[
https://issues.apache.org/jira/browse/COMPRESS-333?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15196233#comment-15196233
]
Dawid Weiss commented on COMPRESS-333:
--------------------------------------
For completeness, COMPRESS-327 talks about seeking through the file -- this
wouldn't yield random-access to 7zip entries if they're block-compressed (I
recall explaining why this is a problem in a comment on another issue, but
can't recall which one it was). In any case, if the blocks are large, providing
random-access to any entry could (and very likely eventually would for
somebody) lead to exponential decompression times because you'd decompress the
same information over and over. The simplest case is reading files "in reverse"
with solid blocks containing many, many files (or very large files).
> bz2 stream decompressor is 10x slower than it could be
> ------------------------------------------------------
>
> Key: COMPRESS-333
> URL: https://issues.apache.org/jira/browse/COMPRESS-333
> Project: Commons Compress
> Issue Type: Improvement
> Reporter: Dawid Weiss
> Fix For: 1.11
>
>
> This is related to COMPRESS-291. In short: decompressing 7z archives was an
> order of magnitude slower in Java than with native tooling.
> My investigation showed that the problematic archive used bz2 streams inside.
> I then did a quick hack-experiment which took bz2 decompressor from the
> Apache Hadoop project (the Java version, not the native one) and replaced the
> default one used for bz2 stream decompression of the 7z archiver in commons.
> I then ran a quick benchmark on this file:
> {code}
> https://archive.org/download/stackexchange/english.stackexchange.com.7z
> {code}
> The decompression speeds are (SSD, the file was essentially fully cached in
> memory, so everything is CPU bound):
> {code}
> native {{7za}}: 13 seconds
> Commons (original): 222 seconds
> Commons (patched w/Hadoop bz2): 30 seconds
> Commons (patched w/BufferedInputStream): 28 seconds
> {code}
> Yes, it's still 3 times slower than native code, but it's no longer glacially
> slow...
> My patch is a quick and dirty proof of concept (not committable, see [1]),
> but it passes the tests. Some notes:
> - Hadoop's stream isn't suited for handling concatenated bz2 streams, it'd
> have to be either patched in the code or (better) decorated at a level above
> the low-level decoder,
> - I only substituted the decompressor in 7z, but obviously this could benefit
> in other places (zip, etc.); essentially, I'd remove
> BZip2CompressorInputStream entirely.
> - while I toyed around with the above idea I noticed a really annoying thing
> -- all streams are required to extend {{CompressorInputStream}}, which only
> adds one method to count the number of consumed bytes. This complicates the
> code and makes plugging in other implementations of InputStreams more
> cumbersome. I could get rid of CompressorInputStream entirely with a few
> minor changes to the code, but obviously this would be backward incompatible
> (see [2]).
> References:
> [1] GitHub fork, {{bzip2}} branch:
> https://github.com/dweiss/commons-compress/tree/bzip2
> [2] Removal and cleanup of CompressorInputStream:
> https://github.com/dweiss/commons-compress/commit/6948ed371e8ed6e6b69b96ee936d1455cbfd6458
--
This message was sent by Atlassian JIRA
(v6.3.4#6332)
