Re: What exactly is our CRC algorithm?

At 2014-11-20 13:47:00 +0530, ams(at)2ndQuadrant(dot)com wrote:
>
> > Suggestions for how to address (b) are welcome.

With help from Andres, I set up a workload where XLogInsert* was at the
top of my profiles: server with fsync and synchronous_commit off, and
pgbench running a multiple-row insert into a single-text-column table
with -M prepared -c 25 -t 250000 -f script.

Unfortunately I can't see much difference despite running things with
slightly different parameters a few dozen times. For example, here are
real/user/sys times from three runs each with HEAD and slice-by-8 on an
otherwise-idle i7-3770 server with a couple of undistinguished Toshiba
7200rpm SATA disks in RAID-1:

HEAD:
2m24.822s/0m18.776s/0m23.156s
3m34.586s/0m18.784s/0m24.324s
3m41.557s/0m18.640s/0m23.920s

Slice-by-8:
2m26.977s/0m18.420s/0m22.884s
3m36.664s/0m18.376s/0m24.232s
3m43.930s/0m18.580s/0m24.560s

I don't know how to interpret these results (especially the tendency for
the tests to slow down as time passes, with every version). At best, it
shows that the new CRC code doesn't hurt, at worst it's just irrelevant.
Supporting the latter interpretation, using the hardware-CRC patch also
gives similar results (but XLogInsert is not at the top of the profile).

Hardware-CRC:
2m29.090s/0m18.652s/0m23.764s
3m30.188s/0m18.692s/0m25.332s
3m38.110s/0m20.424s/0m24.532s

If anyone has other suggestions, I'm all ears.

-- Abhijit