When you put two graphics cards together in a computer using Crossfire or SLI why don’t they generate twice the rendering power?
In: Technology
Because at no point is any part of your computer operating at 100% capacity. This would wear down the part quickly, or possibly instantly destroy it (such as overloading your power supply.)
A properly-functioning computer never allocates any and all resources to any given task. That’s because a computer is constantly performing tons of tasks and needs to always have some free just in case more are added. As such, any given task your PC performs is harshly limited in how much memory, CPU, or GPU it gets to take up. Only malware will try to eat up *all* the resources your PC has, or at least succeed in doing so.
In the specific case of having multiple CPUs or GPUs, it’s quite rare for one task to use more than one of them to begin with, and many code libraries don’t even support multi-threading. Multi-cores tend to optimize running many tasks, one per CPU / GPU, not several tasks all piled on the one.
You can generally expect going from one to two CPUs or GPUs to give you more like a 50% increase in that particular part’s performance, probably less, and this is spread over the entire PC’s performance, not reflected in a single game or program.
In theory it could do something close to 2X the power. However, most software isn’t optimized to use any number of GPUs in the most mathematically efficient way. Furthermore, even with optimally efficient software the 2-GPU system will always see an absolutely minute loss of performance due to the clock cycles required to task multiple devices with processing instead of one.
It would probably be possible to handle this disadvantage by treating the GPUs as a cluster and mediating between them with a purpose-built microprocessor. I do not know if this has been done.
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