RAM speeds and latency

Beas1987

Beas1987

Beas1987

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Joined
9 Apr 2010
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9
I have been reading guides on RAM and the affects of different speeds and levels of latency and wondered what peoples thoughts were on choices of RAM for given tasks. For example is low latency more important than clock cycles for gaming or encoding etc.

As far as I am aware, DDR3 RAM tends to have higher latency than DDR2 but obviously supports the faster clock speeds with 1066Mhz, 1333Mhz and even 1600Mhz but how much difference do these choices really make to performance? I have heard as much as 2-5% but what do these numbers mean in practice? If one was forced to choose between a system supplied with 1066Mhz DDR3 or 1333Mhz DDR3 is there really any odds at all to save a couple of pounds?

These questions are raised in my mind as I am in the process of making my mind up over upgrades and being on a tight budget means I need to decide what I need and forsake the bells and whistles.

I apologise if i'm flogging an old horse here as I seem to have made it my mission lately to pester the world about technical questions before parting with my cash.
 
It’s slightly foolish (i'm not saying your a fool btw) to believe that DDR3 is just a speed bump variant of DDR2 providing higher speeds at the cost of lower latencies. Although partially true, it’s not the full picture and DDR3 is more than just a simple upgrade on its predecessor. New technology and fabrication of the modules have provided the ability to extend the modules beyond DDR2, which mechanically is at its peak.

As for latenency over frequency, the higher frequencies allow for higher system processor clock and system bus speeds thus increasing overall system performance.

The higher frequency of DDR3 more than compensates for the higher latency over DDR2. Therefore, the issue whether latencies are indeed important anymore remain debatable since the I/O bus clock are much sorter and the time taken is similar.

To answer your question, i would go for the highest frequency that's available to suit your pocket. Hope that answer your question.
:)
 
I am more than a fool, I don't know nearly as much about hardware these days as I used to. Things are nowhere near as cut and dry as I remember with so much choice it's hard to tell what you need and what is worth the extra cash. Thanks a lot for your input though, definitely answered my question :).
 
When comparing modules with different clock rates you need to do some math to be able to compare the latencies.

Pay attention that we are talking about “clock cycles”. When the clock is higher, each clock cycle is shorter (i.e. lower period). For example, on a DDR2-800 memory, each clock cycle takes 2.5 ns (1 ns = 0.000,000,001 second) – the math is simple, clock period that is "wait time" = 1 / frequency (note that you need to use the real clock, not the DDR clock on this formula; (see table below)

So supposing a DDR2-800 memory with CL 5, this initial wait time corresponds to 12.5 ns (CL 5 x 2.5 ns).
Now suppose a DDR3-1333 memory with CL 7. With this memory each clock cycle will have a period "wait time" of 1.5 ns (see table below). So the total wait time (latency) will be of 10.5 ns (CL 7 x 1.5 ns).

So even though the latency of this DDR3 memory appears to be higher (7 vs. 5), the wait time is actually lower. So don’t go around thinking that DDR3 memories have worse latencies than DDR2 memories: it will depend on the clock rate you are talking about.

That also means DDR3-2000 memory with CL 10 has a better latency than DDR3-1333 with CL 7........(CL10 x 1ns) = latency of 10 ns, compared to latency of 10.5 ns of DDR3-1333 CL 7 (CL 7 x 1.5 ns)



DDR Clock........ Real Clock........ Clock Period
200 MHz.......... 100 MHz.............. 10 ns
266 MHz.......... 133 MHz.............. 7.5 ns
333 MHz.......... 166 MHz.............. 6 ns
400 MHz.......... 200 MHz.............. 5 ns
533 MHz.......... 266 MHz.............. 3.75 ns
666 MHz.......... 333 MHz.............. 3 ns
800 MHz.......... 400 MHz.............. 2.5 ns
1066 MHz........ 533 MHz...............1.875 ns
1333 MHz........ 666 MHz...............1.5 ns
1600 MHz........ 800 MHz...............1.25 ns
2000 MHz.........1000 MHz............. 1 ns
 
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flying grouse said:
When comparing modules with different clock rates you need to do some math to be able to compare the latencies.

Pay attention that we are talking about “clock cycles”. When the clock is higher, each clock cycle is shorter (i.e. lower period). For example, on a DDR2-800 memory, each clock cycle takes 2.5 ns (1 ns = 0.000,000,001 second) – the math is simple, clock period that is "wait time" = 1 / frequency (note that you need to use the real clock, not the DDR clock on this formula; (see table below)

So supposing a DDR2-800 memory with CL 5, this initial wait time corresponds to 12.5 ns (CL 5 x 2.5 ns).
Now suppose a DDR3-1333 memory with CL 7. With this memory each clock cycle will have a period "wait time" of 1.5 ns (see table below). So the total wait time (latency) will be of 10.5 ns (CL 7 x 1.5 ns).

So even though the latency of this DDR3 memory appears to be higher (7 vs. 5), the wait time is actually lower. So don’t go around thinking that DDR3 memories have worse latencies than DDR2 memories: it will depend on the clock rate you are talking about.

That also means DDR3-2000 memory with CL 10 has a better latency than DDR3-1333 with CL 7........(CL10 x 1ns) = latency of 10 ns, compared to latency of 10.5 ns of DDR3-1333 CL 7 (CL 7 x 1.5 ns)



DDR Clock........ Real Clock........ Clock Period
200 MHz.......... 100 MHz.............. 10 ns
266 MHz.......... 133 MHz.............. 7.5 ns
333 MHz.......... 166 MHz.............. 6 ns
400 MHz.......... 200 MHz.............. 5 ns
533 MHz.......... 266 MHz.............. 3.75 ns
666 MHz.......... 333 MHz.............. 3 ns
800 MHz.......... 400 MHz.............. 2.5 ns
1066 MHz........ 533 MHz...............1.875 ns
1333 MHz........ 666 MHz...............1.5 ns
1600 MHz........ 800 MHz...............1.25 ns
2000 MHz.........1000 MHz............. 1 ns
Click to expand...

Cheers Mate, Thats also helped me while i'm looking around for some ram info for a build i am doing. Thinking i am gonna go with something like OCZ Gold 4GB (2x2GB) DDR3 PC3-16000C10 2000MHz if thats a good choice for the Asus Crosshair IV Formula and AMD Phenom II X6 mix match ?
 
I was having trouble with ram on my motherboard, and it was suggested that I lower its speed from 1600 to 1333, as this would have a minimal effect on the system speed (less than 5% was quoted).

so if you can get ddr3 1333 a lot cheaper than 1600, you may want to weigh up whether that small gain is worth it.
 
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flying grouse said:
you need to do some math
Click to expand...
Nice post flying grouse! :)

Beas1987 said:
wondered what peoples thoughts were on choices of RAM for given tasks. For example is low latency more important than clock cycles for gaming or encoding etc
Click to expand...
There is only two factors I pay attention to with memory
  1. Latency
  2. Bandwidth
Out of the two factors above I think I choose Latency for the win! . . . this is because Latency effects everything you do on your PC at all times (a responsive "Snappy" feel) whereas Bandwidth only works or is noticable during benchmarks or for Real-World software that blitzes the BUS with Data (Video-Encoding springs to mind as one such task!) . . .

Here's a quick ready reckoner for any Dual-Channel users that are weighing up Bandwidth vs Latency . . . if I was using DDR3 I'm pretty sure DDR3-1600 Cas 6 is what I would be aiming for! :cool:


  • DDR3-1066 CAS 5 9.4 ns (128-bit) 8,528 MB/s


  • DDR3-1333 CAS 9 13.5 ns (128-bit) 10,664 MB/s
  • DDR3-1333 CAS 8 12.0 ns (128-bit) 10,664 MB/s
  • DDR3-1333 CAS 7 10.5 ns (128-bit) 10,664 MB/s
  • DDR3-1333 CAS 6 9.00 ns (128-bit) 10,664 MB/s


  • DDR3-1600 CAS 9 11.3 ns (128-bit) 12,800 MB/s
  • DDR3-1600 CAS 8 10.0 ns (128-bit) 12,800 MB/s
  • DDR3-1600 CAS 7 8.80 ns (128-bit) 12,800 MB/s
  • DDR3-1600 CAS 6 7.50 ns (128-bit) 12,800 MB/s


  • DDR3-2000 CAS 9 9.0 ns (128-bit) 16,000 MB/s
  • DDR3-2000 CAS 8 8.0 ns (128-bit) 16,000 MB/s
  • DDR3-2000 CAS 7 7.0 ns (128-bit) 16,000 MB/s
 
Brill so the ram i was first looking at..

CMG4GX3M2A1600C6 - DOMINATOR GT, 4GB would be a good choice then thinking about it ? with the standard latency of 6-6-6-18
 
Admiral Huddy said:
You're right, sometimes too much choice isn't a good thing :)
Click to expand...

So true my head is really burned out from trying to choose RAM, I just dont understand it

Yellowbeard said:
My opinion is that if you intend to OC, get the 1600 or faster for really high OCs. If you do not intend to OC, get 1333.
Click to expand...

Does that apply to this

http://forums.overclockers.co.uk/showthread.php?t=18146655&highlight=username_Relentless81

If anyone could help me I would really appreciate it
 
Yellowbeard said:
My opinion is that if you intend to OC, get the 1600 or faster for really high OCs. If you do not intend to OC, get 1333.
Click to expand...
Hi Yellowbeard,

is your opinion based soley on INTEL® systems? . . . I ask because the end user has the choice of running DDR3-1066, DDR3-1333 and DDR3-1600 on a bone stock AMD® system? . . . if the prices were similar shouldn't one take the faster RAM?

am3stockmemory.gif
 
Big.Wayne said:
Hi Yellowbeard,

is your opinion based soley on INTEL® systems? . . . I ask because the end user has the choice of running DDR3-1066, DDR3-1333 and DDR3-1600 on a bone stock AMD® system? . . . if the prices were similar shouldn't one take the faster RAM?
Click to expand...

If it was my personal machine, I would buy 1600 or faster regardless of the cost difference. However, I was addressing this person's specific situation in this post.

Beas1987 said:
These questions are raised in my mind as I am in the process of making my mind up over upgrades and being on a tight budget means I need to decide what I need and forsake the bells and whistles.

I apologise if i'm flogging an old horse here as I seem to have made it my mission lately to pester the world about technical questions before parting with my cash.
Click to expand...
 
Oh I see thanks! . . . locked on now! :D

Beas1987 said:
If one was forced to choose between a system supplied with 1066Mhz DDR3 or 1333Mhz DDR3 is there really any odds at all to save a couple of pounds?
Click to expand...
Totally depends where your coming from and what you do with your life . . . the difference between DDR3-1333 and DDR3-1600 could mean nothing, zip, zilch *or* it could mean possible world or at least forum domination and a good few extra inches of e-peen gained from stOnking benchmarks! :p

based on you saying your intending to buy a pre-built machine I guess it makes no odds at all in the slightest . . . . I'd say all you need to maybe do is specify a good brand of ram such as Corsair! ;)

Beas1987 said:
being on a tight budget means I need to decide what I need and forsake the bells and whistles
Click to expand...
So I'm thinking that you should be a) using DDR2 memory and b) building the system yourself out of a combo of new and used parts!

What's your budget and what tasks/software do you need to run? :cool:
 
If buying AMD Quad or lower then low latency 1333 will likely give you a sweet spot on price/performance.

If you have hex then you will want a higher speed (going towards 1600).

Of course there is also the issue of needing a higher CPU-NB clock to really use that higher bandwidth.
 
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