[mythtv-users] What is a "dual core processor"?

[Daniel Kristjansson]

On Thu, 2008-12-04 at 10:51 -0800, David Brodbeck wrote:
> Jim Carter wrote:
> > Some or all Pentium 4's have hyperthreading, which means that one core can 
> > hop between two instruction streams.
> 
> Has anyone ever found this to be a benefit?  Every time I've tested
> hyperthreading under real-world workloads, I've found it decreased
> performance, so I now turn it off in the BIOS as a matter of course.

Whether HT helps depends on the workload. MythTV video playback is one
of the workloads it does help. If you enable HT in the BIOS, compile
your kernel with >=1000 slices per second and with a HT aware scheduler,
you will eliminate stutter on 1080i MPEG-2 playback on a P4 2.8Ghz
machine. But this is generally no longer a moot topic since modern
processors don't need any special tricks to get 1080i MPEG-2 playback
to work.

Note: You can also see tremendous improvements with HT if you write an
application from the ground up to take advantage of it, i.e. you segment
the workload of the primary processing threads so that one thread
primarily uses SIMD/MMX kernels and the other thread primarily uses
ALU/FPU kernels. (kernel here refers to processing kernels, not an OS
kernel; generally this is hand-written assembly which goes in the inner
most processing loop of a CPU intensive algorithm.)

-- Daniel