Instructor: Prof. Jason Fritts

Instructor: Prof. Jason Fritts
Storage Technologies and Memory Hierarchy CSCI 224 / ECE 317: Computer Architecture Instructor: Prof. Jason Fritts Slides adapted from Bryant & O’Hallaron’s slides

Storage Technologies and Memory Hierarchy
Storage Technologies and Trends RAM Nonvolatile memory Disk Trends Locality, Cache, and Memory Hierarchy to the Rescue!

Random-Access Memory (RAM)
Basic storage unit is normally a cell (one bit per cell). Static RAM (SRAM) Each cell stores a bit with a four or six-transistor circuit. Retains value indefinitely, as long as it is kept powered. Dynamic RAM (DRAM) Each cell stores bit with a capacitor; one transistor used for access Value must be refreshed every ms Trans. Access Needs per bit time refresh? Cost Applications SRAM 4 or 6 1X No 100x Cache memories DRAM X Yes 1X Main memories, frame buffers

Conventional DRAM Organization
D x W DRAM: organized as D supercells of size W bits access desired supercell by first sending row address (RAS), then column address (CAS) 16 x 8 DRAM chip RAS = 2 Cols Memory controller 1 2 3 CAS = 1 2 / addr To CPU 1 Rows supercell (2,1) 2 3 8 / data supercell (2,1) Internal row buffer

64-bit doubleword at main memory address A
Memory Modules addr (row = i, col = j) : supercell (i,j) DRAM 0 64 MB memory module consisting of eight 8Mx8 DRAMs 31 7 8 15 16 23 24 32 63 39 40 47 48 55 56 64-bit doubleword at main memory address A bits 0-7 8-15 16-23 24-31 32-39 40-47 48-55 56-63 DRAM 7 64-bit doubleword 31 7 8 15 16 23 24 32 63 39 40 47 48 55 56 Memory controller

Enhanced DRAMs Basic DRAM cell has not changed since its invention in 1966 Commercialized by Intel in 1970 DRAM cores with better interface logic and faster I/O : Synchronous DRAM (SDRAM) Uses a conventional clock signal instead of asynchronous control Double data-rate synchronous DRAM (DDR SDRAM) Double edge clocking sends two bits per cycle per pin Different types distinguished by size of small prefetch buffer: DDR (2 bits), DDR2 (4 bits), DDR3 (8 bits) DDR RAM standard for most server and desktop systems

Nonvolatile Memories DRAM and SRAM are volatile memories
Data lost when powered off Nonvolatile memories retain value even if powered off Read-only memory (ROM) – programmed during production Programmable ROM (PROM) – can be programmed once Electrically-Eraseable PROM (EEPROM) – electronic erase capability Flash memory – EEPROMs with partial (sector) erase capability Wears out after about 100,000 erasings Uses for Nonvolatile Memories Firmware programs stored in a ROM (BIOS, disk controllers, …) Solid state disks (USB sticks, smart phones, tablets, laptops, …) Disk caches

Traditional Bus Structure Connecting CPU and Memory
A bus is a collection of parallel wires that carry address, data, and control signals bus width in CPU defines architecture width (32-bit, 64-bit, etc.) Buses are typically shared by multiple devices CPU chip Register file ALU System bus Memory bus Main memory Bus interface I/O bridge

Memory Read Transaction (1)
CPU places address A on the memory bus. CPU chip Register file Load operation: movl A, %eax ALU %eax Main memory I/O bridge A Bus interface A x

Main memory reads A from the memory bus, retrieves word x, and places it on the bus. CPU chip Register file ALU Load operation: movl A, %eax %eax Main memory I/O bridge x Bus interface A x

CPU read word x from the bus and copies it into register %eax. CPU chip Register file Load operation: movl A, %eax ALU %eax x Main memory I/O bridge Bus interface A x

Memory Write Transaction (2)
CPU places data word y on the bus. CPU chip Register file Store operation: movl %eax, A ALU %eax y Main memory I/O bridge y Bus interface A

Memory Write Transaction (3)
Main memory reads data word y from the bus and stores it at address A. CPU chip Register file ALU Store operation: movl %eax, A %eax y main memory I/O bridge Bus interface A y

What’s Inside A Disk Drive?
Spindle Arm Platters Actuator Electronics (including a processor and memory!) SCSI connector Image courtesy of Seagate Technology

Disk Geometry Disks consist of platters, each with two surfaces.
Each surface consists of concentric rings called tracks. Each track consists of sectors separated by gaps. Spindle Surface Tracks Track k Sectors Gaps

Disk Structure - top view of single platter
Surface organized into tracks Tracks divided into sectors Goal: Show the inefficeincy of current disk requests. Conveyed Ideas: Rotational latency is wasted time that can be used to service tasks Background Information: None. Slide Background: Kill text and arrows

Disk Geometry (Muliple-Platter View)
Aligned tracks form a cylinder. Cylinder k Surface 0 Platter 0 Surface 1 Surface 2 Platter 1 Surface 3 Surface 4 Platter 2 Surface 5 Spindle

Disk Capacity Capacity – maximum number of bits that can be stored.
Vendors express capacity in units of gigabytes (GB) Capacity is determined by these technology factors: Recording density (bits/in) – number of bits per segment of a track Track density (tracks/in) – number of tracks per radial segment Areal density (bits/in2) – product of recording and track density Modern disks partition tracks into disjoint subsets called recording zones Each track in a zone has the same number of sectors, determined by the circumference of innermost track Each zone has a different number of sectors/track

Computing Disk Capacity
Capacity = (# bytes/sector) x (avg. # sectors/track) x (# tracks/surface) x (# surfaces/platter) x (# platters/disk) Example: 512 bytes/sector 300 sectors/track (on average) 20,000 tracks/surface 2 surfaces/platter 5 platters/disk Capacity = 512 x 300 x x 2 x 5 = 30,720,000,000 = GB

Disk Operation (Single-Platter View)
The disk surface spins at a fixed rotational rate By moving radially, the arm can position the read/write head over any track. The read/write head is attached to the end of the arm and flies over the disk surface on a thin cushion of air. spindle spindle spindle spindle spindle

Disk Operation (Multi-Platter View)
Read/write heads move in unison from cylinder to cylinder Arm Spindle

Disk Access Head in position above a track Goal:
Show the inefficeincy of current disk requests. Conveyed Ideas: Rotational latency is wasted time that can be used to service tasks Background Information: None. Slide Background: Kill text and arrows

Disk Access Rotation is counter-clockwise Goal:

Disk Access – Read About to read blue sector Goal:

Disk Access – Read After reading blue sector After BLUE read Goal:

Disk Access – Read Red request scheduled next After BLUE read Goal:

Disk Access – Seek Seek to red’s track After BLUE read Seek for RED
Goal: Show the inefficeincy of current disk requests. Conveyed Ideas: Rotational latency is wasted time that can be used to service tasks Background Information: None. Slide Background: Kill text and arrows

Disk Access – Rotational Latency
After BLUE read Seek for RED Rotational latency Wait for red sector to rotate around Goal: Show the inefficeincy of current disk requests. Conveyed Ideas: Rotational latency is wasted time that can be used to service tasks Background Information: None. Slide Background: Kill text and arrows

Disk Access – Read Complete read of red After BLUE read Seek for RED
Rotational latency After RED read Complete read of red Goal: Show the inefficeincy of current disk requests. Conveyed Ideas: Rotational latency is wasted time that can be used to service tasks Background Information: None. Slide Background: Kill text and arrows

Disk Access – Service Time Components
After BLUE read Seek for RED Rotational latency After RED read Goal: Show the inefficeincy of current disk requests. Conveyed Ideas: Rotational latency is wasted time that can be used to service tasks Background Information: None. Slide Background: Kill text and arrows Data transfer Seek Rotational latency Data transfer

Disk Access Time Average time to access some target sector approximated by : Taccess = Tavg seek + Tavg rotation + Tavg transfer Seek time (Tavg seek) Time to position heads over cylinder containing target sector. Typical Tavg seek is 3—9 ms Rotational latency (Tavg rotation) Time waiting for first bit of target sector to pass under r/w head. Tavg rotation = 1/2 x 1/RPMs x 60 sec/1 min Typical Tavg rotation = 7200 RPMs Transfer time (Tavg transfer) Time to read the bits in the target sector. Tavg transfer = 1/RPM x 1/(avg # sectors/track) x 60 secs/1 min.

Disk Access Time Example
Given: Rotational rate = 7,200 RPM Average seek time = 9 ms. Avg # sectors/track = 400. Derived: Tavg rotation = 1/2 x (60 secs/7200 RPM) x 1000 ms/sec = 4 ms. Tavg transfer = 60/7200 RPM x 1/400 secs/track x 1000 ms/sec = 0.02 ms Taccess = 9 ms + 4 ms ms Important points: Access time dominated by seek time and rotational latency. First bit in a sector is the most expensive, the rest are free. SRAM access time is about 4 ns/doubleword, DRAM about 60 ns Disk is about 40,000 times slower than SRAM, and about 2,500 times slower then DRAM

I/O Bus CPU chip Register file ALU System bus Memory bus Main memory
Bus interface I/O bridge I/O bus Expansion slots for other devices such as network adapters. USB controller Graphics adapter Disk controller Mouse Keyboard Monitor Disk

Reading a Disk Sector (1)
CPU chip CPU initiates a disk read by writing a command, logical block number, and destination memory address to a port (address) associated with disk controller. Register file ALU Main memory Bus interface I/O bus USB controller Graphics adapter Disk controller keyboard mouse Monitor Disk

CPU chip Disk controller reads the sector and performs a direct memory access (DMA) transfer into main memory. Register file ALU Main memory Bus interface I/O bus USB controller Graphics adapter Disk controller Mouse Keyboard Monitor Disk

CPU chip When the DMA transfer completes, the disk controller notifies the CPU with an interrupt (i.e., asserts a special “interrupt” pin on the CPU) Register file ALU Main memory Bus interface I/O bus USB controller Graphics adapter Disk controller Mouse Keyboard Monitor Disk

Solid State Disks (SSDs)
I/O bus Requests to read and write logical disk blocks Solid State Disk (SSD) Flash translation layer Flash memory Block 0 Block B-1 … … … Page 0 Page 1 Page P-1 Page 0 Page 1 Page P-1 Pages: 512KB to 4KB, Blocks: 32 to 128 pages Data read/written in units of pages. Page can be written only after its block has been erased A block wears out after 100,000 repeated writes.

SSD Performance Characteristics
Sequential read tput 250 MB/s Sequential write tput 170 MB/s Random read tput 140 MB/s Random write tput 14 MB/s Rand read access 30 us Random write access 300 us Why are random writes so slow? Erasing a block is slow (around 1 ms) Write to a page triggers a copy of all useful pages in the block Find an used block (new block) and erase it Write the page into the new block Copy other pages from old block to the new block

Solid State Disks (SSDs) vs Rotating Disks
Advantages No moving parts  faster, less power, more rugged Disadvantages Have the potential to wear out Mitigated by “wear leveling logic” in flash translation layer E.g. Intel X25 guarantees 1 petabyte (1015 bytes) of random writes before they wear out As of 2014, about 15 times more expensive per byte Applications Smart phones and tablets Found in some laptops, desktops, and servers

Storage Trends Metric 1980 1985 1990 1995 2000 2005 2010 2010:1980
SRAM Metric :1980 $/MB 19,200 2, access (ns) DRAM Metric :1980 $/MB 8, ,000 access (ns) typical size (MB) ,000 8, ,000 Disk Metric :1980 $/MB ,600,000 access (ms) typical size (MB) ,000 20, ,000 1,500,000 1,500,000

CPU Clock Rates Inflection point in computer history
when designers hit the “Power Wall” :1980 CPU Pentium P-III P-4 Core 2 Core i Clock rate (MHz) Cycle time (ns) Cores Effective cycle ,000 time (ns)

The CPU-Memory Gap The gap widens between DRAM, disk, and CPU speeds.
SSD DRAM CPU

Locality and Cache to the Rescue!
The key to bridging this Processor-Memory gap is a fundamental property of computer programs known as… Locality

Locality Principle of Locality: Programs tend to use data and instructions with the same or neighboring addresses as those they have used recently Temporal locality: Recently referenced items are likely to be referenced again in the near future Spatial locality: Items with nearby addresses tend to be referenced close together in time (will discuss locality in more detail later in semester…)

Caches Cache memory takes advantage of locality … the Memory Hierarchy
A smaller, faster memory device that acts as a staging area for a subset of the data from a larger, slower device that subset of data frequently exhibits good locality Cache memory can be made very fast using SRAM BUT, fast caches are small Making caches larger quickly reduces speed How to get BOTH size and speed? … the Memory Hierarchy

Memory Hierarchy L0: Registers L1: L1 cache (SRAM) Smaller, faster,
CPU registers hold words retrieved from L1 cache Registers L1: L1 cache (SRAM) Smaller, faster, more expensive per byte L1 cache holds cache lines retrieved from L2 cache L2: L2 cache (SRAM) L2 cache holds cache lines retrieved from main memory L3: Main memory (DRAM) Larger, slower, cheaper per byte Main memory holds disk blocks retrieved from local disks L4: Local secondary storage (local disks) Local disks hold files retrieved from disks on remote network servers Remote secondary storage (tapes, distributed file systems, Web servers) L5:

Caches Fundamental idea of a memory hierarchy:
For each k, the faster, smaller device at level k serves as a cache for the larger, slower device at level k+1. Why do memory hierarchies work? Because of locality, programs tend to access the data at level k more often than they access the data at level k+1. Thus, the storage at level k+1 can be slower, and thus larger and cheaper per bit. Big Idea: The memory hierarchy creates a large pool of storage that costs as much as the cheap storage near the bottom, but that serves data to programs at the rate of the fast storage near the top.

Intel Core i7 Cache Hierarchy
Processor package Core 0 Core 3 L1 i-cache and d-cache: 32 KB, 8-way, Access: 4 cycles L2 unified cache: 256 KB, 8-way, Access: 11 cycles L3 unified cache: 8 MB, 16-way, Access: cycles Block size: 64 bytes for all caches. Regs Regs L1 d-cache L1 i-cache L1 d-cache L1 i-cache … L2 unified cache L2 unified cache L3 unified cache (shared by all cores) Main memory

Cache Performance Metrics
Miss Rate Fraction of memory references not found in cache (misses / accesses) = 1 – hit rate Typical numbers (in percentages): 3-10% for L1 can be quite small (e.g., < 1%) for L2, depending on size, etc. Hit Time Time to deliver a line in the cache to the processor includes time to determine whether the line is in the cache Typical numbers: 1-2 clock cycle for L1 5-20 clock cycles for L2 Miss Penalty Additional time required because of a miss typically cycles for main memory (Trend: increasing!)

Lets think about those numbers
Huge difference between a hit and a miss Could be 100x, if just L1 and main memory Would you believe 99% hits is twice as good as 97%? Consider: cache hit time of 1 cycle miss penalty of 100 cycles Average access time: 97% hits: 1 cycle * 100 cycles = 4 cycles 99% hits: 1 cycle * 100 cycles = 2 cycles This is why “miss rate” is used instead of “hit rate”

Summary The speed gap between processor, memory, and mass storage continues to widen. Memory hierarchies based on caching close the gap by exploiting locality.

Instructor: Prof. Jason Fritts

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