What is the Maximum Performance Gain for 12 vs 16 vs 24 vs 28 CPU Cores for the Apple Mac Pro? (Intel Xeon W-3245 etc)
The 2019 Mac Pro is the best computer I have ever owned, by far. Which is why I want 28 cores. But what is the real maximum performance difference between 8/12/16/24/28 CPU cores? Far less than it appears, that is, 16 cores are not twice as fast as 8 cores, and 24 cores are not 50% faster than 16 cores.
First, the speedup depends a great deal on the software in use, in particular how well it is coded to scale across CPU cores, which includes both programming competence along with how much resource contention there is (disk I/O, memory bandwidth).
Clock speed vs core utilization
The other major factor: CPU core clock speed is not a constant; the more CPU cores that are in use together, the lower the clock speed drops for all CPU cores.
A good rule of thumb metric for total computation cycles is CoresInUse X ClockSpeed. The clock speed cited for a CPU is the speed at which the CPU cores run when they are all in use. (Turbo Boost is also higher on the 12/16/24/28 core CPUs than on the 8-core).
I don’t have figures for actual CPU clock speed when 3 or 7 or 13 or whatever cores in use, but the graph below captures the rolloff in per-core computing power as the number of cores in use increases.
The issue is TDP (total dissipated power), which is around 200 watts for the Intel Xeon CPUs used in the 2019 Mac Pro. Each CPU core must run at a slower clock speed so that total power consumption stays within the total power budget.
8-Core 16t 3.5GHz Intel Xeon W-3223 Turbo Boost up to 4.0GHz
12-Core 24t 3.3GHz Intel Xeon W-3235 Turbo Boost up to 4.4GHz
16-Core 32t 3.2GHz Intel Xeon W-3245 Turbo Boost up to 4.4GHz
24-Core 48t 2.7GHz Intel Xeon W-3265M Turbo Boost up to
28-Core 56t 2.5GHz Intel Xeon W-3275M Turbo Boost up to 4.4GHz
That total gigahertz is a best case computing power number. It correlates very strongly with actual tests and is thus an excellent proxy for best possible performance but only when all CPU cores are utilized with minimal resource contention.
Bottom line: 12 cores is excellent for most users, 16 cores is a sweet spot, 24 cores is worthwhile but very pricey and 28 vs 24 is negligible. For most users, more than 16 cores is throwing money away.
Relative total potential computing power:
8 cores in use: 1.0
12 cores in use: 1.4X
16 cores in use: 1.8X
24 cores in use: 2.3X
28 cores in use: 2.5X