5.8.2 Determining the optimal global
The global work size is the total number of work-items required for all dimensions combined.
You can change the global work size by processing multiple
data items in a single work-item. The new global work size is then
the original global work size divided by the number of data items
processed by each work-item.
The global work size must be large if you want to ensure high
performance. Typically, the number is several thousand, but the
ideal number depends on the number of shader cores in your device.