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gzip: set default compression level to 6 when CONFIG_FEATURE_GZIP_LEVELS=n

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With this change, GNU gzip -n and BusyBox gzip now produce identical output
assuming that CONFIG_GZIP_FAST=2.

>> Excuse me, but I wonder one thing: Why should we follow
>> strictly with gzip on the no-options default behavior?

> First, the default 6 compression level is a de-facto standard. BSD gzip
> and Apple gzip (on macOS) use this default as well. So there is a
> reasonable expectation that different gzip implementations act the same.
> For instance, if the default for busybox gzip becomes 9, then someone
> writing a script using busybox gzip could reasonably expect that the
> compression level will still be 9 when the same script is run on another
> system. That would be wrong. Implementations should not deviate from
> de-facto standards without a strong reason.
>
> Second, the inherent reason for this default has not gone away. While
> processor speeds have exploded since the default was set, so has the
> typical size of compressed files. Multiple gigabytes are nothing unusual
> these days. And gzip is often used for compression on the fly, precisely
> because it offers a good compromise between speed and compression ratio.
> So I believe 6 continues to be a reasonable default.

function                                             old     new   delta
deflate                                              939     927     -12

Signed-off-by: Daniel Edgecumbe <git@esotericnonsense.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
This commit is contained in:
Daniel Edgecumbe 2019-09-02 22:09:15 +01:00 committed by Denys Vlasenko
parent ca5d86d52c
commit c660cc1b77
1 changed files with 6 additions and 6 deletions
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12
archival/gzip.c
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@ -52,7 +52,7 @@ aa: 85.1% -- replaced with aa.gz
//config: help
//config: Enable support for compression levels 4-9. The default level
//config: is 6. If levels 1-3 are specified, 4 is used.
//config: If this option is not selected, -N options are ignored and -9
//config: If this option is not selected, -N options are ignored and -6
//config: is used.
//config:
//config:config FEATURE_GZIP_DECOMPRESS
@ -259,13 +259,13 @@ enum {
#if !ENABLE_FEATURE_GZIP_LEVELS
comp_level = 9,
max_chain_length = 4096,
comp_level = 6,
max_chain_length = 128,
/* To speed up deflation, hash chains are never searched beyond this length.
* A higher limit improves compression ratio but degrades the speed.
*/
max_lazy_match = 258,
max_lazy_match = 16,
/* Attempt to find a better match only when the current match is strictly
* smaller than this value. This mechanism is used only for compression
* levels >= 4.
@ -277,7 +277,7 @@ enum {
* max_insert_length is used only for compression levels <= 3.
*/
good_match = 32,
good_match = 8,
/* Use a faster search when the previous match is longer than this */
/* Values for max_lazy_match, good_match and max_chain_length, depending on
@ -286,7 +286,7 @@ enum {
* found for specific files.
*/
nice_match = 258, /* Stop searching when current match exceeds this */
nice_match = 128, /* Stop searching when current match exceeds this */
/* Note: the deflate() code requires max_lazy >= MIN_MATCH and max_chain >= 4
* For deflate_fast() (levels <= 3) good is ignored and lazy has a different
* meaning.