Re: hyperthreaded cpu still an issue in 8.4?

On 7/21/09 9:22 AM, "Scott Marlowe" <scott(dot)marlowe(at)gmail(dot)com> wrote:

>> But, the real point is that "thread" (whether "CoolThread" or "HT" thread),
>> is not the same as core, which is not the same as processor. X 2 threads is
>> usually significantly less benefit than X 2 cores. X 2 cores is probably
>> less benefit than X 2 processors.
>
> Actually, given the faster inter-connect speed and communication, I'd
> think a single quad core CPU would be faster than the equivalent dual
> dual core cpu.

Its very workload dependant and system dependant. If the dual core dual cpu
setup has 2x the memory bandwidth of the single quad core (Nehalem,
Opteron), it also likely has higher memory latency and a dedicated
interconnect for memory and cache coherency. And so some workloads will
favor the low latency and others will favor more bandwidth.

If its like the older Xeons, where an extra CPU doesn't buy you more memory
bandwidth alone (but better chipsets do), then a single quad core is usually
faster than dual core dual cpu (if the same chipset). Even more so if there
is a lot of lock contention, since that can all be handled on the same CPU
rather than communicating across the bus.

But back on topic for HT -- HT doesn't like spin-locks much unless they use
the right low level instruction sequence rather than actually spinning.
With the right instruction, the spin will allow the other thread to do
work... With the wrong one, it will tie up the pipeline. I have no idea
what Postgres' spin-locks and tool chain compile down to.