Re: Memory Alignment in Postgres

On Thu, Sep 11, 2014 at 11:27 AM, Andres Freund <andres(at)2ndquadrant(dot)com>
wrote:

> On 2014-09-11 10:32:24 -0300, Arthur Silva wrote:
> > Unaligned memory access received a lot attention in Intel post-Nehalen
> era.
> > So it may very well pay off on Intel servers. You might find this blog
> post
> > and it's comments/external-links interesting
> >
> http://lemire.me/blog/archives/2012/05/31/data-alignment-for-speed-myth-or-reality/
>
> FWIW, the reported results of imo pretty meaningless for postgres. It's
> sequential access over larger amount of memory. I.e. a perfectly
> prefetchable workload where it doesn't matter if superflous cachelines
> are fetched because they're going to be needed next round anyway.
>
> In many production workloads one of the most busy accesses to individual
> datums is the binary search on individual pages during index
> lookups. That's pretty much exactly the contrary to the above.
>
> Not saying that it's not going to be a benefit in many scenarios, but
> it's far from being as simple as saying that unaligned accesses on their
> own aren't penalized anymore.
>
> Greetings,
>
> Andres Freund
>
> --
> Andres Freund http://www.2ndQuadrant.com/
> PostgreSQL Development, 24x7 Support, Training & Services
>

I modified the test code to use a completely random scan pattern to test
something that completely trashes the cache. Not realistic but still
confirms the hypothesis that the overhead is minimal on modern Intel.

------------------ test results compiling for 32bit ------------------
processing word of size 2
offset = 0
average time for offset 0 is 422.7
offset = 1
average time for offset 1 is 422.85

processing word of size 4
offset = 0
average time for offset 0 is 436.6
offset = 1
average time for offset 1 is 451
offset = 2
average time for offset 2 is 444.3
offset = 3
average time for offset 3 is 441.9

processing word of size 8
offset = 0
average time for offset 0 is 630.15
offset = 1
average time for offset 1 is 653
offset = 2
average time for offset 2 is 655.5
offset = 3
average time for offset 3 is 660.85
offset = 4
average time for offset 4 is 650.1
offset = 5
average time for offset 5 is 656.9
offset = 6
average time for offset 6 is 656.6
offset = 7
average time for offset 7 is 656.9

------------------ test results compiling for 64bit ------------------
processing word of size 2
offset = 0
average time for offset 0 is 402.55
offset = 1
average time for offset 1 is 406.9

processing word of size 4
offset = 0
average time for offset 0 is 424.05
offset = 1
average time for offset 1 is 436.55
offset = 2
average time for offset 2 is 435.1
offset = 3
average time for offset 3 is 435.3

processing word of size 8
offset = 0
average time for offset 0 is 444.9
offset = 1
average time for offset 1 is 470.25
offset = 2
average time for offset 2 is 468.95
offset = 3
average time for offset 3 is 476.75
offset = 4
average time for offset 4 is 474.9
offset = 5
average time for offset 5 is 468.25
offset = 6
average time for offset 6 is 469.8
offset = 7
average time for offset 7 is 469.1