On Aug 27, 2010, at 10:25 AM, Greg Smith wrote:
> Scott Carey wrote:
>> But the select count(*) query, cached in RAM is 3x faster in one system than the other. The CPUs aren't 3x
differentperformance wise. Something else may be wrong here.
>>
>> An individual Core2 Duo 2.93Ghz should be at most 50% faster than a 2.2Ghz Opteron for such a query. Unless there
aresome compile options that are set wrong. I would check the compile options.
>>
>
> Sure, it might be. But I've seen RAM on an Intel chip like the E7500
> here (DDR3-1066 or better, around 10GB/s possible) run almost 3X as fast
> as what you'll find paired with an Opteron 2427 (DDR2-800, closer to
> 3.5GB/s). Throw in the clock differences and there you go.
The 2427 should do 12.8 GB/sec theoretical peak (dual channel 800Mhz DDR2) per processor socket (so 2x that if
multithreadedand 2 Sockets).
A Nehalem will do ~2x that (triple channel, 1066Mhz) and is also significantly faster clock for clock.
But a Core2 based Xeon on Socket 775 at 1066Mhz FSB? Nah... its theoretical peak bandwidth is 33% more and real world
nomore than 40% more.
Latency and other factors might add up too. 3x just does not make sense here.
Nehalem would be another story, but Core2 was only slightly faster than Opterons of this generation and did not scale
aswell with more sockets.
>
> I've been wandering around for years warning that the older Opterons on
> DDR2 running a single PostgreSQL process are dog slow compared to the
> same thing on Intel.
This isn't an older Opteron, its 6 core, 6MB L3 cache "Istanbul". Its not the newer stuff either. The E7500 is
basicallythe end of line Core2 before Nehalem based processors took over.
> So that alone might actually be enough to account
> for the difference. Ultimately the multi-processor stuff is what's more
> important to most apps, though, which is why I was hinting to properly
> run that instead.
>
> --
> Greg Smith 2ndQuadrant US Baltimore, MD
> PostgreSQL Training, Services and Support
> greg@2ndQuadrant.com www.2ndQuadrant.us
>