Re: LWLock optimization for multicore Power machines

On Wed, Feb 8, 2017 at 5:00 PM, Bernd Helmle <mailings(at)oopsware(dot)de> wrote:

> Am Dienstag, den 07.02.2017, 16:48 +0300 schrieb Alexander Korotkov:
> > But win isn't
> > as high as I observed earlier. And I wonder why absolute numbers are
> > lower
> > than in our earlier experiments. We used IBM E880 which is actually
> > two
>
> Did you run your tests on bare metal or were they also virtualized?
>

I run tests on bare metal.

> nodes with interconnect. However interconnect is not fast enough to
> > make
> > one PostgreSQL instance work on both nodes. Thus, used half of IBM
> > E880
> > which has 4 sockets and 32 physical cores. While you use IBM E850
> > which is
> > really single node with 4 sockets and 48 physical cores. Thus, it
> > seems
> > that you have lower absolute numbers on more powerful hardware. That
> > makes
> > me uneasy and I think we probably don't get the best from this
> > hardware.
> > Just in case, do you use SMT=8?
>
> Yes, SMT=8 was used.
>
> The machine has 4 sockets, 8 Core each, 3.7 GHz clock frequency. The
> Ubuntu LPAR running on PowerVM isn't using all physical cores,
> currently 28 cores are assigned (=224 SMT Threads). The other cores are
> dedicated to the PowerVM hypervisor and a (very) small AIX LPAR.
>

Thank you very much for the explanation.

Thus, I see reasons why in your tests absolute results are lower than in my
previous tests.
1. You use 28 physical cores while I was using 32 physical cores.
2. You run tests in PowerVM while I was running test on bare metal.
PowerVM could have some overhead.
3. I guess you run pgbench on the same machine. While in my tests pgbench
was running on another node of IBM E880.

Therefore, having lower absolute numbers in your tests, win of LWLock
optimization is also lower. That is understandable. But win of LWLock
optimization is clearly visible definitely exceeds variation.

I think it would make sense to run more kinds of tests. Could you try set
of tests provided by Tomas Vondra?
If even we wouldn't see win some of the tests, it would be still valuable
to see that there is no regression there.

------
Alexander Korotkov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company