Mailinglist Archive: opensuse (2217 mails)
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Re: [opensuse] Re: RAM
- From: Randall R Schulz <rschulz@xxxxxxxxx>
- Date: Wed, 19 Sep 2007 17:05:03 -0700
- Message-id: <200709191705.03558.rschulz@xxxxxxxxx>
On Wednesday 19 September 2007 15:53, Aaron Kulkis wrote:
> Randall R Schulz wrote:
> > On Wednesday 19 September 2007 12:53, JJB wrote:
> >> Price for the system is the same with quad core 1.6 or dual core
> >> 3.0 ghz,
> >
> > That's an interesting pair of options. I wonder how to analyze
> > one's applications to make the better choice.
>
> The more cores contending for memory access, the more your system
> falls short of theoretical maximum throughput.
>
> So, go with the high-speed dual core rather than the low-speed
> quad-core. Memory contention issues will more than destroy the
> theoretical 0.4 GHz*CPU advantage of the quad core.
Not necessarily. Some instruction mixes have a much higher ratio of CPU
internal instruction cycles to memory accesses than others.
For example, tight inner loops (where all the instructions remain in the
level 1 cache) that perform lots of floating-point operations on values
that were computed by the immediately preceding instruction (again
benefitting from the on-chip cache) will exhibit relatively few memory
accesses per clock cycle.
In contrast, processing mixes that involve a lot of data movement and
relatively little calculation (especially mixes that use few
instructions that take multiple clock cycles) will benefit most from
fast RAM.
There's no one-size-fits-all answer. If you really want to optimize a
particular application, you must understand and analyze it carefully.
For "general-purpose" applications (not really meaningful without
_some_ characterization of the processing mix), there presumably are
some kinds of rules of thumb, but I'm not sure what they are.
(I know that for the application that absorbs most of my attention these
days the dominant factor is definitely RAM speed. I've observed that a
2.0 GHz Core Duo (_not_ Core 2) beats a 3.0 GHz Pentium 4 HT simply
because the former has faster memory. In fact, the ratio of the speed
of my current project is almost exactly the ratio of the RAM speed
between the two systems. It's as if the CPU speed didn't even matter!
> ...
Randall Schulz
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> Randall R Schulz wrote:
> > On Wednesday 19 September 2007 12:53, JJB wrote:
> >> Price for the system is the same with quad core 1.6 or dual core
> >> 3.0 ghz,
> >
> > That's an interesting pair of options. I wonder how to analyze
> > one's applications to make the better choice.
>
> The more cores contending for memory access, the more your system
> falls short of theoretical maximum throughput.
>
> So, go with the high-speed dual core rather than the low-speed
> quad-core. Memory contention issues will more than destroy the
> theoretical 0.4 GHz*CPU advantage of the quad core.
Not necessarily. Some instruction mixes have a much higher ratio of CPU
internal instruction cycles to memory accesses than others.
For example, tight inner loops (where all the instructions remain in the
level 1 cache) that perform lots of floating-point operations on values
that were computed by the immediately preceding instruction (again
benefitting from the on-chip cache) will exhibit relatively few memory
accesses per clock cycle.
In contrast, processing mixes that involve a lot of data movement and
relatively little calculation (especially mixes that use few
instructions that take multiple clock cycles) will benefit most from
fast RAM.
There's no one-size-fits-all answer. If you really want to optimize a
particular application, you must understand and analyze it carefully.
For "general-purpose" applications (not really meaningful without
_some_ characterization of the processing mix), there presumably are
some kinds of rules of thumb, but I'm not sure what they are.
(I know that for the application that absorbs most of my attention these
days the dominant factor is definitely RAM speed. I've observed that a
2.0 GHz Core Duo (_not_ Core 2) beats a 3.0 GHz Pentium 4 HT simply
because the former has faster memory. In fact, the ratio of the speed
of my current project is almost exactly the ratio of the RAM speed
between the two systems. It's as if the CPU speed didn't even matter!
> ...
Randall Schulz
--
To unsubscribe, e-mail: opensuse+unsubscribe@xxxxxxxxxxxx
For additional commands, e-mail: opensuse+help@xxxxxxxxxxxx
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