Published online by Cambridge University Press: 05 June 2012
We reviewed the basics of caches in Chapter 2. In subsequent chapters, when we looked at instruction fetch in the front-end and data load–store operations in the back-end, we assumed most of the time that we had cache hits in the respective first-level instruction and data caches. It is time now to look at the memory hierarchy in a more realistic fashion. In this chapter, our focus is principally on the cache hierarchy.
The challenge for an effective memory hierarchy can be summarized by two technological constraints:
With processors running at a few gigahertz, main memory latencies are now of the order of several hundred cycles.
In order to access first-level caches in 1 or 2 cycles, their size and associativity must be severely limited.
These two facts point to a hierarchy of caches: relatively small-size and small-associativity first-level instruction and data caches (L1 caches); a large second-level on-chip cache with access an order of magnitude slower than L1 accesses (L2 cache generally unified, i.e., holding both instructions and data); often in high-performance servers a third-level cache (L3) off chip, with latencies approaching 100 cycles; and then main memory, with latencies of a few hundred cycles. The goal of the design of a cache hierarchy is to keep a latency of one or two cycles for L1 caches and to hide as much as possible the latencies of higher cache levels and of main memory.
To save this book to your Kindle, first ensure no-reply@cambridge.org is added to your Approved Personal Document E-mail List under your Personal Document Settings on the Manage Your Content and Devices page of your Amazon account. Then enter the ‘name’ part of your Kindle email address below. Find out more about saving to your Kindle.
Note you can select to save to either the @free.kindle.com or @kindle.com variations. ‘@free.kindle.com’ emails are free but can only be saved to your device when it is connected to wi-fi. ‘@kindle.com’ emails can be delivered even when you are not connected to wi-fi, but note that service fees apply.
Find out more about the Kindle Personal Document Service.
To save content items to your account, please confirm that you agree to abide by our usage policies. If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account. Find out more about saving content to Dropbox.
To save content items to your account, please confirm that you agree to abide by our usage policies. If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account. Find out more about saving content to Google Drive.