question about multi core processors

[OhioGuy]

I'm thinking of doing a processor upgrade in the next couple of
months. Currently I have a single core Athlon BE running at about 2.6
GHz. I've got a fairly new (~ 5 months old) motherboard that is AM2+.

I do a lot of video compression into h.264/mpeg4 using Nero Recode,
and occasionally WinAVI Video Converter.


I read recently that if you are encoding a single file, multi cores
don't help you out at all. Is this true? Supposedly, you only get a
benefit if you are encoding multiple files, because then the other cores
can each tackle one of the files.

I had always assumed that if you have 4 cores, each core would
somehow do 1/4 of the work encoding a single file, and the computer
would "distribute" the processing chore between the cores, allowing you
to get done maybe 3 times as fast as a single core computer would.
However, I've been told that is not the case.

If this is true, then the multiple core upgrade would only very
occasionally help me out at all, and perhaps I should just stick with my
current single core.

Anyone have further information on this? Thanks!

 

[Robert Redelmeier]

I'm thinking of doing a processor upgrade in the next couple
of months. Currently I have a single core Athlon BE running
at about 2.6 GHz. I've got a fairly new (~ 5 months old)
motherboard that is AM2+.

Go for it. Often I find motherboards un-upgradeable
in practice. This appears an exception.

I do a lot of video compression into h.264/mpeg4 using
Nero Recode, and occasionally WinAVI Video Converter.

RTFM. Video compression is one of the few remaining home
apps that is CPU bound. I expect them to be highly optimized.

I read recently that if you are encoding a single file,
multi cores don't help you out at all. Is this true?

No. As 'tripper mentioned, it all depends on the software.
A quick look at the Nero Recode manual (online) says it can
take advantage of multicore by using the "Threads" option.

Personally, I have often found that threads = 2*CPUs is
optimal, but you should test. It depends entirely on OS
and application read-ahead. If a thread stalls on a read,
it is good to have another to run. But too many threads per
CPU just thrashes caches and may increase task switch overhead.


-- Robert R

 

[Oliver Nowak]

OhioGuy machte das folgende Beispiel :

I had always assumed that if you have 4 cores, each core would somehow do
1/4 of the work encoding a single file, and the computer would "distribute"
the processing chore between the cores, allowing you to get done maybe 3
times as fast as a single core computer would. However, I've been told that
is not the case.

A processor can't by itself split up a single stream of instructions
into multiple ones that can be executed in parallel. This has to be
done by the programmer.

One more comment on your possible upgrade: If you don't have to upgrade
immediately, maybe you should wait until early next year and
investigate whether AMD's upcoming 45nm Deneb quad cores work on your
AM2+ board. Thouse should be faster with lower power consumption than
the current (65nm) ones - and they'll have a larger cache, although
that probably won't increase video encoding performance.

O.

 

[krw]

OhioGuy machte das folgende Beispiel :

A processor can't by itself split up a single stream of instructions
into multiple ones that can be executed in parallel. This has to be
done by the programmer.

Be careful with your wording, here. Processors can and do split a
single stream of instructions into several that are executed in
parallel. They cannot (yet) split a single thread into separate
contexts that can be split across processors.

One more comment on your possible upgrade: If you don't have to upgrade
immediately, maybe you should wait until early next year and
investigate whether AMD's upcoming 45nm Deneb quad cores work on your
AM2+ board. Thouse should be faster with lower power consumption than
the current (65nm) ones - and they'll have a larger cache, although
that probably won't increase video encoding performance.

....and if you wait a little longer...

 

[Robert Redelmeier]

Quad core. Color me skeptical. For most users, is not a
dual a better value, giving a higher clock rate (and better
overall performance) for your dollar?

For the general user, certainly. But the OP was clearly ...

Maybe if you're a real "multitasker", who is ripping DVD's
while compiling a program, while surfing the Web...

His specific application was video encoding (compression),
a task which can be efficiently parallelized, nearly
clock-for-clock. So quad might be worthwhile over dual
even 20% lower MHz.


-- Robert R