Operating Systems

** 4.8.2 and 4.8.3: Segmentation With (demand) Paging

(Tanenbaum gives two sections to explain the differences between Multics and the Intel Pentium. These notes cover what is common to all segmentation+paging systems).

Combines both segmentation and demand paging to get advantages of both at a cost in complexity. This is very common now.

Although it is possible to combine segmentation with non-demand paging, I do not know of any system that did this.

• A virtual address becomes a triple: (seg#, page#, offset).
  
  
• Each segment table entry (STE) points to the page table for that segment. Compare this with a multilevel page table.
  
  
• The physical size of each segment is a multiple of the page size (since the segment consists of pages). The logical size is not; instead we keep the exact size in the STE (limit value) and shoot the process if it referenced beyond the limit. In this case the last page of each segment is partially valid (internal fragmentation).
  
  
• The page# field in the address gives the entry in the chosen page table and the offset gives the offset in the page.
  
  
• From the limit field, one can easily compute the size of the segment in pages (which equals the size of the corresponding page table in PTEs).
  
  
• A straightforward implementation of segmentation with paging would requires 3 memory references (STE, PTE, referenced word) so a TLB is crucial.
  
  
• Some books carelessly say that segments are of fixed size. This is wrong. They are of variable size with a fixed maximum and with the requirement that the physical size of a segment is a multiple of the page size.
  
  
• The first example of segmentation with paging was Multics.
  
  
• Keep protection and sharing information on segments. This works well for a number of reasons.
  1. A segment is variable size.
  2. Segments and their boundaries are user (i.e., linker) visible.
  3. Segments are shared by sharing their page tables. This eliminates the problem mentioned above with shared pages.
  
  
• Since we have paging, there is no placement question and no external fragmentation.
  
  
• Do fetch-on-demand with pages (i.e., do demand paging).
  
  
• In general, segmentation with demand paging works well and is widely used. The only problems are the complexity and the resulting 3 memory references for each user memory reference. The complexity is real, but can be managed. The three memory references would be fatal were it not for TLBs, which considerably ameliorate the problem. TLBs have high hit rates and for a TLB hit there is essentially no penalty.

Homework: 38.

4.9: Research on Memory Management

Skipped

4.10: Summary

Read

Some Last Words on Memory Management

• Segmentation / Paging / Demand Loading (fetch-on-demand)
  • Each is a yes or no alternative.
  • Gives 8 possibilities.
• Placement and Replacement.
• Internal and External Fragmentation.
• Page Size and locality of reference.
• Multiprogramming level and medium term scheduling.

Chapter 5: Input/Output

5.1: Principles of I/O Hardware

5.1.1: I/O Devices

• Not much to say. Devices are varied.
• Block versus character devices:
  • Devices, such as disks and CDROMs, with addressable chunks (sectors in this case) are called block devices,
    These devices support seeking.
  • Devices, such as Ethernet and modem connections, that are a stream of characters are called character devices.
    These devices do not support seeking.
  • Some cases, like tapes, are not so clear.

5.1.2: Device Controllers

These are the “devices” as far as the OS is concerned. That is, the OS code is written with the controller spec in hand not with the device spec.

• Also called adaptors.
  
  
• The controller abstracts away some of the low level features of the device.
  
  
• For disks, the controller does error checking and buffering.
  
  
• (Unofficial) In the old days it handled interleaving of sectors. (Sectors are interleaved if the controller or CPU cannot handle the data rate and would otherwise have to wait a full revolution. This is not a concern with modern systems since the electronics have increased in speed faster than the devices.)
  
  
• For analog monitors (CRTs) the controller does a great deal. Analog video is very far from a bunch of ones and zeros.
  
  

5.1.3: Memory-Mapped I/O

Think of a disk controller and a read request. The goal is to copy data from the disk to some portion of the central memory. How do we do this?

• The controller contains a microprocessor and memory and is connected to the disk (by a cable).
  
  
• When the controller asks the disk to read a sector, the contents come to the controller via the cable and are stored by the controller in its memory.
  
  
• The question is how does the OS, which is running on another processor, let the controller know that a disk read is desired and how is the data eventually moved from the controller's memory to the general system memory.
  
  
• Typically the interface the OS sees consists of some device registers located on the controller.
  • These are memory locations into which the OS writes information such as sector to access, read vs. write, length, where in system memory to put the data (for a read) or from where to take the data (for a write).
  • There is also typically a device register that acts as a “go button”.
  • There are also devices registers that the OS reads, such as status of the controller, errors found, etc.
  
  
• So now the question is how does the OS read and write the device register
  • With Memory-mapped I/O the device registers appear as normal memory. All that is needed is to know at which address each device regester appears. Then the OS uses normal load and store instructions to write the registers.
  • Some systems instead have a special “I/O space” into which the registers are mapped and require the use of special I/O space instructions to accomplish the load and store.
  • From a conceptual point of view there is no difference between the two models.

5.1.4: Direct Memory Access (DMA)

• With or without DMA, the disk controller pulls the desired data from the disk to its buffer (and pushes data from the buffer to the disk).
  
  
• Without DMA, i.e., with programmed I/O (PIO), the cpu then does loads and stores (or I/O instructions) to copy the data from the buffer to the desired memory location.
  
  
• With a DMA controller, the controller writes the memory without intervention of the CPU.
  
  
• Clearly DMA saves CPU work. But this might not be important if the CPU is limited by the memory or by system buses.
  
  
• Very important is that there is less data movement so the buses are used less and the entire operation takes less time.
  
  
• 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.
  Answer: 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.

Homework: 12

5.1.5: Interrupts Revisited

Skipped.

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, unaware of the characteristics of the specific devices attached to the system. Indeed we also want the OS to be largely unaware of the CPU type itself.

Due to this device independence, programs are written to read and write generic devices and then at run time specific devices are assigned. Writing to a disk has differences from writing to a terminal, but Unix cp and DOS copy do not see these differences. Indeed, most of the OS, including the file system code, is unaware of whether the device is a floppy or hard disk.

Homework: 5.9

Uniform naming

Recall that we 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.

Error handling

1. Detection should be done as close to where the error occurred as possible before more damage is done (fault containment). This is not trivial.
   
   
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 schedule 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. Unix/Linux in particular is horrible in this area.

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 notification 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 is assigned.
  
  
• 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.

Buffering

• Often needed to hold data for examination prior to sending it to its desired destination.
  
  
• But this involves copying and takes time.
  
  
• 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. User space --> kernel space as part of the write system call
  2. kernel space to kernel I/O buffer.
  3. I/O buffer to buffer on the network adapter/controller.
  4. From adapter on the source to adapter on the destination.
  5. From adapter to I/O buffer.
  6. From 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 tape drives, only one user at a time is permitted. These are called serially reusable devices, and were studied in the deadlocks chapter. Devices like disks and Ethernet ports can be shared by processes running concurrently.

5.2.2: Programmed I/O

• As mentioned just above, with programmed I/O the processor moves the data between memory and the device.
  
  
• How does the process know when the device is ready to accept or supply new data?
  
  
• In the simplest implementation, the processor loops continually asking the device. This is called polling or busy waiting.
  
  
• If we poll infrequently, there can be a significant delay between when the I/O is complete and the OS uses the data or supplies new data.
  
  
• If we poll frequently and the device is (sometimes) slow, polling is clearly wasteful, which leads us to ...

5.2.3: Interrupt-Driven (Programmed) I/O

• The device interrupts the processor when it is ready.
  
  
• An interrupt service routine then initiates transfer of the next datum.
  
  
• Normally better than polling, but not always. 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 an interrupt handler before when studying page faults. Then it was called “assembly language code”.

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. Possible methods are.

• Releasing a semaphore on which the process is waiting.
• Sending a message to the process.
• Inserting the process table entry onto the ready list.

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

5.3.2: Device Drivers

The portion of the OS that “knows” the characteristics of the controller.

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

1. 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 call the “top” part.
2. 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.

Tanenbaum describes the actions of the driver assuming it is implemented as a process (which he recommends). I give both that view point and the self-service paradigm in which the driver is invoked by the OS acting in behalf of a user process (more precisely the process shifts into kernel mode).

Driver in a self-service paradigm

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 (Tanenbaum) (less detailed than above)

• The user issues an I/O request. 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).
  1. 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).
  2. The driver blocks waiting for an interrupt or for more requests.
• An interrupt arrives (i.e., 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 does most of the functionality, but not necessarily most of the code since there can be many drivers all doing essentially the same thing in slightly different ways due to slightly different controllers.

• Naming. Again an important O/S functionality. Must offer a consistent interface to the device drivers.
  • In Unix this is done by associating each device with a (special) file in the /dev directory.
  • The i-nodes for these files contain an indication that these are special files and also contain so called major and minor device numbers.
  • The major device number gives the number of the driver. (These numbers are rather ad hoc, they correspond to the position of the function pointer to the driver in a table of function pointers.)
  • The minor number indicates for which device (e.g., which scsi cdrom drive) the request is intended
  
  
• Protection. A wide range of possibilities are actually done in real systems. Including both extreme examples of everything is permitted and nothing is permitted (directly).
  • In ms-dos any process can write to any file. Presumably, our offensive nuclear missile launchers do not run dos.
  • In IBM and other mainframe OS's, normal processors do not access devices. Indeed the main CPU doesn't issue the I/O requests. Instead an I/O channel is used and the mainline constructs a channel program and tells the channel to invoke it.
  • Unix uses normal rwx bits on files in /dev (I don't believe x is used).
  
  
• Buffering is necessary since requests come in a size specified by the user and data is delivered in a size specified by the device.
  
  
• Enforce exclusive access for non-shared devices like tapes.

5.3.4: User-Space Software

A good deal of I/O code is actually executed in user space. Some is in library routines linked into user programs and some is in daemon processes.

• Some library routines are trivial and just move their arguments into the correct place (e.g., a specific register) and then issue a trap to the correct system call to do the real work.
  
  
• Some, notably standard I/O (stdio) in Unix, are definitely not trivial. For example consider the formatting of floating point numbers done in printf and the reverse operation done in scanf.
  
  
• Printing to a local printer is often performed in part by a regular program (lpr in Unix) and part by a daemon (lpd in Unix). The daemon might be started when the system boots or might be started on demand. I guess it is called a daemon because it is not under the control of any user. A student has suggested that the usage comes from the meaning of daemon in Greek mythology, namely a guardian spirit. Does anyone know the reason.
  
  
• Printing uses spooling, i.e., the file to be printed is copied somewhere by lpr and then the daemon works with this copy. Mail uses a similar technique (but generally it is called queuing, not spooling).

Homework: 10, 13.