慶應義塾大学
2007年度 春学期

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

2007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19日 入出力
Lecture 11, June 19: Input/Output Systems

Outline

Talking to the Hardware

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

Buses and I/O Ports

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

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:

The following code should give you the number of clock cycles since the Pentium last rebooted:

#include 

__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());
  }
}

宿題 Homework

Your only homework this week is to report on the progress of your term project.

Your project is not complete until I have received a written report on it, in PDF format (日本語はOK). Your report should probably be 4-6 pages, depending on your project, writing format, and type and number of graphs, etc.

Next Lecture

Next lecture:

第12回 6月26日 ハイパーバイザー
Lecture 12, June 26: Hypervisors

Readings for next week:

Followup for this week:

その他 Additional Information