Additional Cache Notes

1. Additional Cache Notes Dan Nguyen Spring 2005 - Lee

2. Cache Review • Cache is used to improve computer performance by giving faster access times. IE. Internet web browsers, or even simple examples such as shops • Cache RAM is fast, operates at CPU level • LRU and FIFO replacement policies are the most common way to increase cache hit ratio

3. Common Cache Found In Computers • Memory Cache • Disk Cache

4. MEMORY CACHE • A bank of memory that connects main memory and CPU • Data is transferred in cache blocks • Uses “look-ahead” algorithm • Basically, the more instructions executed, the better the system performance • LRU, FIFO

5. L1, L2, L3 Caches A type of memory cache. Computer systems use multi-level cache hierarchies. • L1 cache is embedded on processor chip • L2 cache is external to the processor • L3 is built into physical memory These “level” caches increase functionality in chips. Multi-level caches make the cost of accessing memory approximately the same as the cost of accessing a register. Example: going to a movie rental store A)Customer asks clerk for a popular movie. Clerk checks main storage room of all movies (one level cache) B)Customer checks the movie shelves, not found->asks clerk. Clerk checks in storage room (two level cache)

7. More on L1, L2, L3 • Closer to the CPU • If closer to the CPU, stores recently accessed data from RAM • Contains instructions that are executed next Processor L1 L2 L3 Itanium 32KB 256KB 3MB, 4MB, 6MB Itanium 32KB 96KB 2 MB, 4MB Xeon MP 8KB 256KB, 512KB 512KB,1MB,2Mb P4 8KB 512KB --

8. L1,L2,L3 Summary • The main concept is that we want to have more instructions executed in a fast memory location that is close to CPU.

9. DISK CACHE • A section of memory on disk controller that connects disk to CPU • When disk is read, it is copied into disk cache. This is usually a larger block of data.

11. Data Writing • During idle times, if cache data needs to be written, that data will be lined up to be written to disk at high speeds. • Example: MS Word Auto Save feature

12. Write Back Cache • Supports cache writing • Data written to disk, is first written to cache • Improves performance, faster than RAM • Example: MS Word Temp files

13. Write Through Cache • Supports cache writing • Slower for writing, but very good for reading data • Example: floppy disks

14. Pipeline Burst Cache • For data that is read much, less access times to those memory locations for that data. • Related to pipelining later in semester

15. Look-a-side Cache • Cache that shares the computer system bus with main memory

16. Inline Cache • Next to the CPU • Faster than look-a-side because next to the CPU • Slower than back side cache

17. Backside Cache • Very fast • Has a special connection to the CPU • Can operate at CPU level

18. Disk Cache Summary • Cache used directly with the disk, ie hard disks • backside > inline > look-a-side • Used to increase data transfer between different systems of the computer

19. Sources • Class textbook • www.answers.com • www.howstuffworks.com