[crististm]

Yes. I don't know where he gets the idea that a large L1 cache is to a CPU the same as a 150mx150m desk to a human. Address decoding is done in parallel, not sequentially. And desks are as large as people are comfortable to produce and use.

Likewise, if the RAM would be as cheap to produce as SRAM like it is as DRAM, it would be as fast as the CPU (since it is using the same technology as the CPU) and we would not need the cache at all. Imagine gigabytes of L1 cache!

[Symmetry]

Well, address decoding can be started in parallel if your page size lets you do virtually indexed, physically tagged caches which applies to only some processors. But that's a separate issue from the relationship between cache size and cache speed. That's governed by three things.

First, the larger your cache the more layers of muxing you need to select the data you need, meaning more FO4s of transistor delay.

Second, the larger your cache the physically bigger it is. That means more physical distance between the memory location and where it is used. That means more speed of light delay.

And third there's the issue of resolving contention for shared versus unshared caches.

So despite the fact that you're using the same SRAM in both your L1 and L3 but access to the former takes 4 clock cycle but access to the later takes 80.

[gchadwick]

There's also the fact that as you get down the cache hierachy the cache becomes more complicated. An L1 does lookups for a single processor, and responds to snoops. An L3 probably has several processors hanging it off and may deal with running the cache coherency protocol (e.g. implements a directory of what lines are where and sends clean or invalidation snoops when someone wants to upgrade a line from shared to unique). As a result you've got layers of buffering, arbitration and hazarding to get through before you can even touch the memory array.

[smallnamespace]

> And desks are as large as people are comfortable to produce and use.

Think about what this implies though -- a desk that is too large becomes difficult for a person to use (for one, the person would have to start walking to access certain parts of it).

Likewise, L1 cache sizes are bounded, because the larger the cache becomes, the more difficult it is to address a particular location, and the cache also becomes physically larger such that speed-of-light propagation delays will slow the entire cache down.