Re: [PATCHv5] Add Gitweb support for XZ compressed snapshots

["J.H." <warthog19@eaglescrag.net>]

Mark A Rada wrote:
> 
>  >Note that for me the above results are not aligned in table.
>  >This is a cosmetic issue.
> 
> The table formatting issue was due to a bad habit of mixing tabs and 
> spaces,
> I decided to go with spaces this time :)
> 
> 
>  >One thing that would concern me greatly, is not so much the CPU time 
> (though that's a *huge* change in comparison to gz) but the memory 
> usage.  Where gzip and bzip2 are chewing 4M and 13M respectively, xz 
> chews 102M.  >From a 'beefy' server perspective chewing up that much 
> memory per snapshot for that long could be bad.  This is likely 
> something that needs to have some sort of enable/disable switch if it's 
> going to be included.
> 
> True, and there are two solutions I can think of for this "problem".
> 
>     1. My tests were at the default compression level, the XZ documentation
>     says that at lower levels you will get resource usage and compression
>     ratios that are comparable to BZip2. However, I'm not sure where you
>     would change the compression level variable for this (globally for the
>     system, somewhere in $GITWEB_CONFIG, a git config variable). Does
>     someone know the correct answer here?

Well you can always call xz with -[1-9] to change the compression level 
(same as gzip and bzip2) though I think a full disabling would be 'more' 
preferable, though I'm not sure I like Jakub's suggestion of just 
deleting it after the fact, it would work.

>     2. Implement snapshot caching for Gitweb.

I think it's slightly broken in my version (binary files don't work 
right apparently, it's on my todo list to fix in my upcoming update) but 
both Lea and I have done this long ago (caching layers in Gitweb), which 
would be an acceptable workaround for this, create once and serve many - 
though this has the downside of trading cpu for diskspace.  At least 
with xz there's less diskspace used ;-)

I think more my concern is more what's enabled by default, and since xz 
is still new (as was pointed out) it's probably worth only enabling if 
the admin selects it to be enabled.

- John 'Warthog9' Hawley


> -- 
> Mark A Rada (ferrous26)
> marada@uwaterloo.ca
> 
> 
> From: Mark Rada <marada@uwaterloo.ca>
> Date: Thu, 30 Jul 2009 08:56:42 -0400
> Subject: [PATCH] Add Gitweb support for XZ compressed snapshots
> 
> The XZ compression format uses the LZMA2 compression algorithm, which
> often yields higher compression ratios than both GZip and BZip2 at the
> cost of using more CPU time and RAM. Though, while XZ is the slowest
> for compression, it is much faster than BZip2 for decompression, almost
> comparable to GZip (see benchmarks below).
> 
> You can enable XZ compressed snapshots by adding 'txz' to the list of
> default options for snapshots in your $GITWEB_CONFIG.
> 
> I did some simple benchmarks, starting with an already tarballed
> archive of the repos listed below. Memory usage seemed to be consistent
> for any given algorithm at their default compression level. Timings were
> gathered using the `time' command.
> 
> CPU: AMD Sempron 3400+ (1 core @ 1.8GHz with 256K L2 cache)
> Virtual Memory Usage
>     GZip: 4152K    BZip2: 13352K    XZ: 102M
> 
> Linux 2.6 series (f5886c7f96f2542382d3a983c5f13e03d7fc5259)   349M
> gzip    23.70s user   0.47s system  99% cpu   24.227 total    76M
> gunzip  3.74s user    0.74s system  94% cpu   4.741 total
> bzip2   130.96s user  0.53s system  99% cpu   2:11.97 total   59M
> bunzip2 31.05s user   1.02s system  99% cpu   32.355 total
> xz      448.78s user  0.91s system  99% cpu   7:31.28 total   51M
> unxz    7.67s user    0.80s system  98% cpu   8.607 total
> 
> Git (0a53e9ddeaddad63ad106860237bbf53411d11a7)                11M
> gzip    0.77s user    0.03s system  99% cpu   0.792 total     2.5M
> gunzip  0.12s user    0.02s system  98% cpu   0.142 total
> bzip2   3.42s user    0.02s system  99% cpu   3.454 total     2.1M
> bunzip2 0.95s user    0.03s system  99% cpu   0.984 total
> xz      12.88s user   0.14s system  98% cpu   13.239 total    1.9M
> unxz    0.27s user    0.03s system  99% cpu   0.298 total
> 
> XZ (669413bb2db954bbfde3c4542fddbbab53891eb4)                 1.8M
> gzip    0.12s user    0.00s system  95% cpu   0.132 total     442K
> gunzip  0.02s user    0.00s system  97% cpu   0.027 tota
> bzip2   1.28s user    0.01s system  99% cpu   1.298 total     363K
> bunzip2 0.15s user    0.01s system  100% cpu  0.157 total
> xz      1.62s user    0.03s system  99% cpu   1.652 total     347K
> unxz    0.05s user    0.00s system  99% cpu   0.058 total
> 
> Purely from a time and memory perspective, nothing compares to GZip in
> each of the three tests. Though, if you have an average upload speed of
> 20KB/s, it would take ~400 seconds longer to transfer the kernel
> snapshot that was BZip2 compressed than it would the XZ compressed
> snapshot, the transfer time difference is even greater when compared to
> the GZip compressed snapshot. The wall clock time savings are
> relatively the same for all test cases, but less dramatic for the
> smaller repositories.
> 
> The obvious downside for XZ compressed snapshots is the large CPU and
> memory load put on the server to actualy generate the snapshot. Though
> XZ will eventually have good threading support, and I suspect then that
> the wall clock time for making an XZ compressed snapshot would go down
> considerably if the server had a beefy multi-core CPU.
> 
> I have not enabled XZ compression by default because the current default
> is GZip, and XZ is only really competitive with BZip2. Also, the XZ
> format is still fairly new (the format was declared stable about 6
> months ago), and there have been no "stable" releases of the utils yet.
> 
> Signed-off-by: Mark Rada <marada@uwaterloo.ca>
> ---
>  gitweb/gitweb.perl |    8 ++++++++
>  1 files changed, 8 insertions(+), 0 deletions(-)
> 
> diff --git a/gitweb/gitweb.perl b/gitweb/gitweb.perl
> index 7fbd5ff..3398163 100755
> --- a/gitweb/gitweb.perl
> +++ b/gitweb/gitweb.perl
> @@ -176,6 +176,13 @@ our %known_snapshot_formats = (
>          'format' => 'tar',
>          'compressor' => ['bzip2']},
> 
> +    'txz' => {
> +        'display' => 'tar.xz',
> +        'type' => 'application/x-xz',
> +        'suffix' => '.tar.xz',
> +        'format' => 'tar',
> +        'compressor' => ['xz']},
> +
>      'zip' => {
>          'display' => 'zip',
>          'type' => 'application/x-zip',
> @@ -188,6 +195,7 @@ our %known_snapshot_formats = (
>  our %known_snapshot_format_aliases = (
>      'gzip'  => 'tgz',
>      'bzip2' => 'tbz2',
> +    'xz'    => 'txz',
> 
>      # backward compatibility: legacy gitweb config support
>      'x-gzip' => undef, 'gz' => undef,