[Bug 10552] Sender checksum calculation significantly slower with compression enabled

[samba-bugs at samba.org]

https://bugzilla.samba.org/show_bug.cgi?id=10552

--- Comment #1 from John Pierman <haqthat at gmail.com> 2014-04-17 12:34:03 UTC ---
Confirmed:

Without -z avg over 3 runs (real    0m5.998s)
With -z avg over 3 runs (real    0m8.490s)


On Mon, Apr 14, 2014 at 4:10 PM, <samba-bugs at samba.org> wrote:

> https://bugzilla.samba.org/show_bug.cgi?id=10552
>
>            Summary: Sender checksum calculation significantly slower with
>                     compression enabled
>            Product: rsync
>            Version: 3.1.1
>           Platform: All
>         OS/Version: All
>             Status: NEW
>           Severity: normal
>           Priority: P5
>          Component: core
>         AssignedTo: wayned at samba.org
>         ReportedBy: dougmiles at cox.net
>          QAContact: rsync-qa at samba.org
>
>
> I've noticed that with the -z option enabled, comparing two files that are
> identical (or nearly so) but have different modification times takes
> significantly longer than without the -z option.  It looks like the entire
> file
> is compressed as the checksum is calculated even when no data needs to be
> transmitted to the receiver.
>
> To replicate:
>
> Create test folders/files (using incompressible data to maximize effect):
> mkdir a b
> dd if=/dev/urandom of=a/a.tst bs=1M count=250
> cp a/a.tst b/
>
> run rsync without compression:
> touch a/a.tst
> time rsync -av a/ b
>
> run rsync with compression:
> touch a/a.tst
> time rsync -avz a/ b
>
> The second time with the -z option will take significantly longer, even
> though
> the source and destination files are identical apart from the modification
> times.  I've also found that the latter uses much more CPU, but only on
> one of
> the two processes- the sender process I believe.
>
> Even when I added --skip-compress=tst, it made rsync much faster than -z
> alone,
> but it still took about 30% longer than omitting -z entirely.
>
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