Os solaris memory management

MEMORY MANAGEMENT

9/3/2012 1

Agenda
 Introduction to Solaris
 History of solaris
 Solaris memory architecture.
 Backing store
 VM system.
 Solaris memory management.
 Swapping
 Demand paging.

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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.
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 Its source model is Mixed open source/closed

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.

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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.

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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.
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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.

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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.
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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……

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Process memory allocation

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Solaris virtual to physical memory
management

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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.

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Shared Mapped File

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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.
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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.

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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.

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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.

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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).

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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.
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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

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.
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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.

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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

Page Faults
When 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

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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.

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Memory sharing

Multiple users’ processes can share memory

 Multiple processes can sharing program binaries
and application data.
 The Solaris kernel introduced dynamically linked
libraries.

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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.

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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

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.

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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

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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.

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 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.

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References
Book:
 SOLARIS Internals,Core Kernel
Architecture,Sun Microsystems.
Website:
www.sun.com

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THANK YOU….

9/3/2012 35

Os solaris memory management

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