【10.5疑問一籮筐】64位元與32位元

MEMORY BOTTLENECK?
Our friend, Lloyd Chambers, thinks there's a memory bus bottleneck for the 8-core, which he discussed back in March on his blog. He wrote, "Memory bandwidth is inadequate for 8-cores. It’s already a limiting factor with the current quad-core 3.0 GHz Mac Pro. Memory copy speed is at best 2.9GB/sec on the Mac Pro, in spite of Apple’s highly misleading claims of 21.3 GB/sec (“maximum processor bandwidth of up to 21.3 GB/s”—bandwidth is a bit more than double the memory copy speed). That’s a measly 700MB/sec per core on a quad-core machine, and only 350MB/sec per core on an octa-core machine. By comparison, a 6-drive hard disk RAID array can easily perform at over 400MB/sec!"

TIGER IS "CHASING ITS TAIL"
"Yes, the memory bus is weak, but the biggest problem is that OS 10.4 (Tiger) does not know how to deal with so many cores. Perhaps 10.5 (Leopard) will. the problem is that the OS does not know that it should keep a thread on the SAME core instead of swapping it around to any one of the 8-cores. When a thread moves to a different core-group, the cache has to be reloaded on the new core. Since Intel quad-cores are two Core 2 Duos "duct-taped" together, the problem is worse than it would otherwise have been. When this thread/core shift happens repeatedly, it makes the cache ineffective and floods the memory bus with activity." (Though put forth by an anonymous BF reader, this analysis has been confirmed by Lloyd Chambers where he discussed "core swapping" on his latest blog entries regarding the 8-core Mac Pro.)