11/9/17 CS 261 Caching

equation for total cache capacity

S * E * B

what are the parts of a cache line?

Tag, then Set Index, then Block Offset

working set

a collection of elements needed repeatedly for a particular computation

cache set

a collection of one or more cache lines

cache

a small, fast memory that acts as a buffer or staging area for a larger, slower memory, data is transferred in blocks

correlation between stride and read throughput

as stride increases, read throughput decreases

correlation between working set size and read throughput

as working set size increases (from main memory to L1) read throughput increases

if a cache set is full, a cache miss requires...

blocks to be replaced or evicted

a cache always begins _______

cold (empty)

write-back

defer update until replacement/eviction, typically used for lower level of memory hierarchy

hit time

delay in accessing data for a cache hit

miss penalty

delay in loading data for a cache miss

what data structure can we use to implement caches?

hash table

larger cache ->

higher hit rate but higher hit time

lower miss rates ->

higher read throughput

capacity miss

if the working set doesn't fit in cache

write-though

immediately update to lower level, typically used for higher level of memory hierarchy

valid flag/bit (on cache line)

indicates whether the value is up-to-date

write-allocate

load then update, typically used write-back caches

read throughput (or "bandwith")

the rate that a program reads data from a memory system

no-write-allocate

update without loading, typically used for write-through caches

how should we handle write misses?

write-allocate, no-write-allocate

how should we handle writes to a cached value?

write-through, write-back

hit ratio (for caches)

# hits / # memory address

miss rate/ratio

# misses / # memory accesses

What does B equal (cache parameter)

# of blocks

What does S equal (cache parameter)

# of cache sets

What does E equal (cache parameter)

# of lines per cache

what is E for Set-associate caches

1 < E < C/B

what is E for direct-mapped caches

E = 1

what is E for Fully-associative caches

E = C/B