Re: Hyperthreading (was: Two identical systems, radically different performance)

Sent this to Claudio rather than the whole list ... here it is.

On Wed, Oct 10, 2012 at 7:44 AM, Claudio Freire <klaussfreire(at)gmail(dot)com>wrote:

> On Wed, Oct 10, 2012 at 9:52 AM, Shaun Thomas <sthomas(at)optionshouse(dot)com>
> wrote:
> > On 10/09/2012 06:30 PM, Craig James wrote:
> >
> >> ra:8192 walb:1M ra:256 walb:1M ra:256 walb:256kB
> >> ---------------- ---------------- -----------------
> >> -c -t Run1 Run2 Run3 Run4 Run5 Run6 Run7 Run8 Run9
> >> 40 2500 4261 3722 4243 9286 9240 5712 9310 8530 8872
> >> 50 2000 4138 4399 3865 9213 9351 9578 8011 7651 8362
> >
> >
> > I think I speak for more than a few people here when I say: wat.
> >
> > About the only thing I can ask, is: did you make these tests fair? And by
> > fair, I mean:
> >
> > echo 3 > /proc/sys/vm/drop_caches
> > pg_ctl -D /your/pg/dir restart
>
>
I showed the exact commands I used -- if it's not there, I didn't do it.
So the answer is no, I didn't drop caches.

On the other hand, I wanted to know what happened on cold start and after
running for a while. Running pgbench once isn't as interesting as running
it three times.

> Yes, I was thinking the same. Especially if you check the tendency to
> perform better in higher-numbered runs. But, as you said, that doesn't
> explain that jump to twice the TPS. I was thinking, and I'm not
> pgbench expert, could it be that the database grows from run to run,
> changing performance characteristics?
>
> > My head hurts.
>
> I'm just confused. No headache yet.
>
> But really interesting numbers in any case. It these results are on
> the level, then maybe the kernel's read-ahead algorithm isn't as
> fool-proof as we thought? Gotta read the source. BRB
>

Big numbers, little numbers ... I'm just reporting what pgbench tells me
and how I got them. I'm good at chemical databases, you guys are the
Postgres performance experts.