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LOSTCIRCUITS
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| Intel Pentium4 840 Extreme Edition and 840D .... the name of the rose ... | |
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(Review by MS June 20, 2005) |
| AMD Athlon 64 3800+ (Venice) |
Memory Subsystem Benchmarks
As we mentioned in the beginning of this article, the dual core CPUs are somewhat handicapped with respect to memory bandwidth. In the case of the Athlon64 X2- each core has access to a single channel of memory that are overall still combining to generate something similar to a dual channel scenario. In the case of the P4, the entire CPU runs over a shared PSB to the memory controller, which features the dual channel or super-wide channel interface. The oddball in this roundup is the ASUS K8N-DL with the two Opteron 252 that can take advantage of ccNUMA to nearly double the bandwidth.
HyperThreading or not does not matter for the P4 840. Basically, the '840 is on par with the 600 series running on an asynchronous memory bus with the extra latencies incurred from internal buffering and synchronization of the different bus speeds. Compared to the in-house competition in form of the 3.73 GHz Extreme Edition that effectively eliminates the bottleneck of the PSB by increasing the host bus frequency to 1066 MHz, the '840 looks especially undernourished. The AMD processors maintain solid midrange placements but nothing gets even close to the ccNUMA enabled dual Opteron system
Memory Latencies
We used Cachemem 2.65
Memory access latencies for the 840 (transparent) and the 660 (solid columns). Both configurations are running an 800 MHz PSB and 1066 MHz (3:3:3:10) memory bus. The P4 840, despite having a nominal 2 MB L2 cache, cannot fit data blocks larger than 1 MB into the L2, since the latter consists of two separate, non-combinable units, that is 1 MB/core. In other words, the two caches can hold 2 pieces of 1 MB of data each but they cannot combine their space to hold larger blocks of data. The graph shows the cutoff for data being moved into main memory at > 1 MB (840; five large columns) and > 2 MB (660, four large columns).
Because of the reduced memory bandwidth and the smaller block size of coherent data that can be stored within each of the Level2 cache blocks, it is conceivable that the '840 may suffer some performance penalty. On the other hand, in terms of raw chipset latencies for decoding the virtual addresses generated by the CPU's memory management unit into address and command signals, the '955 appears to be slightly faster than the 925XE. Keep in mind that both configurations are running the memory in asynchronous mode, that is, the host bus is running at 800 MHz whereas the memory beats at 1066 MHz.
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Pentium 4 840D (dual core) |
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