Cache Memory

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Cache Memory

• By Sean Hunter

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What is Cache?

• A computer memory with very short access time
  used for storage of frequently used instructions
  or data webster.com

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Types of Cache

• L1/L2/L3 Cache
• RAM Cache
• Disk Cache
• Software Level Cache

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L1/L2/L3 Cache (Cache Memory)

• Cache closer to the CPU that stores recently
  accessed data from RAM
• Holds instructions to be executed next and
  variables for the CPU

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L2 Cache
COAST Cache on a Stick
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Cache Memory Continued

• Level 1 Cache is stored on the CPU
• L2/L3 Cache stored near, but not on the CPU
• L1/L2/L3 Cache is more expensive than RAM

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Cache Memory Replacement Policies

• CPU requests data from cached memory
• Cache memory has data, sends to CPU (Hit)
• RAM has data, sends to CPU (Miss)
• Cache performance measured by hits/misses where
  the ideal algorithm has a ratio close to 1.

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Replacement Policies

• On a miss the data from RAM is loaded in to Cache
• Due to the nature of most programs (Variables are
  accessed multiple times in the same program) this
  cached data will have a high chance of being
  accessed often.
• One possible replacement algorithm is a FIFO
  based one where the oldest cached data is
  replaced first.

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CPU Cache - Write Policies

• CPU writes data in cache (hit)
• Write-through Cache and RAM updated
• Write-back Cache updated, RAM updated when new
  data is going to replace that data in cache
• Dirty bit keeps track of if a part of cache has
  changed

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CPU Cache - Write Policies

• CPU writes but data not in cache (Miss)
• Write-no-allocate RAM updated
• Write-allocate RAM updated, data loaded into
  cache

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CPU Cache SMP Problems

• Multiprocessor systems have trouble with
  write-through and write-back
• IE. One processor updates data cached by both
  processors
• Possible Solutions
• Cache only unshared memory
• Update all cache simultaneously

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RAM Cache

• Same as Cache Memory or CPU Cache, except instead
  of the L1/L2 cache caching RAM, RAM caches data
  from the hard disk.
• For example, Windows loads multiple required
  libraries for running in memory, but code isnt
  executed from them constantly.

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Disk Cache

• Like cache memory, disk cache holds data recently
  read or written to from a hard disk. This is to
  speed up subsequent reads from recently accessed
  data.
• Most newer hard drives have around 8mb of cache
  memory.

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Software Cache

• Software also needs to cache data into RAM that
  it uses often however, more often than not,
  this data isnt directly from the hard disk.
• The data software caches is often processed data
  from a file translated by the program into
  another data structure. The process of
  translating may take a long time, so the cache in
  RAM is used to speed up the rest of the programs
  run time.

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Software Cache - Example

• Games such as Half-Life and Unreal Tournament
  2004 have a loading time before each game where
  the map, models, sounds, and other important data
  are all loaded into RAM.
• Example Website loading lots of data from an SQL
  server each page load may take a few seconds, but
  if the first load caches all the data, the load
  may only take a few milliseconds.

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Software Cache - Example

• Internet Explorer
• History Keeps a cache of recently viewed
  websites for easy access
• Cache Keeps images and other files on disk so
  websites load faster the second time you visit
  them. These files are updated when there is a
  newer version of them on the server, or the cache
  is cleared.

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Interesting Facts

• The first Celeron CPU had no caching, however it
  performed almost as fast as the PII with cache.
• Because some chipsets couldnt cache more than
  the first 64mb of RAM a rumor was spread that
  Windows 98 could only use up to 64mb of RAM,
  however in reality it supports up to 2GB of RAM.
• In these chipsets, adding more than 64MB of RAM
  decreased performance.

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More interesting facts

• Some motherboards ship with fake cache memory
  (Pictures are of fake cache memory) to both
  deceive the user and to sell cheaper non-cache
  motherboards without having to manufacture
  completely different motherboard designs.

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References

• http//searchstorage.techtarget.com/sDefinition/0,
  ,sid5_gci211730,00.html
• http//searchstorage.techtarget.com/sDefinition/0,
  ,sid5_gci211963,00.html
• http//www.karbosguide.com/hardware/module3b2.htm
• http//arstechnica.com/paedia/c/caching/caching-1.
  html
• http//www.gb.tomshardware.com/howto/01q1/010212/s
  ocket5-06.html
• http//www.hardwaresite.net/wb.html
• http//www.pcwebopedia.com/TERM/D/disk_cache.html