|P is for Practical|
Re^2: Improving performance of checksum calculationby Crackers2 (Parson)
|on May 30, 2009 at 16:36 UTC||Need Help??|
Interesting. Here's the results of applying some of this to the real code. My real file is about 200MB.
The full code also has a few more things; it gets a list of filehandles to chain together, and a maximum length to read in total; this complicates things slightly but doesn't seem to affect performance above noise levels.
Going higher than 8K blocks doesn't seem to help much on my system; the extra time taken with 32K blocks is probably noise and would smoothen out over multiple runs.
Since the actual input can be a stream (in this case usually the output of some other program piped to mine), I couldn't move the "multiple of 4 bytes" check out of the loop; but since it's no longer in the tightest loop that doesn't appear to matter much either.
And for completeness, here's my time using your C code converted to xs:
One interesting thing at this point is that we've dropped CPU usage from 100% down to about 80%, indicating that at this point reading the file might be the bottleneck.
Of course, that's all relative; after going from 1:38 or 0:00.95 any remaining optimization isn't of any practical purpose for me.
Thanks to both you and Ikegami for showing me I discarded the C-in-Perl too soon. While the end result is a bit more annoying to use, the performance gains are certainly worth it.