55:035 Computer Architecture and Organization

1. 55:035Computer Architecture and Organization Lecture 11

2. Outline • Interrupts • Program Flow • Multiple Interrupts • Nesting • IO • Architecture • Bus Types • Transfer Methods • Disks • Disk Arrays 55:035 Computer Architecture and Organization

3. Interrupts • Mechanism by which other modules (e.g. I/O) may interrupt normal sequence of processing • Program • e.g. overflow, division by zero • Timer • Generated by internal processor timer • Used in pre-emptive multi-tasking • I/O • from I/O controller • Hardware failure • e.g. memory parity error 55:035 Computer Architecture and Organization

4. Interrupt Cycle • Added to instruction cycle • Processor checks for interrupt • Indicated by an interrupt signal • If no interrupt, fetch next instruction • If interrupt pending: • Suspend execution of current program • Save context • Set PC to start address of interrupt handler routine • Process interrupt • Restore context and continue interrupted program

5. Transfer of Control via Interrupts

6. Program Flow Control

7. Program Timing Short I/O Wait

8. Program Timing Long I/O Wait

9. Multiple Interrupts • Disable interrupts • Processor will ignore further interrupts whilst processing one interrupt • Interrupts remain pending and are checked after first interrupt has been processed • Interrupts handled in sequence as they occur • Define priorities • Low priority interrupts can be interrupted by higher priority interrupts • When higher priority interrupt has been processed, processor returns to previous interrupt

10. Multiple Interrupts - Sequential 55:035 Computer Architecture and Organization

11. Multiple Interrupts – Nested 55:035 Computer Architecture and Organization

12. Time Sequence of Multiple Interrupts 55:035 Computer Architecture and Organization

13. Input/Output & System Performance Issues • System Architecture & I/O Connection Structure • Types of Buses/Interconnects in the system. • I/O Data Transfer Methods. • Cache & I/O: The Stale Data Problem • I/O Performance Metrics. • Magnetic Disk Characteristics. • Designing an I/O System & System Performance: • Determining system performance bottleneck. • (which component creates a system performance bottleneck) 55:035 Computer Architecture and Organization

14. The Von-Neumann Computer Model • Partitioning of the computing engine into components: • Central Processing Unit (CPU): Control Unit (instruction decode, sequencing of operations), Datapath (registers, arithmetic and logic unit, buses). • Memory: Instruction (program) and operand (data) storage. • Input/Output (I/O): Communication between the CPU and the outside world - Input Control Memory (instructions, data) I/O Subsystem Datapath registers ALU, buses Output I/O Devices CPU Computer System System performance depends on many aspects of the system (“limited by weakest link in the chain”)

15. Input and Output (I/O) Subsystem • The I/O subsystem provides the mechanism for communication between the CPU and the outside world (I/O devices). • Design factors: • I/O device characteristics (input, output, storage, etc.). • I/O Connection Structure (degree of separation from memory operations). • I/O interface (the utilization of dedicated I/O and bus controllers). • Types of buses (processor-memory vs. I/O buses). • I/O data transfer or synchronization method (programmed I/O, interrupt-driven, DMA). 55:035 Computer Architecture and Organization

16. Typical System Architecture System Bus or Front Side Bus (FSB) Memory Controller (Chipset North Bridge) I/O Controller Hub (Chipset South Bridge) Isolated I/O I/O Subsystem

17. Memory Controller NICs Memory System Components Time(workload) = Time(CPU) + Time(I/O) - Time(Overlap) L1 L2 L3 CPU (possibly on-chip) Important issue: Which component creates a system performance bottleneck? Caches (FSB) System Bus SDRAM PC100/PC133 100-133MHz 64-128 bits wide 2-way inteleaved ~ 900 MBYTES/SEC )64bit) Double Date Rate (DDR) SDRAM PC3200 200 MHz DDR 64-128 bits wide 4-way interleaved ~3.2 GBYTES/SEC (64bit) RAMbus DRAM (RDRAM) 400MHZ DDR 16 bits wide (32 banks) ~ 1.6 GBYTES/SEC Bus Adapter Main I/O Bus Example: PCI, 33-66MHz 32-64 bits wide 133-528 MB/s PCI-X 133MHz 64-bits wide 1066 MB/s Memory Bus I/O Controllers Disks Displays Keyboards Networks Chipset I/O Devices Chipset I/O Subsystem North Bridge South Bridge 55:035 Computer Architecture and Organization

18. I/O Interface I/O Interface, I/O controller or I/O bus adapter: • Specific to each type of I/O device. • To the CPU, and I/O device, it consists of a set of control and data registers (usually memory-mapped) within the I/O address space. • On the I/O device side, it forms a localized I/O bus which can be shared by several I/O devices • (e.g IDE, SCSI, USB ...) • Handles I/O details (originally done by CPU) such as: • Assembling bits into words, • Low-level error detection and correction • Accepting or providing words in word-sized I/O registers. • Presents a uniform interface to the CPU regardless of I/O device. Processing off-loaded from CPU 55:035 Computer Architecture and Organization

19. I/O Controller Architecture Chipset North Bridge Chipset South Bridge Peripheral or Main I/O Bus (PCI, PCI-X, etc.) Peripheral Bus Interface/DMA Host Micro-controller or Embedded processor Memory Buf fer Memory µProc Processor Cache ROM Host I/O Channel Interface Processor I/O Contr oller SCSI, IDE, USB, ….

20. Types of Buses in The System (1/2) • Processor-Memory Bus • System Bus, Front Side Bus, (FSB) • Should offer very high-speed (bandwidth) and low latency. • Matched to the memory system performance to maximize memory-processor bandwidth. • Usually design-specific (not an industry standard). • Examples: • Alpha EV6 (AMD K7), Peak bandwidth = 400 MHz x 8 = 3.2 GB/s • Intel GTL+ (P3), Peak bandwidth = 133 MHz x 8 = 1 GB/s • Intel P4, Peak bandwidth = 800 Mhz x 8 = 6.4 GB/s • HyperTransport 2.0: 200Mhz-1.4GHz, Peak bandwidth up to 22.8 GB/s (point-to-point system interconnect not a bus)

21. Types of Buses in The System (2/2) • I/O buses (sometimes called an interface): • Follow bus industry standards. • Usually formed by I/O interface adapters to handle many types of connected I/O devices. • Wide range in the data bandwidth and latency • Not usually interfaced directly to memory instead connected processor-memory bus via a bus adapter (chipset south bridge). • Examples: • Main system I/O bus: PCI, PCI-X, PCI Express • Storage: SATA, IDE, SCSI. 55:035 Computer Architecture and Organization

22. Intel Pentium 4 System Architecture (Using The Intel 925 Chipset) CPU (Including cache) System Bus (Front Side Bus, FSB) Bandwidth usually should match or exceed that of main memory Memory Controller Hub (Chipset North Bridge) System Memory Two 8-byte DDR2 Channels Graphics I/O Bus (PCI Express) Storage I/O (Serial ATA) Main I/O Bus (PCI) Misc. I/O Interfaces Misc. I/O Interfaces I/O Controller Hub (Chipset South Bridge) I/O Subsystem 55:035 Computer Architecture and Organization

23. Option High performance Low cost/performance Bus width Separate address Multiplex address & data lines & data lines Data width Wider is faster Narrower is cheaper (e.g., 64 bits) (e.g., 16 bits) Transfer size Multiple words has Single-word transfer less bus overhead is simpler Bus masters Multiple Single master (requires arbitration) (no arbitration) Split Yes, separate No , continuous transaction? Request and Reply connection is cheaper packets gets higher and has lower latency bandwidth (needs multiple masters) Clocking Synchronous Asynchronous Bus Characteristics

24. IDE/Ultra ATASCSI Data Width 16 bits 8 or 16 bits (wide) Clock Rate Upto 100MHz 10MHz (Fast) 20MHz (Ultra) 40MHz (Ultra2) 80MHz (Ultra3) 160MHz (Ultra4) Bus Masters 1 Multiple Max no. devices 2 7 (8-bit bus) 15 (16-bit bus) Peak Bandwidth 200 MB/s 320MB/s (Ultra4) Storage IO Interfaces/Buses 55:035 Computer Architecture and Organization

25. Time(workload) = Time(CPU) + Time(I/O) - Time(Overlap) I/O Data Transfer Methods (1/2) • Programmed I/O (PIO): Polling (For low-speed I/O) • The I/O device puts its status information in a status register. • The processor must periodically check the status register. • The processor is totally in control and does all the work. • Very wasteful of processor time. • Used for low-speed I/O devices (mice, keyboards etc.) 55:035 Computer Architecture and Organization

26. I/O Data Transfer Methods (2/2) • Interrupt-Driven I/O (For medium-speed I/O): • An interrupt line from the I/O device to the CPU is used to generate an I/O interrupt indicating that the I/O device needs CPU attention. • The interrupting device places its identity in an interrupt vector. • Once an I/O interrupt is detected the current instruction is completed and an I/O interrupt handling routine (by OS) is executed to service the device. • Used for moderate speed I/O (optical drives, storage, neworks ..) • Allows overlap of CPU processing time and I/O processing time 55:035 Computer Architecture and Organization

27. I/O data transfer methods • Direct Memory Access (DMA) (For high-speed I/O): • Implemented with a specialized controller that transfers data between an I/O device and memory independent of the processor. • The DMA controller becomes the bus master and directs reads and writes between itself and memory. • Interrupts are still used only on completion of the transfer or when an error occurs. • Low CPU overhead, used in high speed I/O (storage, network interfaces) • Allows more overlap of CPU processing time and I/O processing time than interrupt-driven I/O. 55:035 Computer Architecture and Organization

28. DMA transfer step • DMA transfer steps: • The CPU sets up DMA by supplying device identity, operation, memory address of source and destination of data, the number of bytes to be transferred. • The DMA controller starts the operation. When the data is available it transfers the data, including generating memory addresses for data to be transferred. • Once the DMA transfer is complete, the controller interrupts the processor, which determines whether the entire operation is complete. 55:035 Computer Architecture and Organization

29. Cache & I/O: The Stale Data Problem • Three copies of data, may exist in: cache, memory, disk. • Similar to cache coherency problem in multiprocessor systems. • CPU or I/O (DMA) may modify/access one copy while other copies contain stale (old) data. • Possible solutions: • Connect I/O directly to CPU cache: CPU performance suffers. • With write-back cache, the operating system flushes caches into memory (forced write-back) to make sure data is not stale in memory. • Use write-through cache; I/O receives updated data from memory (This uses too much memory bandwidth). • The operating system designates memory address ranges involved in I/O DMA operations as non-cacheable.

30. I/O Connected Directly To Cache This solution may slow down CPU performance DMA I/O A possible solution for the stale data problem However: CPU performance suffers

31. Factors Affecting Performance • I/O processing computational requirements: • CPU computations available for I/O operations. • Operating system I/O processing policies/routines. • I/O Data Transfer/Processing Method: Polling, Interrupt Driven. DMA • I/O Subsystem performance: • Raw performance of I/O devices (i.e magnetic disk performance). • I/O bus capabilities. • I/O subsystem organization. i.e number of devices, array level .. • Loading level of I/O devices (queuing delay, response time). • Memory subsystem performance: • Available memory bandwidth for I/O operations (For DMA) • Operating System Policies: • File system vs. Raw I/O. • File cache size and write Policy.

32. I/O Performance Metrics: Throughput: • Throughput is a measure of speed—the rate at which the I/O or storage system delivers data. • I/O Throughput is measured in two ways: • I/O rate, Measured in: • Accesses/second, • Transactions Per Second (TPS) or, • I/O Operations Per Second (IOPS). • I/O rate is generally used for applications where the size of each request is small, such as in transaction processing. • Data rate, measured in bytes/second or megabytes/second (MB/s). • Data rate is generally used for applications where the size of each request is large, such as in scientific and multimedia applications.

33. Seek Time Magnetic Disks Current Rotation speed 7200-15000 RPM Characteristics: Diameter (form factor): 2.5in - 5.25in • Rotational speed: 3,600RPM-15,000 RPM • Tracks per surface. • Sectors per track: Outer tracks contain more sectors. • Recording or Areal Density: Tracks/in X Bits/in • Cost Per Megabyte. • Seek Time: (2-12 ms) The time needed to move the read/write head arm. Reported values: Minimum, Maximum, Average. • Rotation Latency or Delay: (2-8 ms) The time for the requested sector to be under the read/write head. (~ time for half a rotation) • Transfer time: The time needed to transfer a sector of bits. • Type of controller/interface: SCSI, EIDE • Disk Controller delay or time. • Average time to access a sector of data = average seek time + average rotational delay + transfer time + disk controller overhead (ignoring queuing time) Access time = average seek time + average rotational delay

34. Read Access • Steps • Memory mapped I/O over bus to controller • Controller starts access • Seek + rotational latency wait • Sector is read and buffered (validity check) • Controller DMA’s to memory and says ready • Access time Queue + controller delay +block size/bandwidth + seek time + transfer time + check delay 55:035 Computer Architecture and Organization

35. Basic Disk Performance Example • Given the following Disk Parameters: • Average seek time is 5 ms • Disk spins at 10,000 RPM • Transfer rate is 40 MB/sec • Controller overhead is 0.1 ms • Assume that the disk is idle, so no queuing delay exists • What is Average Disk read or write service time for a 500-byte (.5 KB) Sector? Avg. seek + avg. rot delay + transfer time + controller overhead = 5 ms + 0.5/(10000 RPM/60) + 0.5 KB/40 MB/s + 0.1 ms = 5 + 3 + 0.13 + 0.1 = 8.23 ms Actual time to process the disk request is greater and may include CPU I/O processing Time and queuing time Tservice (Disk Service Time for this request) Time for half a rotation Here: 1KBytes = 103 bytes, MByte = 106 bytes, 1 GByte = 109 bytes

36. Disk Arrays Disk Product Families Conventional: 4 disk designs 14” 3.5” 5.25” 10” High End Low End Disk Array: 1 disk design 3.5” 55:035 Computer Architecture and Organization

37. Array Reliability • Reliability of N disks = Reliability of 1 Disk / N • 50,000 Hours / 70 disks = 700 hours • Disk system MTBF: Drops from 6 years to 1 month! • • Arrays (without redundancy) too unreliable to be useful! Hot spares support reconstruction in parallel with access: very high media availability can be achieved 55:035 Computer Architecture and Organization

38. Redundant Array of Disks • Files are "striped" across multiple spindles • Redundancy yields high data availability Disks will fail Contents reconstructed from data redundantly stored in the array Capacity penalty to store it Bandwidth penalty to update Mirroring/Shadowing (high capacity cost) Horizontal Hamming Codes (overkill) Parity & Reed-Solomon Codes Failure Prediction (no capacity overhead!) VaxSimPlus — Technique is controversial Techniques: 55:035 Computer Architecture and Organization

39. RAID Levels

40. Raid 1: Disk Mirroring recovery group • Each disk is fully duplicated onto its "shadow" Very high availability can be achieved • Bandwidth sacrifice on write: Logical write = two physical writes • Reads may be optimized • Most expensive solution: 100% capacity overhead Targeted for high I/O rate, high availability environments 55:035 Computer Architecture and Organization

41. Raid 3: Parity Disk P 10010011 11001101 10010011 . . . 1 0 0 1 0 0 1 1 1 1 0 0 1 1 0 1 1 0 0 1 0 0 1 1 0 0 1 1 0 0 0 0 logical record Striped physical records • • Parity computed across recovery group to protect against HD failures • 33% capacity cost for parity in this configuration • wider arrays reduce capacity costs, decrease expected availability, increase reconstruction time • • Arms logically synchronized, spindles rotationally synchronized • logically a single high capacity, high transfer rate disk Targeted for high bandwidth applications: Scientific, Image Processing

42. Raid 5+: High I/O Rate Parity Increasing Logical Disk Addresses D0 D1 D2 D3 P A logical write becomes four physical I/Os Independent writes possible because of interleaved parity Reed-Solomon Codes ("Q") for protection during reconstruction D4 D5 D6 P D7 D8 D9 P D10 D11 D12 P D13 D14 D15 Stripe P D16 D17 D18 D19 Stripe Unit D20 D21 D22 D23 P Targeted for mixed applications . . . . . . . . . . . . . . . Disk Columns

43. Subsystem Organization array controller host single board disk controller host adapter manages interface to host, DMA single board disk controller control, buffering, parity logic single board disk controller physical device control single board disk controller striping software off-loaded from host to array controller no applications modifications no reduction of host performance often piggy-backed in small format devices

44. System Availability Array Controller String Controller . . . String Controller . . . String Controller . . . String Controller . . . String Controller . . . Data Recovery Group: unit of data redundancy Redundant Support Components: fans, power supplies, controller, cables End to End Data Integrity: internal parity protected data paths

45. System-Level Availability host host Fully dual redundant I/O Controller I/O Controller Array Controller Array Controller . . . . . . . . . Goal: No Single Points of Failure . . . . . . Recovery Group . . . with duplicated paths, higher performance can be obtained when there are no failures

46. Peripheral Component Interconnect • 2 Types of Agents on the Bus • Initiator (master) • Target • 3 Address Spaces • Memory • IO • Configuration • Transactions done in 2 (or more) phases • Address/Command • Data/Byte Enable Phase(s) • Synchronous Operation (positive edge of clock) 55:035 Computer Architecture and Organization

47. Host Main PCI bridge memory PCI bus Ethernet Disk Printer interf ace Typical PCI Topology 55:035 Computer Architecture and Organization

48. PCI Signals Name F unction CLK A 33-MHz or 66-MHz clo c k. FRAME# Sent by the initiator to indicate the start and duration of a transaction. AD 32 address/data lines, whic h ma y b e optionally increased to 64. 4 command/byte enable lines (8 for a 64-bit bus.) C/BE# IRD Y#, TRD Y# Initiator-ready and T arget-ready signals. DEVSEL# A resp onse from the device indicating that it has recognized its address and is ready for a data transfer transaction. IDSEL# Initialization Device Select. 55:035 Computer Architecture and Organization

49. 1 2 3 4 5 6 7 CLK Frame# Adress #1 #4 #2 #3 AD Cmnd Byte enable C/BE# IRD Y# TRD Y# DEVSEL# PCI Read 55:035 Computer Architecture and Organization