ` Eric Wong
2016-07-24 19:11     ` Lars Schneider
2016-07-25  7:27       ` Eric Wong
2016-07-25 15:48       ` Duy Nguyen
2016-07-22 21:39 ` [PATCH v1 0/3] Git filter protocol Junio C Hamano
2016-07-24 11:24   ` Lars Schneider
2016-07-26 20:11     ` Jeff King
2016-07-27  0:06 ` [PATCH v2 0/5] " larsxschneider
2016-07-27  0:06   ` [PATCH v2 1/5] convert: quote filter names in error messages larsxschneider
2016-07-27 20:01     ` Jakub Narębski
2016-07-28  8:23       ` Lars Schneider
2016-07-27  0:06   ` [PATCH v2 2/5] convert: modernize tests larsxschneider
2016-07-27  0:06   ` [PATCH v2 3/5] pkt-line: extract and use `set_packet_header` function larsxschneider
2016-07-27  0:20     ` Junio C Hamano
2016-07-27  9:13       ` Lars Schneider
2016-07-27 16:31         ` Junio C Hamano
2016-07-27  0:06   ` [PATCH v2 4/5] convert: generate large test files only once larsxschneider
2016-07-27  2:35     ` Torsten Bögershausen
2016-07-27 13:32       ` Jeff King
2016-07-27 16:50         ` Lars Schneider
2016-07-27  0:06   ` [PATCH v2 5/5] convert: add filter.<driver>.process option larsxschneider
2016-07-27  1:32     ` Jeff King
2016-07-27 17:31       ` Lars Schneider
2016-07-27 18:11         ` Jeff King
2016-07-28 12:10           ` Lars Schneider
2016-07-28 13:35             ` Jeff King
2016-07-27  9:41     ` Eric Wong
2016-07-29 10:38       ` Lars Schneider
2016-07-29 11:24         ` Jakub Narębski
2016-07-29 11:31           ` Lars Schneider
2016-08-05 18:55         ` Eric Wong
2016-08-05 23:26           ` Lars Schneider
2016-08-05 23:38             ` Eric Wong
2016-07-27 23:31     ` Jakub Narębski
2016-07-29  8:04       ` Lars Schneider
2016-07-29 17:35         ` Junio C Hamano
2016-07-29 23:11           ` Jakub Narębski [this message]
2016-07-29 23:44             ` Lars Schneider
2016-07-30  9:32               ` Jakub Narębski
2016-07-28 10:32     ` Torsten Bögershausen
2016-07-27 19:08   ` [PATCH v2 0/5] Git filter protocol Jakub Narębski
2016-07-28  7:16     ` Lars Schneider
2016-07-28 10:42       ` Jakub Narębski
2016-07-28 13:29       ` Jeff King
2016-07-29  7:40         ` Jakub Narębski
2016-07-29  8:14           ` Lars Schneider
2016-07-29 15:57             ` Jeff King
2016-07-29 16:20               ` Lars Schneider
2016-07-29 16:50                 ` Jeff King
2016-07-29 17:43                   ` Lars Schneider
2016-07-29 18:27                     ` Jeff King

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