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

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

On Wed, Oct 10, 2012 at 7:44 AM, Claudio Freire <klaussfreire@xxxxxxxxx> wrote:
On Wed, Oct 10, 2012 at 9:52 AM, Shaun Thomas <sthomas@xxxxxxxxxxxxxxxx> 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.


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