Lec9: Virtual Memory

Virtual Memory#

​ 这一章与上一章有什么区别呢？我的理解如下：

1. 上一张强调的是 virtual address 和 physical address 的映射关系，是在内存分配好了的情况下进行的。
2. 这一章我们的任务是在程序刚开始的时候，我们怎么将程序加载进内存中去。

​ 以上是我的理解，不一定正确，可能需要修改。

Background#

• Virtual memory – separation of user logical memory from physical memory.
  • Only part of the program needs to be in memory for execution
  • Logical address space can therefore be much larger than physical address space
  • Allows address spaces to be shared by several processes
  • 允许共享部分物理地址，实现资源共享
  • Allows for more efficient process creation
  • 允许进程更快被创建（分配的资源减少）
• 实际上，对于一个正在运行的程序来说，很多地方都是用不到的。因此，程序员在进行程序设计的时候，可以开出很大的内存。这也是虚拟内存的由来（比物理内存大很多甚至是几倍）

​ 那么，什么是 virtual memory 呢？下面的一段定义相当精辟：

virtual memory isn’t a physical object, but refers to the collection of abstractions and mechanisms the kernel provides to manage physical memory and virtual addresses.

​ virtual memory 怎么实现呢？

• Virtual memory can be implemented via:
  • Demand paging
  • Demand segmentation

Other benefits#

• System libraries can be shared by several processes through mapping of the shared object into a virtual address space
• Shared memory is enabled
• Pages can be shared during process creation (speeds up creation)

Demand Paging#

​ 为什么我们需要 Demand Paging 这个技术呢？这是因为我们想让程序员的编程更简单，不需要考虑内存问题。

​ 从磁盘加载可执行程序到内存的方法有很多。最简单的一种方法是在程序执行的时候将整个程序加载进物理内存。然而，这种方法的一个问题是，最初可能不需要整个程序都处于内存。

​ 另外一种方法是仅在需要时才加载页面，这种技术被称为 demand paging（请求调页）。对于请求调页的虚拟内存，页面只有在程序执行期间被请求时才会被加载，因此，从未访问的那些页从不加载到物理内存中。

• Bring a page into memory only when it is needed
  • Less I/O needed
  • Less memory needed
  • Faster response
  • More users
• Page is needed => reference to it
  • invalid reference => abort
  • not-in-memory => bring to memory
• Lazy swapper – never swaps a page into memory unless page will be needed
• 在这个方法中，我们采用惰性交换器：“非必要不交换”
  • Swapper that deals with pages is a pager

​ 接下来，我们讲一讲 Demand Paging 的运行机制。当换入进程的时候，调页程序会猜测在该进程被再次调用之前会用到哪些页，并将这些页调入内存。显而易见的是，使用这种方案需要一系列的硬件支持，来区分内存的页面和磁盘的页面，我们可以采用之前说过的“有效-无效位”的方案。

• With each page table entry a valid–invalid bit is associated(v => in-memory, i => not-in-memory)
• Initially valid–invalid bit is set to i on all entries
• Example of a page table snapshot:

• During address translation, if valid–invalid bit in page table entry is i => page fault (a trap to the OS)

​ 在 valid 的情况下，显然页面已存在在内存中了，可以正常运行。然而，在 invalid 的情况下，则会出现两种情况：

1. 页面无效，机器地址不在进程的逻辑地址中(base~base+limit)
2. 有效但只在磁盘上，没有被调进内存中

​ 对于 invalid 的情况，我们采用一个中断程序(trap) page fault 来进行处理。

• If there is a reference to a page, first reference to that page will trap to operating system:
  • page fault
• Operating system looks at another table (kept with PCB) to decide:
  • Invalid reference => abort
  • Just not in memory
• Get empty frame
• Swap page into frame
  • 需要看是否有 free frame，若没有，需要腾出一个，这是后面的内容
• Reset tables
• Set validation bit = v
• Restart the instruction that caused the page fault

实际上，以上操作的后半段（交换页面的操作）并不是由 page fault 这个 trap 完成的，因为 trap 要求很快，但 swap 很慢（有I/O），因此需要 bottom-half 来解决，swapper 就是一种 bottom-half

​ 我们可以使用加载第一条指令所在的 page 来达到快速启动进程的方法

Performance of Demand Paging#

• 其中，==page fault overhead + swap page out + swap page in + restart overhead = page-fault service time==

Copy-on-Write#

• Copy-on-Write (COW) allows both parent and child processes to initially share the same pages in memory
• If either process modifies a shared page, only then is the page copied
• COW allows more efficient process creation as only modified pages are copied
• Free pages are allocated from a pool of zeroed-out pages

Page Replacement#

​ 在之前的内容中，我们提到了，在页面有效但不在内存中时，我们需要将页面换进内存中，也就是下面的过程：

• Page replacement – find some page in memory, but not really in use, swap it out
  • algorithm
  • performance – want an algorithm which will result in minimum number of page faults
  • 我们的目标之一就是在交换完之后，发生 page fault 的次数尽量小，也可以这么说，交换完之后，被换出去的页面的访问次数尽量少。
• Same page may be brought into memory several times
• Prevent over-allocation of memory by modifying page-fault service routine to include page replacement
• Use modify (dirty) bit to reduce overhead of page transfers – only modified pages are written to disk
• Page replacement completes separation between logical memory and physical memory – large virtual memory can be provided on a smaller physical memory

• 我们称被选择交换的帧为 victim frame

Algorithm#

​ 为了实现请求调页，必须要解决两个主要问题：

1. free-allocation algorithm：帧分配算法：若有多个进程在内存中，必须决定要为每个进程分配多少帧
2. page-replacement algorithm：页面置换算法：当需要页面置换的时候，必须选择要置换的帧。

​ 一般来说，我们想要的是一个 page fault rate 最小的算法。

FIFO 页面置换#

• 就是很简单的 First-in-First-out 的算法，使用一个队列进行维护

Belady 异常#

Optimal Algorithm#

• 思路：替换策略是替换最长时间不被使用的 page

​ 上面展示的例子详细说明了这个策略是怎么运作的：每一次更新 reference string 的时候，都会向“未来”观看，寻找 reference string 中最长时间不使用的 page，将其作为 victim 替换出去，并且换入新的 page。

Least Recently Used (LRU) Algorithm#

​ 显而易见的是，Optimal Algorithm 需要 reference string 的未来知识，这个问题在之前出现过类似的，就是 SJF 的 CPU 调度。这里我们采用一种更为可行的方法，也是之前很多课程中都有提及的算法：最近最少使用算法（LRU）。

​ 对于 LRU，有两种比较常见的实现方法：

• Counter implementation
  • Every page entry has a counter; every time page is referenced through this entry, copy the clock into the counter
  • When a page needs to be changed, look at the counters to determine which is to change
  • 也就是说我们对每一个 PTE 都定义一个 counter，用于记录上一次调用时间，并在每次引用的时候都寻找 counter 最小（时间离得最远）的 PTE，作为 victim 交换出去
• Stack implementation – keep a stack of page numbers in a double link form:
  • Page referenced:
    • move it to the top
    • requires 6 pointers to be changed
  • No search for replacement

​ 和 Optimal Algorithm 一样，LRU 置换没有 Belady 异常。这两者都属于同一种算法，称为堆栈算法。

LRU Approximation Algorithms#

​ 然而，在实际的情况下，很少能有计算机系统提供足够的硬件来支持真正的 LRU 算法。因此，我们考虑使用近似 LRU 页面置换算法来解决这个问题。

​ 一般而言，我们有两种方法来实现这个算法：

• Reference bit
  • With each page associate a bit, initially = 0
  • When page is referenced bit set to 1
  • Replace the one which is 0 (if one exists)
    • We do not know the order, however

• Second chance
  • Need reference bit
  • Clock replacement
  • If page to be replaced (in clock order) has reference bit = 1 then:
    • set reference bit 0
    • leave page in memory
    • replace next page (in clock order), subject to same rules

Counting-based Algorithms#

• Keep a counter of the number of references that have been made to each page
• LFU Algorithm: replaces page with smallest count
• MFU Algorithm: based on the argument that the page with the smallest count was probably just brought in and has yet to be used

Allocation of Frames#

​ 我们知道，在启动一个进程的时候，需要有一个最小数量的 pages，用于程序的运行。

• Each process needs minimum number of pages － usually determined by computer architecture.
• Example: IBM 370 – 6 pages to handle Storage-to-Storage MOVE instruction:
  • instruction is 6 bytes, might span 2 pages
  • 2 pages to handle from
  • 2 pages to handle to
• Two major allocation schemes
• 有两种常见的分配方式：
  • fixed allocation
  • priority allocation

Fixed Allocation#

• 分为两种方案：
  • Equal allocation
    • 每个进程分配到的 frame 都是相等的
  • Proportional allocation
    • 根据每个进程的大小分配可用的内存

Thrashing#

• If a process does not have “enough” pages, the page-fault rate is very high. This leads to:
  • low CPU utilization
  • Queuing at paging device, the ready queue becomes empty
  • operating system thinks that it needs to increase the degree of multiprogramming
  • another process added to the system
• Thrashing = a process is busy swapping pages in and out

​ 也就是说，若一个进程没有足够的 pages，会导致 page fault rate 很高，也就会导致 CPU 的利用率。同时，因为大量的 swapping 操作，进程会在 paging device 处变得拥挤，但 ready queue 会变得空闲，导致 OS 认为需要增加 degress of multiprogramming，也就是进程的数量，导致又一个进程被加入到系统中，恶性循环。这就是 Thrashing，也就是系统抖动。

​ 以上是 Thrashing 的示意图。

Demand Paging and Thrashing#

• Why does demand paging work?
• Locality model
  • Process migrates from one locality to another
  • Localities may overlap
• Why does thrashing occur?
  • ∑ size of locality > total memory size
• 实际上，正如上面所说的一样，出现 thrashing 的根本原因是 ∑ size of locality > total memory size，也就是要访问局部空间大于所分配的空间，这就导致了一致的 page fault
• To limit the effect of thrashing: local replacement algo cannot steal frames from other processes. But queue in page device increases effective access time.
• To prevent thrashing: allocate memory to accommodate its locality

​ 为了提高 CPU 的利用率并停止抖动，我们必须降低多道程序程度。有两种方法可以解决这个问题：

• local replacement algorithm：局部置换算法。
• priority replacement algorithm：优先权置换算法。

Working-Set Model#

​ 为了防止抖动，应该为进程提供足够多的所需要的 frame。但我们怎么知道 frame 的数量呢？这里我们使用一个名叫 working-set model 的方法

Page-Fault Frequency Scheme#

• Establish “acceptable” page-fault rate for each process
  • If actual rate too low, process loses frame
  • If actual rate too high, process gains frame

Memory-Mapped Files#

• Memory-mapped file I/O allows file I/O to be treated as routine memory access by mapping a disk block to a page in memory
• A file is initially read using demand paging. A page-sized portion of the file is read from the file system into a physical page. Subsequent reads/writes to/from the file are treated as ordinary memory accesses.
• Simplifies file access by treating file I/O through memory rather than read() write() system calls
• Also allows several processes to map the same file allowing the pages in memory to be shared

Allocating Kernel Memory#

• Treated differently from user memory
• Often allocated from a free-memory pool
  • Kernel requests memory for structures of varying sizes – needs to reduce fragmentation
  • Some kernel memory needs to be contiguous (certain h/w device interacts with contiguous physical memory)
• Therefore, many systems do NOT utilize paging for kernel code and data.

​ 对于上述问题，我们有两种解决方法：

• buddy system
• Slab Allocator

buddy system#