Big Data BlackOut: Are Utilities Powering Up Their Data Analytics?
Memory and stuff.. le well drawn pyramid Features Name Volatile Mutable Accessibility Description...
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Transcript of Memory and stuff.. le well drawn pyramid Features Name Volatile Mutable Accessibility Description...
Memory
and stuff.
le well drawn pyramid
Features
Name• Volatile
• Mutable
• Accessibility
Description• Data is lost quickly after
powering down
• Data can be overwritten
• CPU can directly access data in memory
Cache Memory - features
• Top level of memory
• Is volatile• Mutable• Accessible
Cache Memory - description
• Copies data from other places
• Commonly has two levels.– The first level (L1 cache) is directly ‘in’ the CPU
• Smallest size (2-64kB)• Fastest speed (lowest distance to CPU)
– L2 cache was formerly near the CPU on the motherboard• Relatively larger than L1• Pretty fast too!
Cache Memory - purpose
• When the CPU reaches for data, it first looks in the cache.
• If it finds what it wants, it’s called a ‘cache hit’• Else, it will reach to the other levels of
memory, and this is a ‘cache miss’
• Roughly 95% cache hits [citation needed]
Cache Memory - analogy
• Librarian (CPU) looks for things from book storage room
• Cache is like a pouch storing most commonly borrowed books, saving the time needed to– Walk to room and back (data transfer round trip)– Search for the book in the room
• Instead, Librarian first searches inside pouch containing 5 books– Smaller volume => easier to search– Virtually 0 distance
Cache Memory (tl;dr)
• Top level• Volatile, mutable, directly accessible
• Small, fast
• Stores copies of most commonly used data
Primary Storage - features
• Next level after cache. Most common example is RAM (random access memory)
• Is volatile• Mutable• Accessible through memory bus
Primary Storage - description
• DRAM (dynamic)– Data is stored in capacitors that act as bits• Charged = 1, no charge =0
– Capacitors leak over time• => data leaks over time too
– Data has to be periodically rewritten into DRAM• ‘refresh cycle’
– Most common form (cheapest)
Primary Storage - description
• SRAM (static)– Uses 4-6 transistors/bit than DRAM
• DRAM uses 1 transistor + 1 capacitor/bit• Thus, costs more to make
– Because of how it works, data is remanent (‘stays’)• But it is still volatile (when it loses power, data eventually
vanishes)
– Significantly less powerhungry• No need for data refresh• As little as a few microwattsH in idle state
Secondary storage - features
• Non-volatile– Stored on tape/metallic discs– Read/write with magnetic heads (think turntable)
• May be mutable or not
• Not directly accessible– Uses either primary (RAM) or I/O channels
Secondary storage - purpose
• Storing data permanently
• Completely mutable (hard disk drive)– This type is mutable; stored on disk but can be rewritten
• Partially mutable (flash memory)– Electronically rewritable– Used by USBs, cameras, PDAs, phones…
• Completely unmutable (ROM)– Used to store information permanently (CDs, for example)
Secondary storage - purpose
• Difference in mutability, from HDD to ROM
• Speed goes up– Flash memory is about as fast as DRAM– pure ROM is as fast as SRAM
• Portability goes up
Secondary storage - BIOS
• The computer boots up through this
• BIOS is ROM that primary memory reads from• Primary memory passes this info to CPU
Virtual Memory
• Virtualization of different types of storage– Creates a virtual version of memory
• The OS (and other programs) treat the computer as if there is only one type of memory – “Virtual” memory
• Space is borrowed from secondary storage, speed from primary storage
Virtual Memory
• “Page file” is a block of memory– Its size is the size of space borrowed from
secondary storage
• Thrashing occurs when you use up your virtual memory, and the computer repeatedly swaps between virtual and actual memory