Which is faster PC6400 6-6-6-6 or 4-4-4-12 ?

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Otto

Hi,

I have a choice of memory for a Dell Optiplex 745 taking 800MHz PC6400
RAM - 2 timing specs are available: 6-6-6-6 or 4-4-4-12


Any idea if the latter (and a bit more expensive part) is actually any
faster?
 
Otto said:
Hi,

I have a choice of memory for a Dell Optiplex 745 taking 800MHz PC6400
RAM - 2 timing specs are available: 6-6-6-6 or 4-4-4-12


Any idea if the latter (and a bit more expensive part) is actually any
faster?
Click to expand...

Memory has two performance aspects.

Memory bandwidth is measured by "PC2-6400". The maximum rate that
information can come from the memory, is 6400MB/sec. This is useful
when large blocks of memory are being pulled into the processor.
Usually, a "burst" of data is designed to fill a "cache line" inside
the processor, so that is a normal sized transaction for the processor.

The second set of numbers concern latency. That is important for
random access. Not all operations on the computer access large
contiguous blocks of data. Sometimes, the processor effectively picks
just one byte here and one byte there. The CAS Latency defines the delay
from the request, until the data is provided. The lower the number
(your 4-4-4-12 product), the quicker the answer comes back. If you
consider the time duration of each one of those operations, and then
take the inverse of that, you can see how many "queries per second"
the lower latency memory can make. So, for programs which are
referred to as "cache busters", because the data needed never
seems to be in the cache, it is the memory subsystem performance
that determine overall program execution speed. The more
"queries per second" the memory could support, the faster the
"cache buster" program can run. An example of such a program,
is simulator programs, such as logic simulation for the building
of silicon chips.

On some current computers, these numbers don't make a big difference.
If you're mainly an "interactive" computer user, where no program
runs for more than a few seconds, you can fit the crappiest RAM
you want in the computer.

If you're shrinking DVDs for a living, you probably measure the
execution time with a stopwatch. If the 6-6-6-18 RAM took 2 hours 40 minutes,
and the 4-4-4-12 took 2 hours 30 minutes, you'd likely pay the
extra money. For some people, every little bit like that is
important.

In many cases, the only time you can see the difference,
is running benchmark programs, to brag to your friends.

HTH,
Paul
 
In addition to Paul's answer, your BIOS must have the
ability to recognized the proper SPD values for either
of the two memory sticks you mentioned.

It would be my guess that Dell most likely has fixed
BIOS settings that are conservatively rated so as
to be more tolerant of small variation in SPD values
vs. actual memory peak performance values. So I
don't think you will see any difference between your
two choices (if they work and meet the Dell specs).

That said you should go to Crucial's memory advisor's
or use their scan utility on their web site to see what the
best and more likely most compatible memory sticks are for your PC.

http://www.crucial.com/
 
JS said:
In addition to Paul's answer, your BIOS must have the
ability to recognized the proper SPD values for either
of the two memory sticks you mentioned.

It would be my guess that Dell most likely has fixed
BIOS settings that are conservatively rated so as
to be more tolerant of small variation in SPD values
vs. actual memory peak performance values. So I
don't think you will see any difference between your
two choices (if they work and meet the Dell specs).

That said you should go to Crucial's memory advisor's
or use their scan utility on their web site to see what the
best and more likely most compatible memory sticks are for your PC.

http://www.crucial.com/
Click to expand...

Hi, thanks, yes its listed as a part under Crucials selector, but that
doesnt necesarily mean too much as they list 667MHz as the default part
when the system in question (Optiplex 745) takes up to 800MHz.

I did ask them but their 'live help' didnt seem too sure, but plumped
for 'yes you will see a difference' in the end.
 
Paul said:
Memory has two performance aspects.

Memory bandwidth is measured by "PC2-6400". The maximum rate that
information can come from the memory, is 6400MB/sec. This is useful
when large blocks of memory are being pulled into the processor.
Usually, a "burst" of data is designed to fill a "cache line" inside
the processor, so that is a normal sized transaction for the processor.

The second set of numbers concern latency. That is important for
random access. Not all operations on the computer access large
contiguous blocks of data. Sometimes, the processor effectively picks
just one byte here and one byte there. The CAS Latency defines the delay
from the request, until the data is provided. The lower the number
(your 4-4-4-12 product), the quicker the answer comes back. If you
consider the time duration of each one of those operations, and then
take the inverse of that, you can see how many "queries per second"
the lower latency memory can make. So, for programs which are
referred to as "cache busters", because the data needed never
seems to be in the cache, it is the memory subsystem performance
that determine overall program execution speed. The more
"queries per second" the memory could support, the faster the
"cache buster" program can run. An example of such a program,
is simulator programs, such as logic simulation for the building
of silicon chips.

On some current computers, these numbers don't make a big difference.
If you're mainly an "interactive" computer user, where no program
runs for more than a few seconds, you can fit the crappiest RAM
you want in the computer.

If you're shrinking DVDs for a living, you probably measure the
execution time with a stopwatch. If the 6-6-6-18 RAM took 2 hours 40
minutes,
and the 4-4-4-12 took 2 hours 30 minutes, you'd likely pay the
extra money. For some people, every little bit like that is
important.

In many cases, the only time you can see the difference,
is running benchmark programs, to brag to your friends.

HTH,
Paul
Click to expand...

Thanks, for that.

Looking about I have yet to find a simple quantitative calculation which
gives the performance advantage of one set of timings against another in
memory throughput terms.

If you look at the cumulative clocks they add up to the same (24) in
both cases - I appreciate the difference in the 4 cycle phases but its
not apparently a simple thing to state the actual throughput difference,
assuming that the parts will run at the designated cycle timings.
 
Otto said:
Thanks, for that.

Looking about I have yet to find a simple quantitative calculation which
gives the performance advantage of one set of timings against another in
memory throughput terms.

If you look at the cumulative clocks they add up to the same (24) in
both cases - I appreciate the difference in the 4 cycle phases but its
not apparently a simple thing to state the actual throughput difference,
assuming that the parts will run at the designated cycle timings.
Click to expand...

I haven't seen a decent timing diagram in some time, for computer
memory. If such a thing were available, it may have been possible
to look at it at the cycle level. (There are portions of timing
diagrams, in memory chip datasheets, but there aren't the useful kind
that show several transactions back to back. Like say a read-write-read
or a write-read-write. Something where you see precharge and the like,
or where a page is already open. I used to do stuff like that in
digital simulators, for documentation purposes.)

As an end user, about all you can do, is look for benchmark articles,
where they compare the effects of running at various speeds and with
various timings. Even those kinds of articles are hard to find.

http://www.xbitlabs.com/articles/memory/display/ddr3_6.html

Paul
 
PC 6400 = 800MHz FSB

If you CPU has an 800MHz FSB
then you definitely need PC 6400 memory.

Even if your FSB is slower than 800MHz
(say 667MHz) installing PC 6400 ram will
not hurt, but it will not speed things up.
 
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