On 12/20/2016 02:19 PM, Andres Freund wrote:
On 2016-12-20 08:10:29 -0500, Robert Haas wrote:
We could use the GUC assign hook to compute a mask and a shift, so
that this could be written as (CurrPos & mask_variable) == 0.  That
would avoid the division instruction, though not the memory access.


I suspect that'd be fine.


I hope this is all in the noise, though.

Could very well be.


I know this is code is hot but I think it'll be hard to construct a
test case where the bottleneck is anything other than the speed at
which the disk can absorb bytes.

I don't think that's really true. Heikki's WAL changes made a *BIG*
difference. And pretty small changes in xlog.c can make noticeable
throughput differences both in single and multi-threaded
workloads. E.g. witnessed by the fact that the crc computation used to
be a major bottleneck (and the crc32c instruction still shows up
noticeably in profiles).  SSDs have become fast enough that it's
increasingly hard to saturate them.


It's not just SSDs. RAID controllers with write cache (which is typically just DRR3 memory anyway) have about the same effect even with spinning rust.

So yes, this might make a measurable difference.

--
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services


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