cores and threads

[David Holmes]

On 19/09/2013 7:43 AM, Pete Brunet wrote:
> If a machine has 4 cores and 8 threads will the jdk8 build run faster
> than one with 4 cores and 4 threads?

All depends on where the bottlenecks are. Given a build is pretty much 
I/O bound I wouldn't expect much difference.

> If so would it be a 2x decrease in build time?

No. The CPU component of the build will be a fraction of the I/O component.

Even a compute bound task won't see a 2x difference when run on twice 
the number of hardware threads as they share physical resources in the 
core so can't completely run in parallel.

 > Would the build explicitly take advantage of the
> hyper-threading or would any increase in performance be a side effect?

The build like most other software on the system knows nothing about 
processors, cores and (hyper)-threads. The OS presents a model where all 
of those things represent logical processors and it runs native threads 
on each logical.

Trying to do too much in parallel can easily degrade performance - you 
need to understand how Amdahls Law applies to the computation you are doing.

David