X2 vs X4

In message "Zootal"
wrote:

I don't get this - what can hyperthreading do that a good cpu scheduler
can't do? If I have two virtual cores, I have to have two schedulers running
(one for each virtual cpu), each with their own set of queues and each with
50% cpu time. Is that more efficient then one single scheduler that has 100%
cpu time?

The problem that Hyperthreading was designed to solve is that the P4
series has an extremely long pipeline.

In other words, it takes many cycles to get instructions to the CPU, and
for the CPU to send instructions to pull data to/from memory or other
hardware components.

Hyperthreading was designed to help/encourage existing OSes to schedule
multiple threads/workloads so that the CPU can run them, from the OS'
point of view, concurrently, rather then waiting for one workload to
finish before sending another.

In other words, it takes many cycles to get instructions to the CPU, and
for the CPU to send instructions to pull data to/from memory or other
hardware components.

That isn't exactly correct - the long pipeline *is* the cpu, it just takes a
lot of cycles to make it through the pipeline. In order to get the
advertised clock speed, they had to make the pipeline longer. The P4
Prescott 3.8GHz has a 31 cycle pipeline.

Bill wrote:
In article ,
says...
The effective clock speed of a single core in a multiple core chip is
the chip clock speed divided by the number of cores,

Have you got a cite for that?

snip

Bill

The person who told me this is Miles R***, a person who sells computers
for a living. If the cores ran at the chip's nominal clock speed, a
four-core chip would perform 4 times faster than a single core chip at
the same clock speed, which they don't. And the power consumption would
be much higher. So I think Miles is correct.

Dave Feustel writes:
The person who told me this is Miles R***, a person who sells computers
for a living. If the cores ran at the chip's nominal clock speed, a
four-core chip would perform 4 times faster than a single core chip at
the same clock speed, which they don't. And the power consumption would
be much higher. So I think Miles is correct.

No, this is not correct.

Either you misinterpreted "Miles R***", or he is quite ignorant about
his own product (or both).

-Miles

--
Genealogy, n. An account of one's descent from an ancestor who did not
particularly care to trace his own.

On Sun, 21 Sep 2008 08:21:19 -0500, Dave Feustel wrote:

The person who told me this is Miles R***, a person who sells computers
for a living. If the cores ran at the chip's nominal clock speed, a
four-core chip would perform 4 times faster than a single core chip at
the same clock speed, which they don't. And the power consumption would
be much higher. So I think Miles is correct.

The four core chip can only run an application on all four cores if
it's threaded and at least 4 threads have work that can be run
simultaneously. Even in threaded applications this can't always happen
unless the threads are doing something that doesn't depend on others,
say converting a video file where each core can be given a section of
the file to convert.

I can see how he came to the conclusion though if he ran a single
threaded application and it ran four times slower than expected, since
it ran on only one core. Get him to run four of them at the same time
and they should complete in nearly the same time as one providing he
isn't running anything else at that time.

As for power consumption my dual core chip uses 45 watts and the quad
core version uses 95 watts. Taking into account the extra circuitry for
the 4 cores it's about right.

--
Regards - Rodney Pont
The from address exists but is mostly dumped,
please send any emails to the address below
e-mail ngpsm4 (at) infohitsystems (dot) ltd (dot) uk

Rodney Pont wrote:
On Sun, 21 Sep 2008 08:21:19 -0500, Dave Feustel wrote:

The person who told me this is Miles R***, a person who sells computers
for a living. If the cores ran at the chip's nominal clock speed, a
four-core chip would perform 4 times faster than a single core chip at
the same clock speed, which they don't. And the power consumption would
be much higher. So I think Miles is correct.

The four core chip can only run an application on all four cores if
it's threaded and at least 4 threads have work that can be run
simultaneously. Even in threaded applications this can't always happen
unless the threads are doing something that doesn't depend on others,
say converting a video file where each core can be given a section of
the file to convert.

I can see how he came to the conclusion though if he ran a single
threaded application and it ran four times slower than expected, since
it ran on only one core. Get him to run four of them at the same time
and they should complete in nearly the same time as one providing he
isn't running anything else at that time.

As for power consumption my dual core chip uses 45 watts and the quad
core version uses 95 watts. Taking into account the extra circuitry for
the 4 cores it's about right.

One must also bear in mind that a dual-core or quad-core CPU has to
devote some processing time to deciding what to run on which core,
when. This more intricate scheduling task routinely results in a
process running on only one core running more slowly (all things
included) than a process running on a single-core CPU that is lightly
loaded.

Whatever the CPU speed is in GHz or MHz, all cores will work at that
speed unless power management software readjusts the speed. That
does not mean that all that speed is usable, though. You still have
delays due to I/O requirements, scheduling delays, wait states, and a
host of other bottlenecks, real and potential. My home computer is
an AMD 64-bit 5000+ dual-core, and CPU usage typically is in the 1 to
3 percent range when I am not compiling or doing some other
CPU-intensive task. This does not mean that all tasks complete
instaneously nor that response time is zero (though it is very nice,
I will admit).

Specifically with respect to X2 vs X4, the kernel scheduler will do a
fairly good job of using two CPUs, but rarely does well with more
than two unless the applications are specifically tailored for
multi-CPU usage. Thus, the percentage gain in performance from
shifting from single to dual-core cpu is likely to be significantly
greater than the percentage gain from shifting from dual-core to
quad-core, unless you have software tailored for the additional cores.

The big question, of course, is, are your applications CPU-intensive
enough to make use of the available capacity, regardless of number of
cores? If the computer is not heavily loaded at least part of the
time, the answer is likely to be no.

Cheers!

jim b.




--
UNIX is not user unfriendly; it merely
expects users to be computer-friendly.

Jim Beard writes:


Specifically with respect to X2 vs X4, the kernel scheduler will do a
fairly good job of using two CPUs, but rarely does well with more
than two unless the applications are specifically tailored for

maybe with respect to windows, but linux schedulers are O(1) over
large numbers of cores.

scheduler overhead is pretty much non-existent.

scott

Bill wrote:
In article ,
says...
Bill wrote:
In article ,
says...
The effective clock speed of a single core in a multiple core chip is
the chip clock speed divided by the number of cores,
Have you got a cite for that?

snip

Bill
The person who told me this is Miles R***, a person who sells computers
for a living. If the cores ran at the chip's nominal clock speed, a
four-core chip would perform 4 times faster than a single core chip at
the same clock speed, which they don't. And the power consumption would
be much higher. So I think Miles is correct.


You're entitled to your opinion, but as far as "The effective clock
speed of a single core in a multiple core chip is the chip clock
speed divided by the number of cores" is concerned Miles R*** is full
of *****, and you can tell him I said so.

You need to get to Intel's or AMD's website and do some reading.

Bill
I have a X4 and each core at default is 2.5ghz.

In message Dave Feustel
wrote:

The person who told me this is Miles R***, a person who sells computers
for a living.

"Never trust someone trying to sell you something" comes to mind.

If the cores ran at the chip's nominal clock speed, a
four-core chip would perform 4 times faster than a single core chip at
the same clock speed, which they don't.

Depending on your task, a four-core CPU can perform reasonably close to
four times the clock speed of a single core CPU. Unfortunately, few
tasks parrallelize that well, and even less software takes full
advantage of modern CPUs.

That being said, aside from some shady marketing in the past advertising
dual CPU systems as double the clock speed of one CPU rather then
advertising the actual configuration, each core runs at the full clock
speed advertised.

DevilsPGD wrote:
In message Dave Feustel
wrote:

The person who told me this is Miles R***, a person who sells computers
for a living.

"Never trust someone trying to sell you something" comes to mind.

If the cores ran at the chip's nominal clock speed, a
four-core chip would perform 4 times faster than a single core chip at
the same clock speed, which they don't.

Depending on your task, a four-core CPU can perform reasonably close to
four times the clock speed of a single core CPU. Unfortunately, few
tasks parrallelize that well, and even less software takes full
advantage of modern CPUs.

That being said, aside from some shady marketing in the past advertising
dual CPU systems as double the clock speed of one CPU rather then
advertising the actual configuration, each core runs at the full clock
speed advertised.

So the 4 core chip cpu should run 4 independent identical tasks (compute
pi to 1 million digits) in essentially the same time that a single core
runs one instance of that task?