It's surely an engineering tradeoff resulting from weighing the different pros/cons. If you had dedicated cache per core at 1/Nth the size, much of it would be

It's surely an engineering tradeoff resulting from weighing the different pros/cons. If you had dedicated cache per core at 1/Nth the size, much of it would be wasted when you're running less than full tilt -- eg, if you're using 2/4 cores then half of the cache is artificially unavailable instead of doubling the cache available to those 2 cores.