what are the units of the horizontal axis of these graphs? is it time?

^ My understanding is that the area of these graphs represent time, which is essentially a sum of the horizontal axis of the access time in Register, L1, L2 cache and Main memory. The main point of this graph is that the memory access in multiprocessor takes longer than uniprocessor, but the upside is that there is more execution capability.

Could someone remind me as to why its less of a triangle shape?

@german.enik I think it might represent what % of memory operations occur at each level. You'd imagine most well-written programs spend most of their time working with registers and their L1-L2 cache because they don't have to deal with cache invalidation from other processors. @brianamb same idea, since with multiple processors, each has to deal with cache invalidation more often and as shown above, can get a memory access "miss" when chunks of overall memory are distributed across groups of processors.