avishai wool lecture 8 - 1 introduction to systems programming lecture 8 paging design input-output

Avishai Woollecture 8 - 1

Introduction to Systems Programming Lecture 8

Paging Design

Input-Output

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Steps in Handling a Page Fault

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VirtualPhysical mapping

• CPU accesses virtual address 100000

• MMU looks in page table to find physical address– Page table is in memory too

• Unreasonable overhead!

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TLB: Translation Lookaside Buffer

• Idea: Keep the most frequently used parts of the page table in a cache, inside the MMU chip.

• TLB holds a small number of page table entries: Usually 8 – 64

• TLB hit rate very high because, e.g., instructions fetched sequentially.

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A TLB to speed up paging

Example: • Code loops through pages 19,20,21• Uses data array in pages 129,130,140• Stack variables in pages 860,861

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Valid TLB Entries

• TLB miss:– Do regular page lookup– Evict a TLB entry and store the new TLB entry– Miniature paging system, done in hardware

• When OS does context switch to a new process, all TLB entries become invalid: – Early instructions of new process will cause TLB

misses.

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TLB placement/eviction

• Done by hardware

• Placement rule:– TLBIndex = VirtualAddr modulo TLBSize– TLBSize is always 2k

TLBIndex = k least-significant bits

– Keep “tag” (rest of bits) to fully identify virtual addr

• Virtual address can be only in one TLB index

• No explicit “eviction”: simply overwrite what is in TLB[TLBIndex]

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TLB + Page table lookup

In TLB?In pagetable?

Page fault:copy from

disk to memory

Virtual address

Physical address

No

Yes Yes; update TLB

No

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TLB – cont.

• If address is in TLB page is in physical memory– OS invalidates TLB entry when evicting a page– So page fault not possible if we have a TLB hit

• “page fault rate” is computed only on TLB misses

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Example: Average memory access time

• TLB lookup: 4ns• Phys mem access: 10ns• Disk access: 10ms

• TLB miss rate: 1%• Page fault rate: 0.1%

• Assume page table is in memory.

p=0.99, time=4ns+10ns

Page hit: p=0.01*0.999, time=4ns+10ns+10ns

Page fault: p=0.01*0.001, time=4ns+10ns+10ms+10nsTLB miss

TLB hit

Average memory access: 114.1ns (1.141*10-7)

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Design issues in Paging

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Local versus Global Allocation Policies:Physical Memory

a) Original configuration – ‘A’ causes page faultb) Local page replacementc) Global page replacement

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Local or Global?

• Local number of frames per process is fixed– If working set grows thrashing– If working set shrinks waste

• Global usually better

• Some algorithms can only be local (working set, WSClock).

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How many frames to give a process?

• Fixed number

• Proportional to its size (before load)

• Zero, let it issue page faults for all its pages.– This is called pure demand paging.

• Monitor page-fault-frequency (PFF), give more pages if PFF high.

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Page fault rate as a function of the number of page frames assigned

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Load Control• Despite good designs, system may still thrash• When PFF algorithm indicates

– some processes need more memory – but no processes need less

• Solution: Reduce number of processes competing for memory– swap one or more to disk, divide up frames they held– reconsider degree of multiprogramming

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Cleaning Policy

• Need for a background process, paging daemon– periodically inspects state of memory

• When too few frames are free– selects pages to evict using a replacement algorithm

• It can use same circular list (clock) – as regular page replacement algorithm but with diff ptr

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Windows XP Page Replacement• Processes are assigned working set minimum and

working set maximum• Working set minimum is the minimum number of page

frames the process is guaranteed to have in memory• A process may be assigned as many page frames up to

its working set maximum• When the amount of free memory in the system falls

below a threshold, automatic working set trimming is performed to restore the amount of free memory

• Working set trimming removes frames from processes that have more than their working set minimum

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Devices, Controllers, and I/O Architectures

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I/O Device Types

• Block Devices– block size of 512-32768 bytes– block can be read/written individually– typical: disks / floppy / CD

• Character Devices– delivers / accepts a sequential stream of characters– non-addressable – typical: keyboard, mouse, printer, network

• Other: Monitor, Clock

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Typical Data Rates

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Device Controllers

• I/O devices have components:– mechanical component – electronic component

• The electronic component is the device controller– may be able to handle multiple devices

• Controller's tasks– convert serial bit stream to block of bytes– perform error correction as necessary– make available to main memory

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Communicating with Controllers

• Controllers have registers to deliver data, accept data, etc.

• Option 1: special I/O commands, I/O ports in r0, 4

• “4” is not memory address 4, it is I/O port 4

• Option 2: I/O registers mapped to memory addresses

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Memory-Mapped Registers

• Controller connected to the bus

• Has a physical “memory address” like B0000000

• When this address appears on the bus, the controller responds (read/write to its I/O register)

• RAM configured to ignore controller’s address

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Possible I/O Register Mappings

• Separate I/O and memory space (IBM 360)• Memory-mapped I/O (PDP-11)• Hybrid (Pentium, 640K-1M are for I/O)

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Advantages of Memory Mapped I/O

• No special instructions, can be written in C.

• Protection by not putting I/O memory in user virtual address space.

• All machine instructions can access I/O:LOOP: test *b0000004 // check if port_4 is 0 beq READY branch LOOP

READY: ...

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Disadvantages of Memory Mapped I/O

• Memory and I/O controllers have to be on the same bus:– modern architectures have separate memory bus!– Pentium has 3 buses: memory, PCI, ISA

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Bus Architectures

(a) A single-bus architecture(b) A dual-bus memory architecture

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Memory Mapped with Separate Bus

• I/O Controllers do not see memory bus.

• Option 1: all addresses to memory bus. No response I/O bus

• Option 2: Snooping device between buses– speed difference is a problem

• Option 3 (Pentium): filter addresses in PCI bridge

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Structure of a large Pentium system

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Principles of I/O Software

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Goals of I/O Software

• Device independence– programs can access any I/O device – without specifying device in advance

· (floppy, hard drive, or CD-ROM)

• Uniform naming– name of a file or device a string or an integer– not depending on which machine

• Error handling– handle as close to the hardware as possible

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Goals of I/O Software (2)

• Synchronous vs. asynchronous transfers– blocked transfers vs. interrupt-driven

• Buffering– data coming off a device cannot be stored in final

destination

• Sharable vs. dedicated devices– disks are sharable– tape drives would not be

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How is I/O Programmed

• Programmed I/O

• Interrupt-driven I/O

• DMA (Direct Memory Access)

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Programmed I/O

Steps in printing a string

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Polling

Busy-waiting until device can accept another character

Example assumes memory-mapped registers

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Properties of Programmed I/O

• Simple to program

• Ties up CPU, especially if device is slow

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Interrupts Revisited

bus

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Interrupt-Driven I/O

Code executed when print system call is made

Interrupt service procedure

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Properties of Interrupt-Driven I/O

• Interrupt every character or word.

• Interrupt handling takes time.

• Makes sense for slow devices (keyboard, mouse)

• For fast device: use dedicated DMA controller – usually for disk and network.

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Direct Memory Access (DMA)

• DMA controller has access to bus.

• Registers:– memory address to write/read from– byte count– I/O port or mapped-memory address to use– direction (read from / write to device)– transfer unit (byte or word)

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Operation of a DMA transfer

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I/O Using DMA

code executed when the print system call is made

interrupt service procedure

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DMA with Virtual Memory

• Most DMA controllers use physical addresses

• What if memory of buffer is paged out during DMA transfer?

• Force the page to not page out (“pinning”)

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Burst or Cycle-stealing

• DMA controller grabs bus for one word at a time, it competes with CPU bus access. This is called “cycle-stealing”.

• In “burst” mode the DMA controller acquires the bus (exclusively), issues several transfers, and releases. – More efficient – May block CPU and other devices

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Concepts for review• TLB

• Local/Global page replacement

• Demand paging

• Page-fault-frequency monitor

• I/O device controller

• in/out commands

• Memory-mapped registers

• PCI Bridge

• Programmed I/O (Polling)

• Interrupt-driven I/O

• I/O using DMA

• Page pinning

• DMA cycle-stealing

• DMA burst mode