Re: Detecting CPUs and cores
Lew wrote:
It is likely that the two hyper-threaded cores will achieve more
throughput than on one of those "cores". The reason Intel provided
hyperthreading is that it increases throughput on the chip.
Daniel Dyer wrote:
That's the theory, but it's less clear cut than with a "proper" dual
core chip. There is anecdotal evidence of people turning *off*
Hyper-Threading in the BIOS to improve the performance of their
applications.
http://news.zdnet.co.uk/hardware/0,1000000091,39237341,00.htm
http://www.javalobby.org/java/forums/t54590.html
I haven't personally benchmarked the effect of Hyper-Threading but, if
your application is relying on the result of availableProcessors() to
fine tune it's concurrency strategy, it's important to be aware that 1
HT CPU may behave quite differently from 1 dual-core CPU.
There is evidence that multiple CPUs can reduce throughput relative to a
single CPU, too. It depends on the software.
I am not arguing that HyperThread is as effective as multiple cores. In fact,
HT is a hack devised by Intel to counteract excessive pipeline stalling, so it
stands to reason that its performance would differ perhaps significantly from
true multiple-CPU scenarios. The trick is to discern when the number of CPUs
reported by Java is a reliable number and when it isn't.
My point is that most times to the Java world it doesn't matter. You use the
number as reported. Like every performance issue, especially in Java, it is
nigh impossible to make /a priori/ judgments about the impact this will have,
especially cross-platform /a priori/ judgments. So unless we decide in a
particular case that the difference makes a difference, we pretend that it is
no difference. To the JVM, a HT (with both "cores" enabled) acts like two
CPUs, smells like two CPUs, quacks like two CPUs, so we pretend that it is two
CPUs.
Performance optimization comes after the fact.
--
Lew