Massive Speed Gains via Parallelized BZIP2 Compression

http://hackercodex.com/guide/parallel-bzip-compression/

by SnowLprd • 14 years ago

43 38 14 years ago

th0ma52• 14 years ago

Since our move to multicore over faster processors, I'm sure we'll see a lot of this sort of thing, that is, people suddenly realizing that their code will be some multiple faster if they can find a way to do operations in parallel. I imagine that the compression itself might be slightly less optimal however since similar blocks that could be compressed are on different threads? I didn't dig into how this might or might not be a concern with this project, however. Long of the short of it, however, parallel is the reality. In theory one could arbitrarily split the file, and then compress each of the splits and get a speed up that is roughly comparable?

malkia4• 14 years ago

"In theory one could arbitrarily split the file, and then compress each of the splits and get a speed up that is roughly comparable?" - it could. that's what is done, but for LZ type compressor bigger dictionary (e.g. avoiding splits is for the better) - one more reason why there is .tar -> .tgz (or .tbz), while .zip compresses individually.

It would be even better if files are arranged in such way that contentwise they are similar one after each other.

I'm not compression expert, just average user since the early days of what we used to call it back in the day "solid" compression.

wtetzner• 14 years ago

>In theory one could arbitrarily split the file, and then compress each of the splits and get a speed up that is roughly comparable?

It doesn't seem unlikely that that's what they're doing, considering you can't pipe data to it on stdin.

CJefferson• 14 years ago

bz2, out of all current compression methods, is particularly parallisable, as it has already split the files up into 900k (or smaller) blocks, and compressed each individually (well, run BWT on each seperately at least).

stcredzero• 14 years ago

Reimplementation of things like compression algorithms seems very math/algorithm-heavy and thus amenable to functional programming. How about the Haskell/OCaml guys re-implement a bunch of Un^x style utilities for us?

th0ma52• 14 years ago

in theory gnu parallel can be used to automate an idea in place like xargs

wmf• 14 years ago

BTW, bz2 is kinda over. Check out xz and the parallel version pxz.

vasi4• 14 years ago

I've also got my version of parallel xz: https://github.com/vasi/pixz

It doesn't require use of large temporary files like pxz, and the xz files it produces can also be decompressed in parallel.

LogicX• 14 years ago

Looks worth trying out --

Can I suggest adding installation packages for: brew ubuntu (at least through launchpad)

ak217• 14 years ago

vasi, thanks very much for writing pixz, I've had a great experience working with it and it sped up my code very significantly.

The only thing I wish for is more documentation and inclusion in xz-utils (which I know is not up to you, but I'm hopeful).

SnowLprd5• 14 years ago

That looks very promising. As others have pointed out here, pixz would be much more accessible if one could simply install it via "brew install pixz" or "aptitude install pixz".

bacr• 14 years ago

I'm no expert in compression, so its no surprise that I've never heard of xz. What are it's strengths compared to gzip, or b2z? Efficiency? Convention?

shin_lao• 14 years ago

You probably heard of 7Zip and LZMA. xz uses the LZMA2 algorithm to achieve better compression ratios.

keeperofdakeys• 14 years ago

You've probably heard of LZMA, xz is merely an implementation of this (7zip and rar also use LZMA). The most notable differences to bzip2 are significantly longer compression time and rather moderate decompression speed increases. This makes xz quite good for packaging data for distribution, although it takes a lot longer to compress, it decompresses a lot faster (unlike bzip2) and saves a significant amount of space.

james4k• 14 years ago

Recently when compressing some SQL backups, I found xz to be very slow when compared to bz2 (at least with default compression settings).

Edit: And in this case, bz2 even gave better compression ratios if I recall correctly.

ralph• 14 years ago

If speed matters more than compression then consider lzop(1).

SnowLprd5• 14 years ago

I disagree with your assessment. I'm aware of xz and use it when file size is paramount -- which, by the way, is almost never. Most of the time, I'm willing to give up a few megabytes in order to save time, and in my tests pbzip2 crushed xz by a factor of ten when it comes to speed.

Why not use parallelized versions of xz such as pxz or pixz? Because pre-built pxz/pixz packages are nearly non-existent. When that changes, I'll consider switching formats.

vasi4• 14 years ago

Remember that compressors, including xz, support multiple compression levels. The default level for xz is 6, which is perhaps too far on the small-but-slow side. Levels 2 and lower tend to give similar compression levels to bzip2, and are considerably faster.

Also, note that decompressing bzip2 is very slow, xz usually beats it by a factor of two or more.

malkia4• 14 years ago

"The results: 18.7 seconds for bzip2, and… wait for it… 3.5 seconds for pbzip2. That’s an increase of over 80%!"

File cache effect? He should cold reboot first (not sure how you force the file cache out on OSX/linux, on Windows I do it with SysInternals RamMap) and try in different order.

It could still be faster, but he could really be measuring I/O that was done in the first case, and not in the second.

It's also strange that .tar files are used, not tar.bz2 or .tbz (if such extension makes sense)

sciurus2• 14 years ago

On linux, it's

    echo 1 > /proc/sys/vm/drop_caches

http://linux-mm.org/Drop_Caches

minimax• 14 years ago

Awesome tip! Thanks!

SnowLprd5• 14 years ago

I'll do some additional testing to see if the results are affected by caching.

Not sure why you're surprised that I used .tar files for the compression testing. As I mentioned in the article, most of the time I'm creating bzipped tarballs from directories of files, so it made sense to use what is, for me, a common real-world use case. Your mention of tar.bz/.tbz makes me think there's some misunderstanding, since clearly I wouldn't want to test compression of already-compressed files. But perhaps it's I who am misunderstanding your suggestion. Please feel free to enlighten me. :)

malkia4• 14 years ago

Got it!

aphyr• 14 years ago

18.7 seconds for bzip2, and… wait for it… 3.5 seconds for pbzip2. That’s an increase of over 80%!

Er, not really. How about...

"pbzip2 reduced running time by 80%."

"pbzip2 took only 20% as long as bzip2 did."

"pbzip2 is five times faster."

SnowLprd5• 14 years ago

Quite right. Clearly not enough coffee. Updated the article with better wording. Thanks!

sciurus2• 14 years ago

For parallel gzip there's pigz (pronounced pig-zee).

http://www.zlib.net/pigz/

Inufu• 14 years ago

Is there a reason this is not the default?

reidrac• 14 years ago

I'm GNU tar user (I believe that's the version in most Linux distributions, but I may be wrong), so I tend to use -z for gzip, -j for bzip2 and -J for xz.

That said, I guess using the "alternatives" framework in Linux it would be reasonably easy (and transparent) to support the parallel version of each tool as replacement to the regular one.

juiceandjuice• 14 years ago

bzip2 has always been parallelizable. At one point a few years ago I was working on a compressed file format with that included compressed block metadata, because bzip2 is most efficient when it gets about ~900kB to compress at a time. In effect, you split the file up into 900kb chunks, compress them in parallel, and recombine them into one file at the end.

BrainInAJar• 14 years ago

is there a pbzip2 that doesn't eat all your memory ?

mattst88• 14 years ago

I used to use pbzip2 before I learned about lbzip2 (http://lacos.hu/)

lbzip2 is able to decompress single streams using multiple threads, which apparently pbzip2 cannot do. See the thread beginning with http://lists.debian.org/debian-mentors/2009/02/msg00098.html

rorrr• 14 years ago

A GPU implementation would be cool.

dguido• 14 years ago

Parallel gzip, in case anyone wanted it: http://zlib.net/pigz/

I've used it to great effect during incident response when I needed to search through hundreds of gigs of logs at a time.

Massive Speed Gains via Parallelized BZIP2 Compression

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