Cache Poor

How much do you really know about managing high traffic web sites?  I was asked an interesting question recently.  Here it is:

You are operating a web site that receives about 4 million unique visitors per day from around the world seven days a week, 24 hours per day.  The load is spread evenly throughout each day.  Performance was starting to become unacceptable.  You’ve identified that the current bottle neck was your memcached machines, serving as a back cache to your web front ends.  Below is a diagram of your simple architecture.  What would you do about the problem without bringing your busy site down?

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