Nehalem Memory Speed Information

The memory controller in Intel's new Nehalem processor is a radical departure from previous Intel processor architectures, in that the processor die now contains a memory controller. As a result of this change, much of the system builder community is confused as to the proper memory speed to use for Nehalem processors.

This paper provides information to system builders so that the proper memory can be selected.

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This information is extracted from Intel documentation. It is correct, to the best of our knowledge, as of the time it was written.
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Performance Benchmarks

This first generation of Nehalem processors has three channels per socket. When the number of DIMMs installed on all channels is the same, the peak memory bandwidth is achieved. If there are different numbers of DIMMs installed on the various channels, the memory bandwidth will be less.

The SiSoft Sandra benchmark is used to determine the memory bandwidth. The Cinebench benchmark is used to indicate the impact of the changes in memory bandwidth on a memory-intensive application. The benchmarks were performed under 64-bit Windows Vista SP1 with no other patches. The appropriate drivers for the motherboard were installed.

Do not confuse memory bandwidth with real-world application performance.

Once all the CPUs in a system are running with at least two channels, the difference in memory bandwidth usually has no impact on actual application performance. Vastly more important is having enough memory to eliminate paging, and to provide enough memory to cache file I/O.

Uni-Processor Benchmarks

When dealing with a Uni-Processor system, the bandwidth capabilities are rather limited. The processor clocks for the two tested processors are the same. The differences between them are the UnCore clock and the maximum memory clock. With more than three memory modules, both processors will run the memory at 533MHz. The types memory used in these benchmarks are:

 
Memory Bandwidth
W3520 X5550
Unbuffered 533MHz (1)
Single-Rank
Unbuffered 533MHz
Dual-Rank
Unbuffered 533MHz
Dual-Rank
Unbuffered 666MHz
Single-Rank
Unbuffered 666MHz
Dual-Rank
Memory Modules
Sandra
Cinebench
Sandra
Cinebench
Sandra
Cinebench
Sandra
Cinebench
Sandra
Cinebench
1
6.84
GB/S

CB-CPU
7.23
GB/S

CB-CPU

GB/S

CB-CPU

GB/S

CB-CPU

GB/S

CB-CPU
2
12.85
GB/S

CB-CPU
13.76
GB/S

CB-CPU

GB/S

CB-CPU

GB/S

CB-CPU

GB/S

CB-CPU
3
16.51
GB/S

CB-CPU
18.42
GB/S

CB-CPU

GB/S

CB-CPU

GB/S

CB-CPU

GB/S

CB-CPU
4
13.35
GB/S

CB-CPU
13.72
GB/S

CB-CPU

GB/S

CB-CPU

GB/S

CB-CPU

GB/S

CB-CPU
5
15.50
GB/S

CB-CPU
16.00
GB/S

CB-CPU

GB/S

CB-CPU

GB/S

CB-CPU

GB/S

CB-CPU
6
18.17
GB/S

CB-CPU
18.50
GB/S

CB-CPU

GB/S

CB-CPU

GB/S

CB-CPU

GB/S

CB-CPU

Notes:

(1) This memory is underclocked 666MHz DDR3 CAS-9 Unbuffered ECC Single-Rank, which runs at CAS-7.

Dual-Processor Benchmarks

With the exception of the 8-DIMM dual-channel test, the DIMMs were installed as evenly as possible between the two processors, and among the channels of each processor. The first set of benchmarks uses the E5520 processor. The types memory used in these benchmarks are:

Unbuffered 533MHz (1)
Single-Rank
Unbuffered 533MHz
Dual-Rank
Memory Modules
Sandra
Cinebench
Sandra
Cinebench
1
6.42 GB/S
22532 CB-CPU
7.00 GB/S
21376 CB-CPU
2
14.32 GB/S
21794 CB-CPU
15.00 GB/S
21844 CB-CPU
3
14.58 GB/S
22089 CB-CPU
15.32 GB/S
21560 CB-CPU
4
23.27 GB/S
21936 CB-CPU
26.15 GB/S
21365 CB-CPU
5
24.76 GB/S
21894 CB-CPU
26.14 GB/S
22377 CB-CPU
6
29.27 GB/S
21326 CB-CPU
31.19 GB/S
20751 CB-CPU
7
24.90 GB/S
20991 CB-CPU
25.44 GB/S
21307 CB-CPU
8
25.00 GB/S
22535 CB-CPU
25.15 GB/S
20935 CB-CPU
9
23.51 GB/S
21806 CB-CPU
25.40 GB/S
21687 CB-CPU
10
22.56 GB/S
20993 CB-CPU
28.85 GB/S
21735 CB-CPU
11
22.63 GB/S
22233 CB-CPU
28.86 GB/S
20996 CB-CPU
12
24.20 GB/S
21704 CB-CPU
31.20 GB/S
21185 CB-CPU
8 (dual-channel)
20.19 GB/S
22036 CB-CPU
25.40 GB/S
21077 GB-CPU

Notes:

(1) This memory is underclocked 666MHz DDR3 CAS-9 Unbuffered ECC Single-Rank, which runs at CAS-7.

This set of benchmarks uses the X5550 processor. The types memory used in these benchmarks are:

Unbuffered 533MHz
Dual-Rank
Unbuffered 666MHz
Single-Rank
Unbuffered 666MHz
Dual-Rank
Registered 666MHz
Dual-Rank
Memory Modules
Sandra
Cinebench
Sandra
Cinebench
Sandra
Cinebench
Sandra
Cinebench
1
6.72
GB/S
26300
CB-CPU
7.00
GB/S
26677
CB-CPU
8.56
GB/S
26430
CB-CPU
8.42
GB/S
26976
CB-CPU
2
14.52
GB/S
26606
CB-CPU
12.00
GB/S
25083
CB-CPU
18.73
GB/S
26383
CB-CPU
18.46
GB/S
26373
CB-CPU
3
16.00
GB/S
26009
CB-CPU
13.27
GB/S
25258
CB-CPU
18.88
GB/S
27060
CB-CPU
18.77
GB/S
26945
CB-CPU
4
26.68
GB/S
25081
CB-CPU

GB/S

CB-CPU
32.12
GB/S
27068
CB-CPU
31.26
GB/S
27269
CB-CPU
5
27.25
GB/S
26121
CB-CPU

GB/S

CB-CPU
32.32
GB/S
26429
CB-CPU
32.20
GB/S
25848
CB-CPU
6
34.00
GB/S
26909
CB-CPU

GB/S

CB-CPU
38.38
GB/S
26695
CB-CPU
36.37
GB/S
27382
CB-CPU
7
26.28
GB/S
24671
CB-CPU

GB/S

CB-CPU
26.27
GB/S
26915
CB-CPU
26.21
GB/S
26119
CB-CPU
8
28.30
GB/S
25884
CB-CPU

GB/S

CB-CPU
26.40
GB/S
26182
CB-CPU
26.63
GB/S
26199
CB-CPU
9
26.84
GB/S
25892
CB-CPU

GB/S

CB-CPU
26.60
GB/S
26758
CB-CPU
26.22
GB/S
26105
CB-CPU
10
30.38
GB/S
25187
CB-CPU

GB/S

CB-CPU
30.66
GB/S
26497
CB-CPU
31.00
GB/S
26502
CB-CPU
11
32.22
GB/S
26794
CB-CPU

GB/S

CB-CPU
31.86
GB/S
25993
CB-CPU
32.40
GB/S
25493
CB-CPU
12
34.14
GB/S
26755
CB-CPU

GB/S

CB-CPU
35.58
GB/S
26582
CB-CPU
34.65
GB/S
26508
CB-CPU
8 (dual-channel)
25.63
GB/S
26425
CB-CPU

GB/S

CB-CPU
26.14
GB/S
27365
CB-CPU
26.23
GB/S
25461
CB-CPU

It can be seen that there's a significant difference in the memory bandwidth between 533MHz dual-rank memory and 666MHz dual-rank memory. It can also be seen that there's a similar difference in memory bandwidth between single-rank and dual-rank memory modules.

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