Bug#839600: perl: concatenating string instead of sprintf takes all computation power of server

[Niko Tyni]

On Sun, Oct 02, 2016 at 11:59:12PM +0300, Niko Tyni wrote:

> It seems to have to do with the size of the input file rather
> than its contents, but I'm not quite sure yet.

It looks like none of the gory regexps are necessary to trigger it.
Here's a short test case, resulting here in

           Rate  concat  assign sprintf
concat   2928/s      --    -94%    -94%
assign  47733/s   1530%      --     -0%
sprintf 47733/s   1530%      0%      --

so showing the 'concat' option has pathologic performance.
It goes away if the length of the base string $s is varied
even slightly, and seems to reappear in 8-byte increments
or decrements. Some of those, like 2**16 - 3, also blow
up the 'sprintf' performance.

The results don't vary much between jessie (Perl 5.20) and sid (5.24)
for me on amd64.

I'll test/bisect this on upstream code next and try to find out if it's
really copy-on-write related or something else.
-- 
Niko Tyni   ntyni at debian.org
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