Re: What exactly is our CRC algorithm?

On 11/19/2014 06:49 PM, Robert Haas wrote:
> On Wed, Nov 19, 2014 at 11:44 AM, Heikki Linnakangas
> <hlinnakangas@vmware.com> wrote:
>> That's an interesting choice of workload. That sure is heavy on the CRC
>> calculation, but the speed of pg_xlogdump hardly matters in real life.
>
> But isn't a workload that is heavy on CRC calculation exactly what we
> want here?  That way we can see clearly how much benefit we're getting
> on that particular part of the computation.  It'll still speed up
> other workloads, too, just not as much.

Sure. But pg_xlogdump's way of using the CRC isn't necessarily 
representative of how the backend uses it. It's probably pretty close to 
WAL replay in the server, but even there the server might be hurt more 
by the extra cache used by the lookup tables. And a backend generating 
the WAL computes the CRC on smaller pieces than pg_xlogdump and WAL redo 
does.

That said, the speedup is so large that I'm sure this is a big win in 
the server too, despite those factors.

- Heikki