Operating Systems

Start Lecture #26

Since PIO is pure software it is easier to change, which is an advantage.

DMA does need a number of bus transfers from the CPU to the controller to specify the DMA. So DMA is most effective for large transfers where the setup is amortized.

Why have the buffer? Why not just go from the disk straight to the memory?

  1. Speed matching.
    The disk supplies data at a fixed rate, which might exceed the rate the memory can accept it. In particular the memory might be busy servicing a request from the processor or from another DMA controller.
    Alternatively, the disk might supply data at a slower rate than the memory (and memory bus) can handle thus under-utilizing an important system resource.
  2. Error detection and correction.
    The disk controller verifies the checksum written on the disk.

Homework: 12

5.1.5 Interrupts Revisited

Precise and Imprecise Interrupts

5.2 Principles of I/O Software

As with any large software system, good design and layering is important.

5.2.1 Goals of the I/O Software

Device Independence

We want to have most of the OS to be unaware of the characteristics of the specific devices attached to the system. (This principle of device independence is not limited to I/O; we also want the OS to be largely unaware of the CPU type itself.)

This objective has been accomplished quite well for files stored on various devices. Most of the OS, including the file system code, and most applications can read or write a file without knowing if the file is stored on a floppy disk, an internal SATA hard disk, an external USB SCSI disk, an external USB Flash Ram, a tape, or (for reading) a CD-ROM.

This principle also applies for user programs reading or writing streams. A program reading from ``standard input'', which is normally the user's keyboard can be told to instead read from a disk file with no change to the application program. Similarly, ``standard output'' can be redirected to a disk file. However, the low-level OS code dealing with disks is rather different from that dealing keyboards and (character-oriented) terminals.

One can say that device independence permits programs to be implemented as if they will read and write generic or abstract devices, with the actual devices specified at run time. Although writing to a disk has differences from writing to a terminal, Unix cp, DOS copy, and many programs we compose need not be aware of these differences.

However, there are devices that really are special. The graphics interface to a monitor (that is, the graphics interface presented by the video controller—often called a ``video card'') does not resemble the ``stream of bytes'' we see for disk files.

Homework: What is device independence?

Uniform naming

We have already discussed the value of the name space implemented by file systems. There is no dependence between the name of the file and the device on which it is stored. So a file called IAmStoredOnAHardDisk might well be stored on a floppy disk.

More interesting once a device is mounted on (Unix) directory, the device is named exactly the same as the directory was. So if a CD-ROM was mounted on (existing) directory /x/y, a file named joe on the CD-ROM would now be accessible as /x/y/joe.

Error handling

There are several aspects to error handling including: detection, correction (if possible) and reporting.

  1. Detection should be done as close to where the error occurred as possible before more damage is done (fault containment). Moreover, the error may be obvious at the low level, but harder to discover and classify if the erroneous data is passed to higher level software.

  2. Correction is sometimes easy, for example ECC memory does this automatically (but the OS wants to know about the error so that it can request replacement of the faulty chips before unrecoverable double errors occur).

    Other easy cases include successful retries for failed ethernet transmissions. In this example, while logging is appropriate, it is quite reasonable for no action to be taken.

  3. Error reporting tends to be awful. The trouble is that the error occurs at a low level but by the time it is reported the context is lost.

Creating the illusion of synchronous I/O

I/O must be asynchronous for good performance. That is the OS cannot simply wait for an I/O to complete. Instead, it proceeds with other activities and responds to the interrupt that is generated when the I/O has finished.

Users (mostly) want no part of this. The code sequence 

    Read X
    Y = X+1
    Print Y
should print a value one greater than that read. But if the assignment is performed before the read completes, the wrong value can easily be printed.

Performance junkies sometimes do want the asynchrony so that they can have another portion of their program executed while the I/O is underway. That is, they implement a mini-scheduler in their application code.
See this message from linux kernel developer Ingo Molnar for his take on asynchronous IO and kernel/user threads/processes. You can find the entire discussion here.

Buffering

Buffering is often needed to hold data for examination prior to sending it to its desired destination.

Since this involves copying the data, which can be expensive, modern systems try to avoid as much buffering as possible. This is especially noticeable in network transmissions, where the data could conceivably be copied many times.

  1. From user space to kernel space as part of the write system call.
  2. From kernel space to a kernel I/O buffer.
  3. From the I/O buffer to a buffer on the network adaptor.
  4. From the adapter on the source to the adapter on the destination.
  5. From the destination adapter to an I/O buffer.
  6. From the I/O buffer to kernel space.
  7. From kernel space to user space as part of the read system call.

I am not sure if any systems actually do all seven.

Sharable vs. Dedicated Devices

For devices like printers and CD-ROM drives, only one user at a time is permitted. These are called serially reusable devices, which we studied in the deadlocks chapter. Devices such as disks and ethernet ports can, on the contrary, be shared by concurrent processes without any deadlock risk.

5.2.2 Programmed I/O

As mentioned just above, with programmed I/O the main processor (i.e., the one on which the OS runs) moves the data between memory and the device. This is the most straightforward method for performing I/O.

One question that arises is how does the processor know when the device is ready to accept or supply new data.

In the simplest implementation, the processor, when it seeks to use a device, loops continually querying the device status, until the device reports that it is free. This is called polling or busy waiting.

If we poll infrequently (and do useful work in between), there can be a significant delay from when the previous I/O is complete to when the OS detects the device availability. 

    loop
        if device-available exit loop
        do-useful-work

If we poll frequently (and thus are able to do little useful work in between) and the device is (sometimes) slow, polling is clearly wasteful.

The extreme case is where the process does nothing between polls. For a slow device this can take the CPU out of service for a significant period. This bad situation leads us to ... .

5.2.3 Interrupt-Driven (Programmed) I/O

As we have just seen, a difficulty with polling is determining the frequency with which to poll. Another problem is that the OS must continually return to the polling loop, i.e., we must arrange that do-useful-work takes the desired amount of time. Really we want the device to tell the CPU when it is available, which is exactly what an interrupt does.

The device interrupts the processor when it is ready and an interrupt handler (a.k.a. an interrupt service routine) then initiates transfer of the next datum.

Normally interrupt schemes perform better than polling, but not always since interrupts are expensive on modern machines. To minimize interrupts, better controllers often employ ...

5.2.4 I/O Using DMA

We discussed DMA above.

An additional advantage of dma, not mentioned above, is that the processor is interrupted only at the end of a command not after each datum is transferred. Many devices receive a character at a time, but with a dma controller, an interrupt occurs only after a buffer has been transferred.

5.3 I/O Software Layers

Layers of abstraction as usual prove to be effective. Most systems are believed to use the following layers (but for many systems, the OS code is not available for inspection).

  1. User-level I/O routines.
  2. Device-independent (kernel-level) I/O software.
  3. Device drivers.
  4. Interrupt handlers.

We will give a bottom up explanation.

5.3.1 Interrupt Handlers

We discussed the behavior of an interrupt handler before when studying page faults. Then it was called assembly-language code. A difference is that page faults are caused by specific user instructions, whereas interrupts just occur. However, the assembly-language code for a page fault accomplishes essentially the same task as the interrupt handler does for I/O.

In the present case, we have a process blocked on I/O and the I/O event has just completed. So the goal is to make the process ready and then call the scheduler. Possible methods are.

Once the process is ready, it is up to the scheduler to decide when it should run.

5.3.2 Device Drivers

Device drivers form the portion of the OS that is tailored to the characteristics of individual controllers. They form the dominant portion of the source code of the OS since there are hundreds of drivers. Normally some mechanism is used so that the only drivers loaded on a given system are those corresponding to hardware actually present.

Indeed, modern systems often have loadable device drivers, which are loaded dynamically when needed. This way if a user buys a new device, no changes to the operating system are needed. When the device is installed it will be detected during the boot process and the corresponding driver is loaded.

Sometimes an even fancier method is used and the device can be plugged in while the system is running (USB devices are like this). In this case it is the device insertion that is detected by the OS and that causes the driver to be loaded.

Some systems can dynamically unload a driver, when the corresponding device is unplugged.

The driver has two parts corresponding to its two access points. Recall the figure at the upper right, which we saw at the beginning of the course.

  1. The driver is accessed by the main line OS via the envelope in response to an I/O system call. The portion of the driver accessed in this way is sometimes called the top part.
  2. The driver is also accessed by the interrupt handler when the I/O completes (this completion is signaled by an interrupt). The portion of the driver accessed in this way is sometimes call the bottom part.

In some system the drivers are implemented as user-mode processes. Indeed, Tannenbaum's MINIX system works that way, and in previous editions of the text, he describes such a scheme. However, most systems have the drivers in the kernel itself and the 3e describes this scheme. I previously included both descriptions, but have eliminated the user-mode process description (actually I greyed it out).

Driver in a self-service paradigm

The numbers in the diagram to the right correspond to the numbered steps in the description that follows. The bottom diagram shows the state of processes A and B at steps 1, 6, and 9 in the execution sequence described.

What follows is the Unix-like view in which the driver is invoked by the OS acting in behalf of a user process (alternatively stated, the process shifts into kernel mode). Thus one says that the scheme follows a self-service paradigm in that the process itself (now in kernel mode) executes the driver.

  1. The user (A) issues an I/O system call.

  2. The main line, machine independent, OS prepares a generic request for the driver and calls (the top part of) the driver.
    1. If the driver was idle (i.e., the controller was idle), the driver writes device registers on the controller ending with a command for the controller to begin the actual I/O.
    2. If the controller was busy (doing work the driver gave it previously), the driver simply queues the current request (the driver dequeues this request below).

  3. The driver jumps to the scheduler indicating that the current process should be blocked.

  4. The scheduler blocks A and runs (say) B.

  5. B starts running.

  6. An interrupt arrives (i.e., an I/O has been completed) and the handler is invoked.

  7. The interrupt handler invokes (the bottom part of) the driver.
    1. The driver informs the main line perhaps passing data and surely passing status (error, OK).
    2. The top part is called to start another I/O if the queue is nonempty. We know the controller is free. Why?
      Answer: We just received an interrupt saying so.

  8. The driver jumps to the scheduler indicating that process A should be made ready.

  9. The scheduler picks a ready process to run. Assume it picks A.

  10. A resumes in the driver, which returns to the main line, which returns to the user code.

Driver as a Process (Less Detailed Than Above)

Actions that occur when the user issues an I/O request.

  1. The main line OS prepares a generic request (e.g. read, not read using Buslogic BT-958 SCSI controller) for the driver and the driver is awakened. Perhaps a message is sent to the driver to do both jobs.
  2. The driver wakes up.
    1. If the driver was idle (i.e., the controller is idle), the driver writes device registers on the controller ending with a command for the controller to begin the actual I/O.
    2. If the controller is busy (doing work the driver gave it), the driver simply queues the current request (the driver dequeues this below).
  3. The driver blocks waiting for an interrupt or for more requests.

Actions that occur when an interrupt arrives (i.e., when an I/O has been completed).

  1. The driver wakes up.
  2. The driver informs the main line perhaps passing data and surely passing status (error, OK).
  3. The driver finds the next work item or blocks.
    1. If the queue of requests is non-empty, dequeue one and proceed as if just received a request from the main line.
    2. If queue is empty, the driver blocks waiting for an interrupt or a request from the main line.

5.3.3 Device-Independent I/O Software

The device-independent code cantains most of the I/O functionality, but not most of the code since there are very many drivers. All drivers of the same class (say all hard disk drivers) do essentially the same thing in slightly different ways due to slightly different controllers.

Uniform Interfacing for Device Drivers

As stated above the bulk of the OS code is made of device drivers and thus it is important that the task of driver writing not be made more difficult than needed. As a result each class of devices (e.g. the class of all disks) has a defined driver interface to which all drivers for that class of device conform. The device independent I/O portion processes user requests and calls the drivers.

Naming is again an important O/S functionality. In addition it offers a consistent interface to the drivers. The Unix method works as follows

Protection. A wide range of possibilities are actually done in real systems. Including both extreme examples of everything is permitted and nothing is (directly) permitted.