慶應義塾大学
2010年度 春学期

システム・ソフトウェア
System Software / Operating Systems

2010年度春学期　火曜日2時限
科目コード: 60730
開講場所：SFC
授業形態：講義
担当: Rodney Van Meter
E-mail: rdv@sfc.keio.ac.jp

第11回 6月29日 入出力
Lecture 11 (was supposed to be 10), June 29 (was supposed to be 22): Input/Output Systems
(n.b.: lectures were presented out of order)

Outline

Today's Photos

Ajisai in Kamakura Ajisai (Moto Hachimangu)

What's a Disk Drive?

Overview of a disk drive (Wikipedia)

A disk drive stores data in sectors that held on tracks; all of the tracks at the same distance from the spindle are called a cylinder.

It uses a read/write head attached to a slider, mounted on an actuator arm, to read and write the data as it spins past.

Cylinder, head, sector (Wikicommons) Magnetic Media and head

The Importance of a Disk Drive

In an architectural sense, what's important about disk drives?

...and yet, the Information Revolution （情報革命？） can fairly be said to be built on disk drives. Without them, there would be no PCs, no Google.

Disk drive industry shipments, in terabytes

The Access Time Gap

H-P Fig. 6.1

The Insides of a Disk Drive

Hard disk reflection (Wikipedia) Hard disk anatomy (Wikipedia) Head/actuator on platter (Wikipedia) Disk Slider and head (Wikipedia)

Disk Drive Trends

Hard drive capacity over time (Wikipedia) Trends of Disk Transfer Rates

Talking to the Hardware

In order to understand I/O, we need to briefly review the hardware architecture...

Buses and I/O Ports

Basic architecture of a PC
Systems generally consist of multi-level attachments that provide differing types of aggregation. Some physical devices sit on, or close to, the main memory bus; others are kept more distant via some sort of controller.

There are a number of common types of controller:

As it gets easier to put more hardware into the devices themselves, they exhibit more complex behavior, including helping the OS identify them. We will come back to that later. First, some examples of the device types:

Device Types

Okay, so what kind of devices are we actually talking to? There are many, many kinds of I/O devices. Here are the classes that SCSI defines: Here are the USB device types:

And neither of these lists includes graphics devices such as the monitor or graphics display itself. Other devices requiring similar I/O control may include specialized processors, and of course all manner of scientific equipment.

Obviously, all of this requires a lot of software; in Linux, there are almost 3,300 different device drivers! Don't worry, the complexity is actually quite manageable; we'll come back to that when we discuss the drivers themselves below.

Doing the Talking

...okay. Now, how do we talk to a device? There are two basic ways for the CPU to talk to hardware devices:

When using I/O instructions, the CPU executes an IN or OUT instruction, which reads from or writes to a separate address space (namespace) for I/O devices, usually attached to a separate bus.

Those methods refer to how the CPU talks to, or controls the device. In both cases, there are two primary ways to get your actual data out:

DMA involves setting up some other piece of hardware besides the CPU to actually control the I/O and move the data from the device into main memory. DMA may be done to virtual or physical addresses. The primary advantage of virtual is that it supports scatter/gather I/O. These days, most device controllers support scatter/gather directly for physical addresses anyway, and with the MMU incorporated into the CPU it's a little harder to use the address translation hardware, so it's not commonly done any more.

Device Drivers

So far, we've hardly said a word about the operating system. The device driver is the primary piece of the OS that is responsible for managing I/O.

As you might expect from the initial discussion of hardware, there are several levels of device drivers, starting with software to control the actual buses and going on down to the devices. The bus drivers are used more or less as a library of functions for the actual device drivers.

In most modern systems, the device driver that matches a particular device can be loaded as a kernel module after the device is identified by the OS.

A device driver must follow a particular form, which is very dependent on the operating system. Over the last several years, there has been a push for OS-independent device drivers, so that OS developers can share the same code for a device independent of whether it was developed for Windows, Linux, or Mac.

Character and Block Devices

Very early on in the development of Unix, the authors made a brilliant decision: they devided hardware devices into two classes, the block devices and the character devices. The primary difference between the two is that file systems can be mounted on block devices, requiring additional functions from the device driver.

Top Half and Bottom Half

In Unix, the code for a device driver is divided into the top half and the bottom half. (The bottom half is usually much less than half of the total code, though.) The bottom half is essentially the interrupt handler, and it must be prepared to run at any time, with the system in any state. The top half generally runs with the system set to the state (e.g., memory map) of the process that is scheduling (or has scheduled) the I/O.

Naming

Devices used to be named strictly according to their bus address, which was simple and never changed. In today's Plug-and-Play (PNP) world, that's simply not so. Morever, different flash drives can be plugged into the same slot and use the same address (over time), but the OS should treat them differently!

Ideally, devices would always identify themselves completely. Most devices provide some identification, but those that store data could, and should, make more effective use of the volume name, which is generally embedded in the device.

Performance

The overall performance goal, as we discussed in the lecture on process scheduling, is generally balance between keeping the CPU busy and keeping the devices busy. For the moment, we are really only concerned with how to achieve the highest I/O rate (measured in throughput or I/O operations per second).

The principle reasons that I/O slows down are:

Device Stalls

For a disk drive, a common form of disk performance problem is a rotational miss. Disks also must seek, and poor choices in ordering seeks can ruin your performance, but we don't have time to go into that right now.

For tape drives, underflowing or overflowing a buffer results in a tape stall, which is extremely expensive.

Data Copies in the File System and Elsewhere

Data copies in the file system
Data copies for NFS
A couple of weeks ago we talked about file system APIs. At one point, we talked about the alignment of application file read/write buffers. In modern C/Unix APIs, the buffer can be any place in memory, but in older systems, buffers always had to be aligned to the size of the system memory page.

In Unix systems, it is also true that disk I/Os are done in multiples of a page size, and the I/O is also done to page boundaries. So how are the API and the I/O system reconciled? Through the file system buffer cache. The buffer cache serves two important purposes: the first is alignment, and the second is buffering, to allow speed matching of I/O and allow the application to continue while I/O is handled by the kernel on its behalf.

Packets arrive into the system in a variety of sizes. Worse, in general, you don't know which process (if any!) wants the packet until you get it into memory and examine the headers.

These effects cumulatively mean that data copies are common in operating systems, and they have an enormous impact on system performance:

Interrupt Rate and Coalescing Interrupts

How many interrupts per second do you get from a 100Mbps Ethernet card with 1500B frames? What about a 10Gbps Ethernet card with minimum-size frames?

Tools

On Linux, and on some other Unix systems, the following tools are useful:

You may be interested in the following benchmarks:

On most modern Intel (and AMD?) processors, the following code should give you the number of clock cycles since the OS last rebooted:

#include >stdio.h<

__inline__ unsigned long long int rdtsc()
   {
     unsigned long long int x;
     __asm__ volatile (".byte 0x0f, 0x31" : "=A" (x));
     return x;
   }

main()
{
  int i;

  for ( i = 0 ; i < 100 ; i++ ) {
    printf("%ld\n",rdtsc());
  }
}

Homework, Etc.

宿題 Homework

This is the last homework!!! After this week, your homework responsibility is your term project.
  1. Imagine that the bitmap showing which disk blocks are free has become unreliable, so you decide to rebuild it by walking through all of the in-use inodes. Assume on-disk inodes are 128 bytes, and the file system was initialized to hold a maximum of one million files. Your disk has a transfer rate of 40 megabytes/second, and can execute a random operation in 10milliseconds. It holds 100GB.
    1. If all of the inodes are stored in one contiguous chunk of disk, how long will it take to read them all?
    2. If 100,000 of the files each use a single indirect block that is randomly placed on the disk, how long will it take to work through all of them?
    3. Using the same type of disk, how long would it take to read every 4KB block on the disk in random order?
  2. Do you back up the data on your computer? If so, how?
    1. Execute a data backup. This backup may be of any type, using any tool, and may be just your user files or may be the entire system. You may back up to CD/DVD, external disk, tape, or over the network to a server.
    2. Report how long it took to perform your backup, and how much data was transferred.
  3. Report the status of your term project.

Next Lecture

Next lecture:

Followup for this week:

その他 Additional Information