Which would be faster: A triple core processor with L1 cache or a quad core processor without L1 cache?

Question

Which would be faster?

By this I mean, a triple core processor that has L1-L3 cache, or a quad core processor which only has L2-L3 cache.

Both operate at the same clock rates.

Just wondering, since I don't know a ton about computers. Thanks!

Answer 1

A CPU with literally no L1 cache would be a hideously crippled beast. Unlike the L2 and L3 cache (if present), the L1 cache is a vital part of the instruction decoder. A significant fraction of code consists of tight loops, and the CPU would be terribly hobbled trying to execute such loops with no L1 cache.



That's not even looking at the consequences for data accesses. L1 cache latency is about three clock cycles on modern CPUs. L2 cache latency is 12-15 clock cycles. So every read memory access that would be an L1 cache hit would take about four times as long.



The posted write buffers would almost always be full because the L2 can't drain them fast enough. So almost all memory writes would also incur an L2 cache latency penalty as the CPU has to wait for a slot in its posting buffer to open up.



You'd also have another huge problem trying to implement a quad-core without L1 cache. The L2 cache would bottleneck because it would have to implement both all the CPU memory traffic and the cache coherency traffic. So effectively, each core's memory accesses would step on the other cores trying to do the same thing. The four cores would likely operate in lock step, fighting for access to the L2 cache as each core's local accesses fight with the other core's cache coherency operations. The ability to use the L1/L2 cache separation to buffer each CPU from the invalidates and unshares issued by other CPUs would be lost.



A single core CPU with L1 cache would easily beat a quad core without L1 cache.

Answer 2

Faster for what type of tasks? What are the particular CPUs in question?



It depends upon the specific processor models. Just looking at the number of cores and cache doesn't tell you how the processors actually perform. For example, AMD's original Phenom quad-core processors were slow compared to Intel's Core 2 Quad, while Phenom II is much faster.



For gaming, the number of cores is far less important than other factors (most of all, clock speed)- for example a dual-core 3.0Ghz E8400 easily outperforms a 2.5Ghz quad-core Q8300.



Here are some results for the Athlon II X4- as you can see, simply being quad-core does NOT necessarily mean faster!



http://firingsquad.com/hardware/amd_propus_athlon_2_x4_630_620_performance/page4.asp

http://firingsquad.com/hardware/amd_propus_athlon_2_x4_630_620_performance/page9.asp

Answer 3

I would say the quad core

Answer 4

quad core.

Answer 5

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