Operating Systems

Start Lecture #9

3.5.3 Page size

Page size must be a multiple of the disk block size. Why?
Answer: When copying out a page if you have a partial disk block, you must do a read/modify/write (i.e., 2 I/Os).

Characteristics of a large page size.

• Good for demand paging I/O:

  We will learn later this term that the total time for performing 8 I/O operations each of size 1KB is much larger that the time for a single 8KB I/O. Hence it is better to swap in/out one big page than several small pages.

  But if the page is too big you will be swapping in data that are not local and hence might well not be used.

• Large internal fragmentation (1/2 page size).
  
  
• Small page table (process size / page size * size of PTE).
  
  
• These last two can be analyzed together by setting the derivative of the sum equal to 0. The minimum overhead occurs at a page size of

          sqrt(2 * process size * size of PTE)
        

  Since the term inside the sqrt is typically megabytes, we see that modern practice of having the page size a few kilobytes is near the minimum point.
  
  
• A very large page size leads to very few pages. A process will have many faults if it references more regions than the number of (large) frames that the process has been allocated.

A small page size has the opposite characteristics.

Homework: Consider a 32-bit address machine using paging with 8KB pages and 4 byte PTEs. How many bits are used for the offset and what is the size of the largest page table? Repeat the question for 128KB pages.

3.5.4 Separate Instruction and Data (I and D) Spaces

This was used when machine have very small virtual address spaces. Specifically the PDP-11, with 16-bit addresses, could address only 2₁₆ bytes or 64KB, a severe limitation. With separate I and D spaces there could be 64KB of instructions and 64KB of data.

Separate I and D are no longer needed with modern architectures having large address spaces.

3.5.5 Shared pages

Permit several processes to each have the same page loaded in the same frame. Of course this can only be done if the processes are using the same program and/or data.

• Really should share segments.
• Must keep reference counts or something so that, when a process terminates, pages it shares with another process are not automatically discarded.
• Similarly, a reference count would make a widely shared page (correctly) look like a poor choice for a victim.
• A good place to store the reference count would be in a structure pointed to by both PTEs. If stored in the PTEs themselves, we must keep somehow keep the count consistent between processes.
• If you want the pages to be initially shared for reading but want each process's updates to be private, then use so called copy on write techniques.

Homework: Can a page shared between two processes be read-only for one process and read-write for the other?

3.5.6 Shared Libraries (Dynamic-Linking)

In addition to sharing individual pages, process can share entire library routines. The technique used is called dynamic linking and the objects produced are called shared libraries or dynamically-linked libraries (DLLs). (The traditional linking you did in lab1 is today often called static linking).

• With dynamic linking, frequently used routines are not linked into the program. Instead, just a stub is linked.
• When the routine is called (or when the process begins), the stub checks to see if the real routine has been loaded by another program).
  • If it has not been loaded, load it (really page it in as needed).
  • If it is already loaded, share it. The read-write data must be shared copy-on-write.
• Advantages of dynamic linking.
  • Saves RAM: Only one copy of a routing is in memory even when it is used concurrently by many processes. For example even a big server with hundreds of active processes will have only one copy of printf in memory. (In fact with demand paging only part of the routine will be in memory.)
  • Saves disk space: Files containing executable programs no longer contain copies of the shared libraries.
  • A bug fix to a dynamically linked library fixes all applications that use that library, without having to relink these applications.
• Disadvantages of dynamic linking.
  • New bugs in dynamically linked library infect all applications.
  • Applications change even when they haven't changed.
• A Technical Difficulty with dynamic linking. The shared library has different virtual addresses in each process so addresses relative to the beginning of the module cannot be used (they would need to be relocated to different addresses in the multiple copies of the module). Instead position-independent code must be used. For example, jumps within the module would use PC-relative addresses.

3.5.7 Mapped Files

The idea of memory-mapped files is to use the mechanisms in place for demand paging (and segmentation, if present) to implement I/O.

A system call is used to map a file into a portion of the address space. (No page can be part of a file and part of regular memory; the mapped file would be a complete segment if segmentation is present).

The implementation of demand paging we have presented assumes that the entire process is stored on disk. This portion of secondary storage is called the backing store for the pages. Sometimes it is called a paging disk. For memory-mapped files, the file itself is the backing store.

Once the file is mapped into memory, reads and writes become loads and stores.

3.5.8 Cleaning Policy (Paging Daemons)

Done earlier

The only point to add is now that we know replacement algorithms one can suggest an implementation. If a clock-like algorithm is used for victim selection, one can have a two handed clock with one hand (the paging daemon) staying ahead of the other (the one invoked by the need for a free frame).

The front hand simply writes out any page it hits that is dirty and thus the trailing hand is likely to see clean pages and hence is more quickly able to find a suitable victim.

3.6 Implementation Issues

3.6.1 Operating System Involvement with Paging

When must the operating system be involved with paging?

• During process creation. The OS must guess at the size of the process and then allocate a page table and a region on disk to hold the pages that are not memory resident. A few pages of the process must be loaded.
  
  
• The Ready→Running transition. Real memory must be allocated for the page table if the table has been swapped out (which is permitted when the process is not running).
  Some hardware register(s) must be set to point to the page table. There can be many page tables resident, but the hardware must be told the location of the page table for the running process—the active page table.
  The MMU must be cleared (unless it contains a process id field).
  
  
• Processing a page fault. Lots of work is needed; see 3.6.2 just below.
  
  
• Process termination. Free the page table and the disk region for swapped out pages.

3.6.2 Page Fault Handling

What happens when a process, say process A, gets a page fault? Compare the following with the processing for a trap command and for an interrupt.

1. The hardware detects the fault and traps to the kernel (switches to supervisor mode and saves state).
   
   
2. Some assembly language code save more state, establishes the C-language (or another programming language) environment, and calls the OS.
   
   
3. The OS determines that a page fault occurred and which page was referenced.
   
   
4. If the virtual address is invalid, process A is killed. If the virtual address is valid, the OS must find a free frame. If there is no free frames, the OS selects a victim frame. Call the process owning the victim frame, process B. (If the page replacement algorithm is local, then B=A.)
   
   
5. The PTE of the victim page is updated to show that the page is no longer resident.
   
   
6. If the victim page is dirty, the OS schedules an I/O write to copy the frame to disk and blocks A waiting for this I/O to occur. As we know, this is really not what happens since there is always a free frame thanks to the page cleaning daemon.
   
   
7. Assuming process A needed to be blocked (i.e., the victim page is dirty) the scheduler is invoked to perform a context switch.
   • Tanenbaum forgot some here.
   • The process selected by the scheduler (say process C) runs.
   • Perhaps C is preempted for D or perhaps C blocks and D runs and then perhaps D is blocked and E runs, etc.
   • When the I/O to write the victim frame completes, a disk interrupt occurs. Assume processes C is running at the time.
   • Hardware trap / assembly code / OS determines I/O done.
   • The scheduler marks A as ready.
   • The scheduler picks a process to run, maybe A, maybe B, maybe C, maybe another processes.
   • At some point the scheduler does pick process A to run. Recall that at this point A is still executing OS code.
   
   
8. Now the O/S has a free frame (this may be much later in wall clock time if a victim frame had to be written). The O/S schedules an I/O to read the desired page into this free frame. Process A is blocked (perhaps for the second time) and hence the process scheduler is invoked to perform a context switch.
   
   
9. Again, another process is selected by the scheduler as above and eventually a Disk interrupt occurs when the I/O completes (trap / asm / OS determines I/O done). The PTE in process A is updated to indicate that the page is in memory.
   
   
10. The O/S may need to fix up process A (e.g. reset the program counter to re-execute the instruction that caused the page fault).
    
    
11. Process A is placed on the ready list and eventually is chosen by the scheduler to run. Recall that process A is executing O/S code.
    
    
12. The OS returns to the first assembly language routine.
    
    
13. The assembly language routine restores registers, etc. and returns to user mode.

The user's program running as process A is unaware that all this happened (except for the time delay).

3.6.3 Instruction Backup

A cute horror story. The 68000 was so bad in this regard that an early demand paging system for the 68000, used two processors one running one instruction behind. If the first got a page fault, there wasn't always enough information to figure out what to do so (for example did a register pre-increment occur), the system switched to the second processor after bringing in the faulting page. The next generation machine, the 68010, provided extra information on the stack so the horrible 2-processor kludge was no longer necessary.

Don't worry about instruction backup; it is very machine dependent and modern implementations tend to get it right.

3.6.4 Locking (Pinning) Pages in Memory

We discussed pinning jobs already. The same (mostly I/O) considerations apply to pages.

3.6.5 Backing Store

The issue is where on disk do we put pages that are not in frames.

• For program text, which is presumably read only, a good choice is the file executable itself.
  
  
• What if we decide to keep the data and stack each contiguous on the backing store. Data and stack grow so we must be prepared to grow the space on disk, which leads to the same issues and problems as we saw with MVT.
  
  
• If those issues/problems are painful, we can scatter the pages on the disk.
  • That is we employ paging!
  • This is NOT demand paging.
  • Need a table to say where the backing space for each page is located.
    • This corresponds to the page table used to tell where in real memory a page is located.
    • The format of the memory page table is determined by the hardware since the hardware modifies/accesses it. It is machine dependent.
    • The format of the disk page table is decided by the OS designers and is machine independent.
    • If the format of the memory page table were flexible, then we might well keep the disk information in it as well. But normally the format is not flexible, and hence this is not done.
  
  
• What if we felt disk space was too expensive and wanted to put some of these disk pages on say tape?
  Ans: We use demand paging of the disk blocks! That way "unimportant" disk blocks will migrate out to tape and are brought back in if needed.
  Since a tape read requires seconds to complete (because the request is not likely to be for the sequentially next tape block), it is crucial that we get very few disk block faults.

Homework: Assume every memory reference takes 0.1 microseconds to execute providing the reference page is memory resident. Assume a page fault takes 10 milliseconds to service providing the necessary disk block is actually on the disk. Assume a disk block fault takes 10 seconds service. So the worst case time for a memory reference is 10.0100001 seconds. Finally assume the program requires that a billion memory references be executed.

1. If the program is always completely resident, how long does it take to execute?
2. If 0.1% of the memory references cause a page fault, but all the disk blocks are on the disk, how long does the program take to execute and what percentage of the time is the program waiting for a page fault to complete?
3. If 0.1% of the memory references cause a page fault and 0.1% of the page faults cause a disk block fault, how long does the program take to execute and what percentage of the time is the program waiting for a disk block fault to complete?

3.7 Segmentation

Up to now, the virtual address space has been contiguous. In segmentation the virtual address space is divided into a number of variable-size segments. One can view the designs we have studied so far as having just one segment, the entire process.

• Among other issues this makes memory management difficult when there are more that two dynamically growing regions.
• With two regions you start them on opposite sides of the virtual space as we did before.
• Better is to have many virtual address spaces each starting at zero.
• This split up is user visible. So a segment is a logical split up of the address space. Unlike with (user-invisible) paging, segment boundaries occur at logical point, e.g., at the end of a procedure.
• Imagine a system with several large, dynamically-growing, data structures. The same problem we mentioned for the OS when there are more than two growing regions, occurs as well for user programs. The user (or some user-mode tool) must decide how much virtual space to leave between the different tables. With segmentation
• Eases flexible protection and sharing: One places in a single segment a unit that is logically shared. This would be the natural method to implement shared libraries.
• When shared libraries are implemented on paging systems, the design essentially mimics segmentation by treating a collection of pages as a segment. This is more complicated since one must ensure that the end of the unit to be shared occurs on a page boundary (this is done by padding).
• Without segmentation (equivalently said with just one segment) all procedures are packed together so, if one changes in size, all the virtual addresses following this procedure are changed and the program must be re-linked. With each procedure in a separate segment this relinking would be limited to the symbols defined or used in the modified procedure.

Homework: Explain the difference between internal fragmentation and external fragmentation. Which on occurs in paging systems? Which one occurs in systems using pure segmentation?

** Two Segments

Late PDP-10s and TOPS-10

• Each process has one shared text segment, that can also contain shared (normally read only) data. As the name indicates, all process running the same executable share the same text segment.
• The process also contains one (private) writable data segment.
• Permission bits define for each segment.

** Three Segments

Traditional (early) Unix had three segments as shown on the right.

1. Shared text marked execute only.
2. Data segment (global and static variables).
3. Stack segment (automatic variables).

Since the text doesn't grow, this was sometimes treated as 2 segments by combining text and data into one segment. But then the text could not be shared.

** General (Not Necessarily Demand) Segmentation

Segmentation is a user-visible division of a process into multiple variable-size segments, whose sizes change dynamically during execution. It enables fine-grained sharing and protection. For example, one can share the text segment as done in early unix.

With segmentation, the virtual address has two components: the segment number and the offset in the segment.

Segmentation does not mandate how the program is stored in memory.

• One possibility is that the entire program must be in memory in order to run it. Use whole process swapping. Early versions of Unix did this.
• Can also implement demand segmentation (see below).
• More recently, segmentation is combined with demand paging (done below).

Any segmentation implementation requires a segment table with one entry for each segment.

• A segment table is similar to a page table.
• Entries are called STEs, Segment Table Entries.
• Each STE contains the base address of the segment and the limit value (the size of the segment).
• Why is there no limit value in a page table?
• Answer: All pages are the same size so the limit is obvious.

The address translation for segmentation is
    (seg#, offset) --> if (offset<limit) base+offset else error.

3.7.1 Implementation of Pure Segmentation

Pure Segmentation means segmentation without paging.

Segmentation, like whole program swapping, exhibits external fragmentation (sometimes called checkerboarding). (See the treatment of OS/MVT for a review of external fragmentation and whole program swapping). Since segments are smaller than programs (several segments make up one program), the external fragmentation is not as bad as with whole program swapping. But it is still a serious problem.

As with whole program swapping, compaction can be employed.

** Demand Segmentation

Same idea as demand paging, but applied to segments.

• If a segment is loaded, base and limit are stored in the STE and the valid bit is set in the STE.
• The STE is accessed for each memory reference (not really, there is probably a TLB).
• If the segment is not loaded, the valid bit is unset. The base and limit as well as the disk address of the segment is stored in the an OS table.
• A reference to a non-loaded segment generate a segment fault (analogous to page fault).
• To load a segment, we must solve both the placement question and the replacement question (for demand paging, there is no placement question).
• Pure segmentation was once implemented by Burroughs in the B5500. I believe the implementation was in fact demand segmentation but I am not sure. Demand segmentation is not used in modern systems.

The table on the right compares demand paging with demand segmentation. The portion above the double line is from Tanenbaum.

** 3.7.2 and 3.7.3 Segmentation With (Demand) Paging

These two sections of the book cover segmentation combined with demand paging in two different systems. Section 3.7.2 covers the historic Multics system of the 1960s (it was coming up at MIT when I was an undergraduate there). Multics was complicated and revolutionary. Indeed, Thompson and Richie developed (and named) Unix partially in rebellion to the complexity of Multics. Multics is no longer used.

Section 3.7.3 covers the Intel Pentium hardware, which offers a segmentation+demand-paging scheme that is not used by any of the current operating systems (OS/2 used it in the past). The Pentium design permits one to convert the system into a pure damand-paging scheme and that is the common usage today.

I will present the material in the following order.

1. Describe segmentation+paging (not demand paging) generically, i.e. not tied to any specific hardware or software.
2. Note the possibility of using demand paging (again generically).
3. Give some details of the Multics implementation.
4. Give some details of the Pentium hardware, especially how it can emulate straight demand paging.

** Segmentation With (non-demand) Paging

One can combine segmentation and paging to get advantages of both at a cost in complexity. In particular, user-visible, variable-size segments are the most appropriate units for protection and sharing; the addition of (non-demand) paging eliminates the placement question and external fragmentation (at the smaller average cost of 1/2-page internal fragmentation per segment).

The basic idea is to employ (non-demand) paging on each segment. A segmentation plus paging scheme has the following properties.

• 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 terminate the process if it references beyond the limit. In this case the last page of each segment is partially wasted (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.
  
  
• 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-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.
  
  
• The problems are the complexity and the resulting 3 memory references for each user memory reference. The complexity is real. 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.

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

Homework: 36.

Homework: Consider a 32-bit address machine using paging with 8KB pages and 4 byte PTEs. How many bits are used for the offset and what is the size of the largest page table? Repeat the question for 128KB pages. So far this question has been asked before. Repeat both parts assuming the system also has segmentation with at most 128 segments.

** Segmentation With Demand Paging

There is very little to say. The previous section employed (non-demand) paging on each segment. For the present scheme, we employ demand paging on each segment, that is we perform fetch-on-demand for the pages of each segment.

The Multics Scheme

Multics was the first system to employ segmentation plus demand paging. The implementation was as described above with just a few wrinkles, some of which we discuss now together with some of the parameter values.

• The Multics hardware (GE-645) was word addressable, with 36-bit words (the 645 predates bytes).
• Each virtual address was 34-bits in length and was divided into three parts as mentioned above. The seg# field was the high-order 18 bits; the page# field was the next 6 bits; and the offset was the low-order 10 bits.
• The actual implementation was more complicated and the full 34-bit virtual address was not present in one place in an instruction.
• Thus the system supported up to 2¹⁸=256K segments, each of size up to 2⁶=64 pages. Each page is of size 2¹⁰ (36-bit) words.
• Since the segment table can have 256K STEs (called descriptors), the table itself can be large and was itself demand-paged.
• Multics permits some segments to be demand-paged while other segments are not paged; a bit in each STE distinguishes the two cases.

The Pentium Scheme

The Pentium design implements a trifecta: Depending on the setting of a various control bits the Pentium scheme can be pure demand-paging, pure segmentation, or segmentation with demand-paging.

The Pentium supports 2¹⁴=16K segments, each of size up to 2³² bytes.

• This would seem to require a 14+32=46 bit virtual address, but that is not how the Pentium works. The segment number is not part of the virtual address found in normal instructions.
• Instead separate instructions are used to specify which are the currently active code segment and data segment (and other less important segments). Technically, the CS register is loaded with the selector of the active code segment and the DS register is loaded with the selector of the active data register.
• When the selectors are loaded, the base and limit values are obtained from the corresponding STEs (called descriptors).
• There are actually two flavors of segments, some are private to the process; others are system segments (including the OS itself), which are addressable (but not necessarily accessible) by all processes.

Once the 32-bit segment base and the segment limit are determined, the 32-bit address from the instruction itself is compared with the limit and, if valid, is added to the base and the sum is called the 32-bit linear address. Now we have three possibilities depending on whether the system is running in pure segmentation, pure demand-paging, or segmentation plus demand-paging mode.

1. In pure segmentation mode the linear address is treated as the physical address and memory is accessed.

2. In segmentation plus demand-paging mode, the linear address is broken into three parts since the system implements 2-level-paging. That is, the high-order 10 bits are used to index into the 1st-level page table (called the page directory). The directory entry found points to a 2nd-level page table and the next 10 bits index that table (called the page table). The PTE referenced points to the frame containing the desired page and the lowest 12 bits of the linear address (the offset) finally point to the referenced word. If either the 2nd-level page table or the desired page are not resident, a page fault occurs and the page is made resident using the standard demand paging model.

3. In pure demand-paging mode all the segment bases are zero and the limits are set to the maximum. Thus the 32-bit address in the instruction become the linear address without change (i.e., the segmentation part is effectively) disabled. Then the (2-level) demand paging procedure just described is applied.

   Current operating systems for the Pentium use this last mode.

3.9 Summary

Read

Some Last Words on Memory Management

We have studied the following concepts.

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