Saturday, April 18, 2015

Lower CAS latency on a legacy Computer: Is it worth it?

Some Story background
With time going on, i got enough DDR2 modules to play a little with them.
An old question was how CAS latency is important and impacts performace on a legacy computer (with a CPU that doesn't feature any L3 cache)

Clock and CAS Latency
Modules used:

Name PC2- Frequency (MHz) CAS latency Size (MB) per module
Nanya NT2GT64U8HD0BY-AD 6400 800 6-6-6-18 2048
Corsair Value Select VS2GB800D2 6400 800 5-5-5-18 2048
Gskill Extrem2 PC6400PK 6400 800 4-4-4-12 1024
Gskill Extrem2 PC6400HZ 6400 800 4-4-4-12 512
Hynix HYMP564U64BP8-C4 AB-T 4200 533 4-4-4-12 512
Micron MT8HTF6464AY-53EB3 4200 533 4-4-4-12 512
only the green modules will be compared in this article, we'll play with the others later on.

Testing Conditions
Hardware & Software

CPU : AMD Athlon X2 64 5600+ (@2.9GHz)
MotherBoard:  Gigabyte GA-M61PME-S2P
RAM : Tested Criteria
GPU : Geforce 8400GS
HDD : Seagate Barracuda 7200.8 250 GB (7200rpm)
CD/DVD :  writer

The Software used is nothing complicated: Memtest86 v4.20

Single and Dual Channel CAS battle (DDR800)
Both Modules are 2GB DDR800 (PC2-6400), the only discriminating criteria is the Cas Latency : CL5 (5-5-5-18) vs CL6 (6-6-6-18)
Signle Channel (1*2GB) and Dual-Channel (2*2GB) results are the following:

  • SC-5-5-5-18: 2369MB/s
  • SC-6-6-6-18: 2252MB/s
  • DC-5-5-5-18: 2861MB/s
  • DC-6-6-6-18: 2643MB/s

Upgrading from to Single Channel 5-5-5-18 Dual Channel 5-5-5-18
Single Channel 6-6-6-18 +5 %

Dual Channel 6-6-6-18

+8 %

There is definitely a performance increase in pure RAM bandwidth, though this has barely any impact on real life work on a legacy PC.
NB: i also tried to mix the Corsair (5-5-5-18) and Nanya (6-6-6-18) modules, the motherboard set them to dual channel even though the chips are quite differents... and gave the following result:
It down-clocked the RAM to 667MHz (351MHz), and reduced latency to 5-5-5-15; for a 2410MB/s bandwidth.

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