Os solaris memory management


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Os solaris memory management

  1. 1. MEMORY MANAGEMENT 9/3/2012 1
  2. 2. Agenda Introduction to Solaris History of solaris Solaris memory architecture. Backing store VM system. Solaris memory management. Swapping Demand paging. 9/3/2012 2
  3. 3. Introduction to solaris Solaris is a Unix operating system originally developed by Sun Microsystems. Solaris is known for its scalability especially on SPARC systems and for originating many innovative features such as DTrace, ZFS and Time Slider. Solaris supports SPARC-based and x-86 based workstation and servers from Sun and other vendors. Solaris has a reputation for being well-suited to symmetric multiprocessing, supporting a large number of CPUs. Programmed in C. 9/3/2012 3 Its source model is Mixed open source/closed
  4. 4. History of solaris In earlier days SunOS is implemented in uniprocessor workstation. In 1980’s distributed and network-based computing became popular, so they went to multiprocessor system to speedup. In 1987, AT&T bell and Sun joined together for a project and developed a new OS by merging the existing OS.(SunOS,BSD,SVR3,Xenix). That new OS is named as System V Release 4 (SVR4) and then its name changed as Solaris 2. Latest version is Solaris 11. 9/3/2012 4
  5. 5. Solaris Memory Architecture Physical memory is divided into fixed-sized pieces called pages. The size of the page varies from platform to platform. The common size is 8 Kbytes. Each page is associated with a file and offset. The file and offset identify the backing store for the page. 9/3/2012 5
  6. 6. Backing store Backing store is the location to which the physical page contents will be migrated when that page is need to be taken for another process. The pages are migrated to slower medium like disk and that is called swap space. The location where the pages are migrated is called page-out. When again that pages are needed by the process, the location from which the pages are read back again is called page-in. 9/3/2012 6
  7. 7. Virtual memory System Virtual memory system is the core for a Solaris system.Why have a Virtual memory System? It presents simple memory programming model. It allows process to see linear range of bytes, regardless of fragmentation of the real memory. It gives programming model with a larger size than available physical storage. 9/3/2012 7
  8. 8. Jobs of VM System The job of VM system is to keep the most frequently referenced portions of memory in primary storage. When RAM shortage comes, VM is required to free the RAM by transfering infrequently used memory out to the backing store. By doing so,the VM system optimizes the performance. VM system supports multiple users and sharing of pages and thus provide protection. 9/3/2012 8
  9. 9. Process Memory Allocation A process will have a linear virtual address space. The linear Virtual address space of a process is divided into segments like Executable-text : Executable instructions in binary form Executable-Data : Initialized variables in the executable Heap space : Memory allocated by malloc() Process Stack and so on…… 9/3/2012 9
  10. 10. Process memory allocation 9/3/2012 10
  11. 11. Solaris virtual to physical memory management 9/3/2012 11
  12. 12. Virtual memory management unit(MMU) Solaris kernel breaks the process into segments and segments into pages. The hardware MMU maps those pages into physical memory by using a platform-specific set of translation tables. Each entry in the table has the physical address of the page of the RAM,so that memory access can be converted on-the-fly in hardware. 9/3/2012 12
  13. 13. Shared Mapped File 9/3/2012 13
  14. 14. Solaris Memory Management Two basic types of memory management manage the allocation and migration of physical pages of memory to and from swap space : 1.Swapping 2.Demand paging The VM system uses a global paging model that implements a single global policy to manage the allocation of memory between the processes. 9/3/2012 14
  15. 15. Swapping The swapping algorithm uses user process as the granularity for managing memory. If there is a shortage of memory, then all the pages of the least active process will be swapped out to the swap device, freeing memory for other processes. Then the corresponding flag in the process table is set to indicate that this process has been swapped out. 9/3/2012 15
  16. 16. Memory Scheduler The memory scheduler is launched at boot time and does nothing. If the memory is consistently less, it starts looking for processes that it can completely swap out. If shortage is minimal, then soft swap takes place. Otherwise hard-swap. 9/3/2012 16
  17. 17. Soft Swapping If the process have been inactive for atleast maxslp(default 20 seconds) seconds, then memory sceduler swaps out all the pages for that process. 9/3/2012 17
  18. 18. Hard Swapping Hard swapping takes place when all of the following are true : Atleast two processes are on the run queue, waiting for CPU. The average free memory over 30 seconds is consistently less than minimum. Excessive paging (page-out + page-in is high). 9/3/2012 18
  19. 19. Pros and Cons of Swapping Pros: 1.This is the inexpensive way to conserve memory. 2. Easy implementation. Cons: 1. It dramatically affects a process’s performance. So the swapping is used only as a last resort when the system is desperate for memory. 9/3/2012 19
  20. 20. Demand paging The Demand paging model uses a page as the granularity for memory management. Pages of memory are allocated on demand. When memory is first referenced, a page fault occurs and memory is allocated one page at a time. The page scanner and the virtual memory page fault mechanism are the core of the demand paged memory management. 9/3/2012 20
  21. 21. Page Scanner The page scanner is a kernel thread, which is awakened when the amount of memory on the free-page list falls below a system threshhold, typically 1/64 th of total physical memory. Scanner (pageout_scanner) tracks pages by reading the state of the hardware bits in MMU – MMU bits maintain state of referenced and written (dirty). – Uses a twohanded clock algorithm to determine eviction. 9/3/2012 21
  22. 22. Two Handed Clock Algorithm Both hands rotate clock-wise. The front hand clears the referenced and modified bit of each page. The trailing back hand then inspects the referenced and modified bits some time later. 9/3/2012 22
  23. 23. Scan Rate The scan rate changes during the course of the page scanners operation Scanners scans at slowscan when memory is below lotsfree and then increases to fastscan when memory has fallen below minfree threshold Slowscan is set to 100 pages by default. Fastscan is set to physicalmemory/2 capped at 8192 pages 9/3/2012 23
  24. 24. Page FaultsWhen do Page Faults occur? MMU-generated exceptions (trap) tell the operating system when a memory access cannot continue without the kernel’s intervention. Three major types of memory-related hardware exceptions can occur: Major page faults Minor page faults Protection faults 9/3/2012 24
  25. 25. Advantages of Demand paging Loading pages of memory on demand dramatically lowers the memory footprint  Memory footprint refers to the amount of main memory that a program uses or references while running. And startup time of the process. 9/3/2012 25
  26. 26. Memory sharingMultiple users’ processes can share memory Multiple processes can sharing program binaries and application data. The Solaris kernel introduced dynamically linked libraries. 9/3/2012 26
  27. 27. Memory protection A user’s process must not be able access the memory of another process. A program fault in one program could cause another program (or the entire operating system) to fail. The protection is implemented by using protection modes read, write, execute and boundary checking. 9/3/2012 27
  28. 28. Kernel Virtual Memory Layout The kernel uses virtual memory and MMU like the process. The kernel uses top 256 Mbytes or 512 Mbytes in common virtual address space. Most of the kernel memory are not pageable. This characteristics avoids the deadlocks. The kernel cannot rely on the global paging. 9/3/2012 28
  29. 29. Kernel Memory Allocation Kernel memory is allocated at different levels. Page allocator It allocates unmapped pages from the free list to the kernel address space. Solaris uses Resource map allocator to allocate the free memory to the kernel. Resource map allocator uses first-fit algorithm. 9/3/2012 29
  30. 30. Kernel Memory Slab Allocator Solaris provides a general-purpose memory allocator that provides arbitrarily sized memory allocations.We use the slab allocator for memory requests that are: Smaller than a page size Not an even multiple of a page size Frequently going to be allocated and freed, so would otherwise fragment the kernel map 9/3/2012 30
  31. 31. Why Slab Allocator? The reasons for introducing the slab allocator were as follows: The SVR4 allocator was slow to satisfy allocation requests. Significant fragmentation problems arose with use of the SVR4 allocator. The allocator footprint was large, wasting a lot of memory. With no clean interfaces for memory allocation, code was duplicated in many places. 9/3/2012 31
  32. 32. 9/3/2012 32
  33. 33.  The slab allocator uses the term object to describe a single memory allocation unit. Cache to refer to a pool of like objects. Slab to refer to a group of objects that reside within the cache. Slab allocator solves many of the fragmentation issues by grouping different sized memory objects. Many cache will be activate at once in the solaris kernel. 9/3/2012 33
  34. 34. ReferencesBook: SOLARIS Internals,Core Kernel Architecture,Sun Microsystems.Website: www.sun.com 9/3/2012 34
  35. 35. THANK YOU…. 9/3/2012 35