- Mar 10, 2006
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People like to blame X86 for why Atom sucks compared to newer, more modern micro-architectures, but the plain and simple truth is: the underlying design is a kind of a mess and likely due to the fact that it was limited by the constraints of the 45nm process on which it was designed. I dug up this really helpful CPU optimization guide (http://www.agner.org/optimize/microarchitecture.pdf renowned programmer/optimizer Agner Fog, in which he does a really nice job summarizing why Atom is such a POS:
- In-order execution is a bottleneck
- The instruction fetch rate is really low (and you thought Bulldozer's shared fetch was a bad idea!)
- Memory access is limited to one read or one write per clock (can't do both)
- Maximum throughput of 2 instructions/cycle is only realistic if the code is specially optimized for Atom
- Memory latency is HUGE for floating point instructions compared to integer once since the memory unit is only connected to the integer cluster (ever wonder why Atom gets thrashed in the FPU part of Geekbench?)
The good news is that "Silvermont", with a much bigger transistor budget (thanks, 22nm!), probably fixes a lot of these bottlenecks that were introduced courtesy of the 45nm node. But the bad news is that today's Atom still sucks
- In-order execution is a bottleneck
- The instruction fetch rate is really low (and you thought Bulldozer's shared fetch was a bad idea!)
- Memory access is limited to one read or one write per clock (can't do both)
- Maximum throughput of 2 instructions/cycle is only realistic if the code is specially optimized for Atom
- Memory latency is HUGE for floating point instructions compared to integer once since the memory unit is only connected to the integer cluster (ever wonder why Atom gets thrashed in the FPU part of Geekbench?)
The good news is that "Silvermont", with a much bigger transistor budget (thanks, 22nm!), probably fixes a lot of these bottlenecks that were introduced courtesy of the 45nm node. But the bad news is that today's Atom still sucks