Re: Speed up Clog Access by increasing CLOG buffers

On Thu, Oct 20, 2016 at 3:36 AM, Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> On Thu, Oct 13, 2016 at 12:25 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>> I agree with these conclusions. I had a chance to talk with Andres
>> this morning at Postgres Vision and based on that conversation I'd
>> like to suggest a couple of additional tests:
>>
>> 1. Repeat this test on x86. In particular, I think you should test on
>> the EnterpriseDB server cthulhu, which is an 8-socket x86 server.
>
> I have done my test on cthulhu, basic difference is that In POWER we
> saw ClogControlLock on top at 96 and more client with 300 scale
> factor. But, on cthulhu at 300 scale factor transactionid lock is
> always on top. So I repeated my test with 1000 scale factor as well on
> cthulhu.

So the upshot appears to be that this problem is a lot worse on power2
than cthulhu, which suggests that this is architecture-dependent. I
guess it could also be kernel-dependent, but it doesn't seem likely,
because:

power2: Red Hat Enterprise Linux Server release 7.1 (Maipo),
3.10.0-229.14.1.ael7b.ppc64le
cthulhu: CentOS Linux release 7.2.1511 (Core), 3.10.0-229.7.2.el7.x86_64

So here's my theory. The whole reason why Tomas is having difficulty
seeing any big effect from these patches is because he's testing on
x86. When Dilip tests on x86, he doesn't see a big effect either,
regardless of workload. But when Dilip tests on POWER, which I think
is where he's mostly been testing, he sees a huge effect, because for
some reason POWER has major problems with this lock that don't exist
on x86.

If that's so, then we ought to be able to reproduce the big gains on
hydra, a community POWER server. In fact, I think I'll go run a quick
test over there right now...

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company