Cache Performance, Interfacing, Multiprocessors

CPSC 321

Andreas Klappenecker

Today’s Menu

Cache PerformanceReview of Virtual MemoryProcessor and PeripheralsMultiprocessors

Cache Performance

Caching Basics

• What are the different cache placement schemes? • direct mapped• set associative• fully associative

• Explain how a 2-way cache with 4 sets works• If we want to read a memory block whose address is

addr, then we search the set addr mod 4• The memory block could be in either element of the

set• Compare tags with upper n-2 bits of addr

Implementation of a Cache

Sketch an implementation of a 4-way associative

cache

Measuring Cache Performance

• CPU cycle time• CPU execution clock cycles (including cache

hits)• Memory-stall clock cycles (cache misses)• CPU time = (CPU execution clock cycles +

memory stall clock cycles) x clock cycle time

• Memory stall clock cycles • read stall cycles (rsc)• write stall clock cycles (wsc)• Memory stall clock cycles = rsc + wsc

Measuring Cache Performance

• Write-stall cycle [write-through scheme]:• two sources of stalls:• write misses (usually require to fetch the block)• write buffer stalls (write buffer is full when write

occurs)• WSCs are sum of the two:

WSCs = (writes/prg x write miss rate x write miss penalty) + write buffer stalls

• Memory stall clock cycles similar

Cache Performance Example

• Instruction cache rate: 2%• Data miss rate: 4%• Assume that 2 CPI without any memory stalls• Miss penalty 40 cycles for all misses• Instruction count I• Instruction miss cycles = I x 2% x 40 = 0.80 x I

• gcc has 36% loads and stores• Data miss cycles = I x 36% x 4% x 40 = 0.58 x I

Review of Virtual Memory

Virtual Memory

• Processor generates virtual addresses• Memory is accessed using physical

addresses • Virtual and physical memory is broken

into blocks of memory, called pages• A virtual page may be absent from main

memory, residing on the disk or may be mapped to a physical page

Page Tables

Physical memory

Disk storage

Valid

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Page table

Virtual pagenumber

Physical page ordisk address

The page table maps each page to either a page in mainmemory or to a page stored on disk

Pages: virtual memory blocks

• Page faults: if data is not in memory, retrieve it from disk• huge miss penalty, thus pages should be fairly large

(e.g., 4KB)• reducing page faults is important (LRU is worth the

price)• using write-through takes too long so we use writeback• Example: page size 212=4KB; 218 physical pages; main memory <= 1GB; virtual memory <= 4GB

3 2 1 011 10 9 815 14 13 1231 30 29 28 27

Page offsetVirtual page number

Virtual address

3 2 1 011 10 9 815 14 13 1229 28 27

Page offsetPhysical page number

Physical address

Translation

Page Faults

• Incredible high penalty for a page fault• Reduce number of page faults by

optimizing page placement• Use fully associative placement

• full search of pages is impractical• pages are located by a full table that indexes

the memory, called the page table• the page table resides within the memory

Making Memory Access Fast

• Page tables slow us down• Memory access will take at least twice as

long• access page table in memory• access page

• What can we do?

Memory access is local => use a cache that keeps track of recently used address translations, called translation lookaside buffer

Making Address Translation Fast

A cache for address translations: translation lookaside buffer

Valid

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Page table

Physical pageaddressValid

TLB

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TagVirtual page

number

Physical pageor disk address

Physical memory

Disk storage

Processors and Peripherals

Collection of I/O Devices

Communication between I/O devices, processor and

memory use protocols on the bus and interrupts

Mainmemory

I/Ocontroller

I/Ocontroller

I/Ocontroller

Disk Graphicsoutput

Network

Memory– I/O bus

Processor

Cache

Interrupts

Disk

Impact of I/O on Performance

A benchmark executes in 100 seconds• 90 seconds CPU time• 10 seconds I/O timeIf CPU improves 50% per year for next 5 years, howmuch faster does the benchmark run in 5 years?

90/(1.5)5 = 90/7.59 = 11.851

I/O Devices

• Very diverse devices— behavior (i.e., input vs. output)— partner (who is at the other end?)— data rateDevice Behavior Partner Data rate (KB/sec)

Keyboard input human 0.01Mouse input human 0.02Voice input input human 0.02Scanner input human 400.00Voice output output human 0.60Line printer output human 1.00Laser printer output human 200.00Graphics display output human 60,000.00Modem input or output machine 2.00-8.00Network/LAN input or output machine 500.00-6000.00Floppy disk storage machine 100.00Optical disk storage machine 1000.00Magnetic tape storage machine 2000.00Magnetic disk storage machine 2000.00-10,000.00

Communicating with Processor

• Polling• simple• I/O device puts information in a status register• processor retrieves information• check the status periodically

• Interrupt driven I/O• device notifies processor that it has completed some

operation by causing an interrupt• similar to exception, except that it is asynchronous• processor must be notified of the device csng interrupt• interrupts must be prioritized

I/O Example: Disk Drives

• To access data:— seek: position head over the proper track (8 to 20 ms. avg.)— rotational latency: wait for desired sector (.5 / RPM)— transfer: grab the data (one or more sectors) 2 to 15 MB/sec

Platter

Track

Platters

Sectors

Tracks

I/O Example: Buses

• Shared communication link (one or more wires)• Difficult design:

— may be bottleneck— tradeoffs (buffers for higher bandwidth increases latency)— support for many different devices— cost

• Types of buses:— processor-memory (short high speed, custom design)— backplane (high speed, often standardized, e.g., PCI)— I/O (lengthy, different devices, standardized, e.g., SCSI)

• Synchronous vs. Asynchronous— use a clock and a synchronous protocol,

fast and small, but every device must operate at same rate and clock skew requires the bus to be short

— don’t use a clock - use handshaking instead

Asynchronous Handshake Protocol

• ReadyReq: Indicates a read request from memory

• DataRdy: Indicates that data word is now ready on data lines

• Ack: Used to acknowledge the ReadyReq or DataRdy signal of the other party

DataRdy

Ack

Data

ReadReq 13

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Asynchronous Handshake Protocol

1. Memory sees ReadReq, reads address from data bus, raises Ack2. I/O device sees Ack high, releases ReadReq and data lines3. Memory sees ReadReq low, drops Ack to acknowledge ReadReq4. When memory has data ready, it places data from the read request on the data

lines and raises DataRdy5. I/O devices sees DataRdy, reads data from the bus, signals that it has the data

by raising Ack6. Memory sees the Ack signal, drops DataRdy, releases datalines 7. If DataRdy goes low, the I/O device drops Ack to indicate that transmission is

over

DataRdy

Ack

Data

ReadReq 13

4

57

642 2

Synchronous vs. Asynchronous Buses

• Compare max. bandwidth for a synchronous bus and an asynchronous bus

• Synchronous bus • has clock cycle time of 50 ns• each transmission takes 1 clock cycle

• Asynchronous bus• requires 40 ns per handshake

• Find bandwidth for each bus when performing one-word reads from a 200ns memory

Synchronous Bus

1. Send address to memory: 50 ns2. Read memory: 200 ns3. Send data to device: 50ns4. Total: 300 ns5. Max. bandwidth:

1. 4 bytes/300ns = 13.3 MB/second

Asynchronous Bus

• Apparently much slower because each step of the protocol takes 40 ns and memory access 200 ns

• Notice that several steps are overlapped with memory access time

• Memory receives address at step 1• does not need to put address until step 5• steps 2,3,4 can overlap with memory access

• Step 1: 40 ns• Step 2,3,4: max(3 x 40ns =120ns, 200 ns)• Steps 5,6,7: 3x40ns = 120ns• Total time 360ns • max. bandwidth 4bytes/360ns=11.1MB/second

Other important issues

• Bus Arbitration:— daisy chain arbitration (not very fair)— centralized arbitration (requires an arbiter), e.g., PCI— self selection, e.g., NuBus used in Macintosh— collision detection, e.g., Ethernet

• Operating system:— polling, interrupts, DMA

• Performance Analysis techniques:— queuing theory— simulation— analysis, i.e., find the weakest link (see “I/O System

Design”)

Overhead of Polling

Overhead of Polling

Ways to Transfer Data between Memory and Device

Multiprocessors

Idea

Build powerful computers by connecting

many smaller ones.

Multiprocessors

+ Good for timesharing

+ easy to realize

- difficult to write good concurrent programs

- hard to parallelize tasks

- mapping to architecture can be difficult

Questions

• How do parallel processors share data?— single address space— message passing

• How do parallel processors coordinate? — synchronization (locks, semaphores)— built into send / receive primitives— operating system protocols

• How are they implemented?— connected by a single bus — connected by a network

Shared Memory Multiprocessors

Problems???Symmetric multiprocessor (SMP)

Distributed Memory Multiprocessors

• Distributed shared-memory multiprocessor

• Message passing multiprocessor

Multiprocessors

Global Memory Distributed memory

Common Address Space

Symmetric Multiprocessor

Distributed shared-memorymultiprocessor

Distributed Address Space

does not exist Message passing multiprocessor

Connection Network

• Static Network• fixed connections between nodes

• Dynamic Network• packet switching (packets routed from

sender to recipient)• circuit switching (connection between

nodes can be established by crossbar or switching network)

Static Connection Networks

0

01

Circuit Switching: Delta Networks

• Route from any input x to output y by selecting links determined by successive d-ary digits of y’s label.

• This process is reversible; we can route from output y back to x by following the links determined by successive digits of x’s label.

• This self-routing property allows for simple hardware-based routing of cells.

1101

0101

0

11

1

1

x=xk-1 . . . x0 y=yk-1 . . . y0y0

xk-1y1

xk-2

yk-1 yk-2 . . . xk-1y1

yk-3

x2

yk-1 yk-2xk-1 . . . x3yk-3

yk-2

x1

yk-1xk-1 . . . x2yk-2

x0

yk-1

xk-1 . . . x1 yk-1

Network versus Bus

Performance / Unit Cost

Programming

• lock variables• semaphores• monitor• …

Cache Coherency

Outlook

• Distributed Algorithms• Distributed Systems• Parallel Programming• Parallelizing Compilers