Hardware Reference
In-Depth Information
In this analogy, the processor was 14 times faster than the main memory. Memory speeds
have increased from 16MHz (60ns) to 333MHz (3.0ns) or faster in the latest systems,
but processor speeds have also risen to 3GHz and beyond. So even in the latest systems,
memory is still 7.5 or more times
slower
than the processor. Cache is what makes up the
difference.
The main feature of L1 cache is that it has always been integrated into the processor core,
where it runs at the same speed as the core. This, combined with the hit ratio of 90% or
greater, makes L1 cache important for system performance.
Level 2 Cache
To mitigate the dramatic slowdown every time an L1 cache miss occurs, a secondary (L2)
cache is employed.
UsingtherestaurantanalogyIusedtoexplainL1cacheintheprevioussection,I'llequate
the L2 cache to a cart of additional food items placed strategically in the restaurant such
that the waiter can retrieve food from the cart in only 15 seconds (versus 60 seconds from
the kitchen). In an actual Pentium class (Socket 7) system, the L2 cache is mounted on
the motherboard, which means it runs at motherboard speed (66MHz, or 15ns in this ex-
ample). Now, if you ask for an item the waiter did not bring in advance to your table,
instead of making the long trek back to the kitchen to retrieve the food and bring it back
to you 60 seconds later, he can first check the cart where he has placed additional items.
If the requested item is there, he will return with it in only 15 seconds. The net effect in
the real system is that instead of slowing down from 233MHz to 16MHz waiting for the
data to come from the 60ns main memory, the system can instead retrieve the data from
the 15ns (66MHz) L2 cache. The effect is that the system slows down from 233MHz to
66MHz.
All modern processors have integrated L2 cache that runs at the same speed as the pro-
cessor core, which is also the same speed as the L1 cache. For the analogy to describe
these newer chips, the waiter would simply place the cart right next to the table you were
seated at in the restaurant. Then, if the food you desired wasn't on the table (L1 cache
miss), it would merely take a longer reach over to the adjacent L2 cache (the cart, in this
analogy) rather than a 15-second walk to the cart as with the older designs.
Level 3 Cache
Some processors, primarily those designed for high-performance desktop operation or
enterprise-level servers, contain a third level of cache known as
L3 cache
. In the past, rel-
atively few processors had L3 cache, but it is becoming more and more common in newer
and faster multicore processors such as the Intel Core i7 and AMD Phenom II processors.
