# Best way to mix DIMMs of different CAS latencies?



## Lothar

As I posted in another thread here, I have a new system that has at least one bad RAM DIMM, and the vendor has sent me several replacement sticks for troubleshooting purposes.  No matter how I swap and reposition the DIMMs, the bad memory error messages that I'm getting are failing to help me narrow it down to just one DIMM (it's not worth going into the details here, but suffice it to say that tech support is as stumped as I am), and it's possible that I have *two* bad DIMMs.  At the very least, I have narrowed it down to two of the original DIMMs that are definitely working fine, and am debating whether it's worth the effort to troubleshoot it any further, given that I have free replacements that I can use.

The problem is that the original DIMMs have a CAS latency of 555, and the replacements are a slower 666.  These are 1GB DDR2 ECC SDRAM modules (x4 for a total of 4GB).

My question is, what is the best way to mix and match these DIMMS in my 4 memory slots?  Here are my options:

1) Continue the troubleshooting and try to narrow down the bad memory to a single DIMM, and then use three 555 DIMMs plus one 666 DIMM.

2) Don't bother with any further troubleshooting, assume that I have two bad DIMMs, and use two 555 DIMMs plus two 666 DIMMs

3) Replace all of the DIMMs with the slower 666 DIMMs so that they are all the same spec.

Which of the above three options will give me the best performance (if there would even be any noticeable difference), and in what order should I arrange the DIMMs in the four slots for either options 1 or 2?  

According to my system's manual, DIMMs should be installed first into DIMMs 1 and 2, then any additional DIMMs into 3 and 4, so that is how the slots are paired.

Lothar


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## Cromewell

The slowest setting will be used for all 4 so it doesn't matter if you replace 1, 2, 3 or 4. Obviously the best performance would be to get your vendor to replace them with the same 5-5-5s that you have now but 6-6-6 isn't that much slower. Going to 4-4-4 would make some difference (if they have 4-4-4 ECC), it might be noticible outside of benchmarks whereas 5-5-5 to 6-6-6 will really only show up in benchmarking.

From a performance standpoint, there's no difference if you have mixed 6-6-6 with 5-5-5 or have all 6-6-6.


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## Lothar

Cromewell said:


> The slowest setting will be used for all 4 so it doesn't matter if you replace 1, 2, 3 or 4. Obviously the best performance would be to get your vendor to replace them with the same 5-5-5s that you have now but 6-6-6 isn't that much slower. Going to 4-4-4 would make some difference (if they have 4-4-4 ECC), it might be noticeable outside of benchmarks whereas 5-5-5 to 6-6-6 will really only show up in benchmarking.
> 
> From a performance standpoint, there's no difference if you have mixed 6-6-6 with 5-5-5 or have all 6-6-6.



Okay, Cromewell, that all makes sense.  I guess I'll just toss a coin to decide which ones to send back to the vendor.  I appreciate your reply!

Lothar


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## Lothar

Here's something else I just discovered... In the *product brochure* for the Samsung DDR2 SSDRAM FBDIMMs, it lists the following under "Features":

- Posted CAS
- Programmable CAS Latency: 3, 4, 5, 6

I don't see any other discussion anywhere in the technical documents about how this "programming" is accomplished (i.e., is it user-programmable, or is it shipped set locked at a particular latency from the factory, or does it set itself according to what it detects in my system?).

If it is programmable, then I also don't understand why anybody would want to set it at a slower latency than its fastest setting.  Maybe I'm just not understanding what they mean by "programmable" here.


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## Cromewell

You set a slower speed because not all ICs are able to run at faster latencies. Basically, the manufacturer takes a batch of RAM chips and tests for what speed/latency they are stable at and then makes a stick of RAM and sells it.

The manufacturer will program the 'recommended' latencies for certain frequencies into a small EEPROM on the stick of RAM called an SPD module. You can override the settings on the SPD in the BIOS but then you have no gurantee that the RAM will be stable. Sometimes adding voltage will help if you are overclocking the latencies (or if you overlock by increasing the frequency) but only to a certain point (it's a little different for each stick of RAM).


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## Lothar

Cromewell said:


> You set a slower speed because not all ICs are able to run at faster latencies. Basically, the manufacturer takes a batch of RAM chips and tests for what speed/latency they are stable at and then makes a stick of RAM and sells it.
> 
> The manufacturer will program the 'recommended' latencies for certain frequencies into a small EEPROM on the stick of RAM called an SPD module. You can override the settings on the SPD in the BIOS but then you have no gurantee that the RAM will be stable. Sometimes adding voltage will help if you are overclocking the latencies (or if you overlock by increasing the frequency) but only to a certain point (it's a little different for each stick of RAM).



Thanks for that supplemental info, Cromewell.  I don't see any settings in my BIOS to override the latency settings, but from what you just said, it sounds like I don't want to mess with that anyway.  I guess I'll just have to be content with running at the slower CAS6 latency.


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## Cromewell

Here's a little more on memory timings. I know the article is on the older side but the results with higher clocked RAM will be no better. http://www.techwarelabs.com/reviews/memory/memory_timings/index_3.shtml


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## Lothar

Cromewell said:


> Here's a little more on memory timings. I know the article is on the older side but the results with higher clocked RAM will be no better. http://www.techwarelabs.com/reviews/memory/memory_timings/index_3.shtml



Thanks Cromewell, that was most enlightening!  While that article doesn't show the performance decrease changing from CAS5 to CAS6 DIMMs, it looks like it would be something on the order of 0.002%.  Negligible indeed! (though it could add up with memory-intensive operations I suppose).

I was just informed by Dell a few minutes ago that they are sending me a new set of 4 DIMMs (in addition to the 3 replacements that they've already sent me), so that they will "all be the same," though they said there's no guarantee as to what the latency would be.  So I guess I have my pick of which ones I want to install out of a total of 11 sticks (for 4 slots), and will send all the rest back.  

I must say that they are being very accommodating to me, though I wish they would just send me DIMMs that match the original latency.  I hate having to downgrade what I paid for.


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