Download Memory

Memory – The Memory
Hierarchy
CS/COE 1541 (term 2174)
Jarrett Billingsley
Class Announcements
● Here are your quizzes!
o Yaaaaaaay!
● HW takes a long time to grade!!!!
o Gosh!
o Then come exams, which should be quicker.
● Sorry about my voice!
o I hope I can make it through the lecture!
● Exclamation points!!!!!!!!!!!!
o WOW!!!!!!!!
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Let's go over the exam
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Externalities
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Oh yeah, memory.
● Thus far we've focused on making the CPU faster by shoving more
and more instructions into it. But the CPU doesn't work alone.
● Those IF and MEM phases...
o They aren't really one cycle.
o Well, maybe.
o In the best case.
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The elephants in the room
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DRAM
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2010
2005
2000
1995
1990
CPU
1985
1980
● The big issue is.... memory is slow.
● CPUs outpaced memory in the
early 1980s and haven't looked
back.
● This is what we call the
Processor-Memory
Performance Gap.
● Well this sucks.
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Memory Technologies
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Static RAM (SRAM)
● SRAM uses a loop of NOT gates to
store a single bit, relying on some
non-ideal characteristics of circuits
to "flip" the bit.
● This is usually called a 6T SRAM
cell since it uses... 6 Transistors!
● Pros: very fast to read/write; low
power; simple interface
● Cons: volatile (loses data without
power); low density -> more silicon
-> expensive
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Dynamic RAM (DRAM)
● DRAM uses one transistor and one capacitor per
bit. The bit is stored as a charge in the cap.
● But in the real world, capacitors "bleed" charge
and have to be periodically recharged (refreshed).
● DRAM controllers therefore have to spend a
significant portion of their time just making sure
the capacitors stay charged -> less time for the CPU to access it.
● Pros: higher density -> less silicon -> much cheaper than SRAM
● Cons: slower access time; still volatile; more complex interface
● Experimental DRAM tech skips the capacitor and uses stray
capacitance of substrate itself to store data! Can be faster than
SRAM - but not in production
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Spinning magnetic disks (HDD)
● Very old technology taken to an incredible level
of sophistication
● Spinning platter coated with a ferromagnetic
substance which is magnetized in one of two
directions to represent bits
● Extremely high bit densities, especially
with perpendicular and shingled writing
o Becoming more "archival"
● But it's a spinning piece of metal. It's slow.
● Pros: nonvolatile; ridiculously cheap, ridiculously huge (1TB for
$50); can be rewritten a practically unlimited number of times
● Cons: sloooowwwww; sensitive to acceleration/shock/vibration;
very power-hungry (two motors!); did I mention slow?
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Flash Memory (SSD, solid-state drive)
● Works by "stranding" charges
in a special kind of transistor.
● NAND and NOR flash – NAND
denser and more "general-purpose"
while NOR better for "read-only"
applications
● Tech has advanced rapidly, giving us incredible densities
● MLC and TLC cells sacrifice reliability for density
● Pros: nonvolatile, much faster than HDDs, more rugged,
no power use when not being read/written
● Cons: more expensive than HDDs (1TB for $250), writing is
destructive and shortens lifespan, asymmetric read/write speed
and power
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Experimental tech
● Ferroelectric RAM (FeRAM), Magnetoresistive RAM (MRAM), Phasechange memory (PRAM), carbon nanotubes, quantum dots,
holographic memory...
● In varying states of development and maturity
● Some volatile, some nonvolatile
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Outdated tech
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Magnetic disks
Magnetic tapes
Magnetic drums
Magnetic bubbles
Core memory (the origin of "core dump"!)
Delay-line memory
o Mercury, wire
CRT-based memory
Punchcards/paper tape
Well-trained pigeons
(Memory tech history is kinda fun!)
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The memory hierarchy
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From each according to their ability...
● Use the fast memory closest to the CPU, and use the big
memory furthest from it.
CPU
Pipeline
SRAM
(regs)
DRAM
(memory)
HDD/SDD
(long-term
storage)
● Speed tends to be inversely proportional to both price and size.
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Caching and Locality
● Caching is keeping a temporary copy of data for quick access.
● Each level in the hierarchy acts as a cache for the next lower level.
o Registers are a temporary copy of data from memory.
o Memory is a temporary copy of data from long-term storage.
o Long-term storage is a temporary copy of... remote data?
● Caching exploits locality.
o Temporal locality is the idea that you will access the same piece
of data many times in succession.
o Spatial locality is the idea that if you access a piece of data, you
are likely to soon access another piece very close by
● By keeping data in a cache, we can greatly speed up data access by
reusing the temporary copy.
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"There are two hard problems in CS:
● cache invalidation, naming things, and off-by-one errors."
● A cache is said to be valid if its data is an accurate copy of the data
in the level below. If the data in the level below changes, the cache
is now invalid.
● There is also cache coherence: the concept that multiple caches
of the same data are all valid.
o If one cache goes out of sync with another, it is now incoherent.
o This becomes a big issue in multiprocessor systems!
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Caching concerns
● If you find the data in the cache, it's a hit. Your hit rate is what
percentage of the time you have a cache hit when you access it.
● The inverse is a miss and miss rate. (hit rate + miss rate = 100%)
● How big do you make the cache?
o Bigger means more hits, but more cost.
● How many levels of caching do you need?
o More means more speed, but more complexity/cost.
● What do you do when the cache misses?
o Cache is limited size. Have to kick someone out!
● How do you quickly look up things in the cache?
● What about memories that are shared among multiple CPUs?
● Ohhhhhh boy.....
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