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Winter Systems 2007 Module 2 Architectural Support for Operating Systems Ed Lazowska lazowska @cs.washi ngton.edu 570 Allen Center Even coarse architectural trends impact tremendously the design of systems Processing power doubling every 18 months 60% improvement each year factor of 100 every decade 1980: 1 MHz Apple II+ == $2,000 1980 also 1 MIPS VAX-11/780 == $120,000 2006: 3.0GHz Pentium D == $800 01/18/09 2007 Gribble, Lazowska, Levy, Zahorjan 2 Primary memory capacity same story, same reason (Moore s Law) 1972: 1MB = $1,000,000 1982: I remember pulling all kinds of strings to get a special deal: 512K of VAX-11/780 memory for $30,000 2005: 4GB vs. 2GB (@400MHz) = $800 01/18/09 2007 Gribble, Lazowska, Levy, Zahorjan 3 today: 4GB vs. 2GB (@667MHz) = $290 01/18/09 2007 Gribble, Lazowska, Levy, Zahorjan 4 Aside: Where does it all go? Facetiously: What Gordon giveth, Bill taketh away Realistically: our expectations for what the system will do increase relentlessly e.g., GUI Software is like a gas it expands to fill the available space Nathan Myhrvold (1960-) Microsoft Stock Price 01/18/09 2007 Gribble, Lazowska, Levy, Zahorjan 5 Disk capacity, 1975-1989 doubled every 3+ years 25% improvement each year factor of 10 every decade Still exponential, but far less rapid than processor performance doubling every 12 months 100% improvement each year factor of 1000 every decade 10x as fast as processor performance! Disk capacity since 1990 01/18/09 2007 Gribble, Lazowska, Levy, Zahorjan 6 Only a few years ago, we purchased disks by the megabyte (and it hurt!) Today, 1 GB (a billion bytes) costs $1 $0.50 $0.25 from Dell (except you have to buy in increments of 40 80 250 GB) => 1 TB costs $1K $500 $250, 1 PB costs $1M $500K $250K 01/18/09 2007 Gribble, Lazowska, Levy, Zahorjan 7 Optical bandwidth today Doubling every 9 months 150% improvement each year Factor of 10,000 every decade 10x as fast as disk capacity! 100x as fast as processor performance!! What are some of the implications of these trends? Just one example: We have always designed systems so that they spend processing power in order to save scarce storage and bandwidth! 01/18/09 2007 Gribble, Lazowska, Levy, Zahorjan 8 Storage Latency: How Far Away is the Data? Andromeda 10 9 Tape /Optical Robot Pluto 2,000 Years 10 6 Disk 2 Years 100 10 2 1 01/18/09 Memory On Board Cache On Chip Cache Registers Olympia 1.5 hr This Building 10 min This Room My Head 1 min 2 9 2004 Jim Gray, Microsoft Corporation 2007 Gribble, Lazowska, Levy, Zahorjan 01/18/09 2007 Gribble, Lazowska, Levy, Zahorjan 10 01/18/09 2007 Gribble, Lazowska, Levy, Zahorjan 11 01/18/09 2007 Gribble, Lazowska, Levy, Zahorjan 12 01/18/09 2007 Gribble, Lazowska, Levy, Zahorjan 13 01/18/09 2007 Gribble, Lazowska, Levy, Zahorjan 14 Lower-level architecture affects the OS even more dramatically The operating system supports sharing and protection multiple applications can run concurrently, sharing resources a buggy or malicious application can t nail other applications or the system There are many approaches to achieving this The architecture determines which approaches are viable (reasonably efficient, or even possible) includes instruction set (synchronization, I/O, ) also hardware components like MMU or DMA controllers 01/18/09 2007 Gribble, Lazowska, Levy, Zahorjan 15 Architectural support can vastly simplify (or complicate!) OS tasks e.g.: early PC operating systems (DOS, MacOS) lacked support for virtual memory, in part because at that time PCs lacked necessary hardware support Apollo workstation used two CPUs as a bandaid for nonrestartable instructions! Until very recently, Intel-based PCs still lacked support for 64-bit addressing (which has been available for a decade on other platforms: MIPS, Alpha, IBM, etc ) changing rapidly due to AMD s 64-bit architecture 01/18/09 2007 Gribble, Lazowska, Levy, Zahorjan 16 Architectural features affecting OS s These features were built primarily to support OS s: timer (clock) operation synchronization instructions (e.g., atomic test-and-set) memory protection I/O control operations interrupts and exceptions protected modes of execution (kernel vs. user) privileged instructions system calls (and software interrupts) [2006] virtualization architectures (aka Intel discovers the early 1970s) Intel: http://www.intel.com/technology/itj/2006/v10i3/1-hardware/1abstract.htm AMD: http://enterprise.amd.com/us-en/AMD-Business/BusinessSolutions/Consolidation/Virtualization.aspx 01/18/09 2007 Gribble, Lazowska, Levy, Zahorjan 17 Privileged instructions some instructions are restricted to the OS known as protected or privileged instructions e.g., only the OS can: directly access I/O devices (disks, network cards) why? manipulate memory state management page table pointers, TLB loads, etc. why? manipulate special mode bits interrupt priority level why? halt instruction why? 01/18/09 2007 Gribble, Lazowska, Levy, Zahorjan 18 OS protection So how does the processor know if a privileged instruction should be executed? the architecture must support at least two modes of operation: kernel mode and user mode VAX, x86 support 4 protection modes mode is set by status bit in a protected processor register user programs execute in user mode OS executes in kernel mode (OS == kernel) Privileged instructions can only be executed in kernel mode what happens if user mode attempts to execute a privileged instruction? 01/18/09 2007 Gribble, Lazowska, Levy, Zahorjan 19 Crossing protection boundaries So how do user programs do something privileged? e.g., how can you write to a disk if can you t execute I/O instructions? User programs must call an OS procedure OS defines a sequence of system calls how does the user-mode to kernel-mode transition happen? There must be a system call instruction, which: causes an exception (throws a software interrupt), which vectors to a kernel handler passes a parameter indicating which system call to invoke saves caller s state (registers, mode bit) so they can be restored OS must verify caller s parameters (e.g., pointers) must be a way to return to user mode once done 01/18/09 2007 Gribble, Lazowska, Levy, Zahorjan 20 A kernel crossing illustrated Firefox: read(int fileDescriptor, void *buffer, int numBytes) package arguments trap to kernel mode user mode kernel mode trap handler save registers find sys_read( ) handler in vector table sys_read( ) kernel routine restore app state, return to user mode, resume 01/18/09 2007 Gribble, Lazowska, Levy, Zahorjan 21 System call issues What would happen if kernel didn t save state? Why must the kernel verify arguments? How can you reference kernel objects as arguments or results to/from system calls? 01/18/09 2007 Gribble, Lazowska, Levy, Zahorjan 22 Memory protection OS must protect user programs from each other maliciousness, ineptitude OS must also protect itself from user programs integrity and security what about protecting user programs from OS? Simplest scheme: base and limit registers are these protected? Prog A Prog B Prog C 01/18/09 base reg limit reg base and limit registers are loaded by OS before starting program 2007 Gribble, Lazowska, Levy, Zahorjan 23 More sophisticated memory protection coming later in the course paging, segmentation, virtual memory page tables, page table pointers translation lookaside buffers (TLBs) page fault handling 01/18/09 2007 Gribble, Lazowska, Levy, Zahorjan 24 OS control flow After the OS has booted, all entry to the kernel happens as the result of an event event immediately stops current execution changes mode to kernel mode, event handler is called Kernel defines handlers for each event type specific types are defined by the architecture e.g.: timer event, I/O interrupt, system call trap when the processor receives an event of a given type, it transfers control to handler within the OS handler saves program state (PC, regs, etc.) handler functionality is invoked handler restores program state, returns to program 01/18/09 2007 Gribble, Lazowska, Levy, Zahorjan 25 Interrupts and exceptions Two main types of events: interrupts and exceptions exceptions are caused by software executing instructions e.g., the x86 int instruction e.g., a page fault, or an attempted write to a read-only page an expected exception is a trap , unexpected is a fault interrupts are caused by hardware devices e.g., device finishes I/O e.g., timer fires 01/18/09 2007 Gribble, Lazowska, Levy, Zahorjan 26 I/O control Issues: how does the kernel start an I/O? special I/O instructions memory-mapped I/O how does the kernel notice an I/O has finished? polling interrupts Interrupts are basis for asynchronous I/O device performs an operation asynchronously to CPU device sends an interrupt signal on bus when done in memory, a vector table contains list of addresses of kernel routines to handle various interrupt types who populates the vector table, and when? CPU switches to address indicated by vector index specified by interrupt signal 01/18/09 2007 Gribble, Lazowska, Levy, Zahorjan 27 Timers How can the OS prevent runaway user programs from hogging the CPU (infinite loops?) use a hardware timer that generates a periodic interrupt before it transfers to a user program, the OS loads the timer with a time to interrupt quantum how big should it be set? when timer fires, an interrupt transfers control back to OS at which point OS must decide which program to schedule next very interesting policy question: we ll dedicate a class to it Should the timer be privileged? for reading or for writing? 01/18/09 2007 Gribble, Lazowska, Levy, Zahorjan 28 Synchronization Interrupts cause a wrinkle: may occur any time, causing code to execute that interferes with code that was interrupted OS must be able to synchronize concurrent processes Synchronization: guarantee that short instruction sequences (e.g., readmodify-write) execute atomically one method: turn off interrupts before the sequence, execute it, then re-enable interrupts architecture must support disabling interrupts another method: have special complex atomic instructions read-modify-write test-and-set load-linked store-conditional 01/18/09 2007 Gribble, Lazowska, Levy, Zahorjan 29 Concurrent programming Management of concurrency and asynchronous events is biggest difference between systems programming and traditional application programming modern event-oriented application programming is a middle ground Arises from the architecture Can be sugar-coated, but cannot be totally abstracted away Huge intellectual challenge Unlike vulnerabilities due to buffer overruns, which are just sloppy programming 01/18/09 2007 Gribble, Lazowska, Levy, Zahorjan 30 Some questions Why wouldn t you want a user program to be able to access an I/O device (e.g., the disk) directly? OK, so what keeps this from happening? What prevents user programs from directly accessing the disk? So, how does a user program cause disk I/O to occur? What prevents a user program from scribbling on the memory of another user program? What prevents a user program from scribbling on the memory of the operating system? What prevents a user program from running away with the CPU? 01/18/09 2007 Gribble, Lazowska, Levy, Zahorjan 31

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