Transcript Advanced Operating Systems

Advanced
Operating Systems
Lecture 2: Design principles
University of Tehran
Dept. of EE and Computer Engineering
By:
Dr. Nasser Yazdani
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How to design a system
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Some general guides.
References
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“Hints for Computer System Design”, by
Butler W. Lampson
Read “The Interaction of Architecture and
Operation System Design”, Thomas E.
Anderson, et al.
“Lisp: Good News, Bad news, How to Win
Big”, Richard P. Gabriel
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Outline
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Introduction
Lampson general rules
The worse is better
Interaction between OS and Architecture.
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Why this discussion
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sea of possibilities, unclear about how one choice
will limit his freedom to make other choices, or
affect the size and performance of the entire
system.
In system design not try the best way (is there?)
avoid terrible choices.
We, as designer, forgot principles, obvious
problems. Usually, saying something and acting
another.
Pervious discussion, methods, top-down,
abstraction, modularity, etc., are there.
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Main and basic questions?
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Why it helps in making a good system?
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with functionality (does it work?),
speed (is it fast enough?), or
fault-tolerance (does it keep working?).
Where in the system design it helps?
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in ensuring completeness,
in choosing interfaces,
or in devising implementations.
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Functionality
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The most important hints, and the vaguest.
Obtaining the right functionality that is, with
getting it to do the things you want it to do.
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Separates Interface and implementation of some
abstraction from the clients who use the
abstraction.
Three conflicting requirements on interface:
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should be simple,
complete, and
admit a sufficiently small and fast implementation.
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Functionality (Simplicity1)
Perfection is reached not when there is no longer
anything to add, but when there is no longer anything
to take away. (A. Saint-Exupery).
 Do one thing at a time, and do it well. An interface
should capture the minimum essentials of an
abstraction. Don’t generalize; generalizations are
generally wrong. Too much its implementation =>
probably be large, slow and complicated.
KISS: Keep It Simple, Stupid. (Anonymous)
If in doubt, leave if out. (Anonymous)
Exterminate features. (C. Thacker)
Everything should be made as simple as possible, but no simpler. (A. Einstein)
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Functionality (Simplicity2)
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Services must have a fairly predictable cost,
and the interface must not promise more than
the implementer knows how to deliver.
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PL/1 got into serious trouble by attempting to
provide consistent meanings for a large number
of generic operations across a wide variety of
data types.
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Functionality (Simplicity3)

The Tenex system: reports a reference to an unassigned virtual page by a
trap to the user program. One of arguments of CONNECT to obtain access
to another directory is a string containing the password. If the password is
wrong, the call fails after a three second delay, to prevent guessing
passwords at high speed.
for i := 0 to Length(directoryPassword) do
if directoryPassword[i] ¹ passwordArgument[i] then
Wait three seconds; return BadPassword
connect to directory; return Success
 The following trick finds a password of length n in 64n tries on the average,
rather than 128n/2 Arrange the passwordArgument so that its first character
is the last character of a page and the next page is unassigned, and try each
possible character as the first. If CONNECT reports BadPassword, the guess
was wrong; if an unassigned page, it was correct. Now arrange the
passwordArgument so that its second character is the last character of the
page, and proceed in the obvious way.
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Functionality (Simplicity4)
An engineer is a man who can do for a dime what any
fool can do for a dollar. (Anonymous)
 Get it right. Neither abstraction nor simplicity is a
substitute for getting it right. In fact, abstraction can
be a source of severe difficulties.
for i := 0 to numberofFields do
FindIthField; if its name is name then exit
end loop
In {name: contents} Takes O(n2) to find name field?
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Functionality (Other
consideration 1)
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Make it fast, rather than general or powerful.
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RISC [39] doing general instructions like store,
load, etc., quick can run programs faster than
machines like the VAX with more general and
powerful instructions that take longer in the simple
cases. It is easy to lose a factor of two in the
running time of a program.
To find the places where time is being spent in a
large system. Need measurement tools that will
pinpoint the time-consuming code.
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Functionality (Other
consideration 1)
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Make it fast, Other people opinion
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“Make it simple”
Beware premature optimization
Ousterhout: The biggest speedup is when a system
goes from working to not working. That’s infinite
speedup.
it is necessary to have measurement tools ...
Design a system with a structure that facilitates
incremental tuning
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Functionality (Other
consideration 2)
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Don’t hide power.
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When a low level of abstraction allows something
to be done quickly, higher levels should not bury
this power inside something more general.
Alto disk hardware [53] can transfer a full cylinder
at disk speed, use it wherever possible.
Use procedure arguments to provide flexibility
in an interface. This technique can greatly
simplify an interface, eliminating unnecessary
parameters.
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Functionality (Other
consideration 3)
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Leave it to the client.
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An interface can combine simplicity, flexibility and high
performance by solving only one problem and leaving the
rest to the client.
Success of monitors as a synchronization device is partly
due to the fact that the locking and signaling mechanisms
do very little, leaving all the real work to the client
programs.
Unix encourages building small programs taking one or
more character streams, produce one or more streams as
output, and do one operation. Each program has a simple
interface and does one thing well, leaving the client to
combine a set of such programs with its own code.
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Functionality
(Continuity1)
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Keep basic interfaces stable.
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An interface embodies assumptions that are shared
by more than one part of a system or many
systems. It is very desirable not to change the
interface.
Keep a place to stand if you have to change the
interface.
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IBM 360/370 systems provided emulation of the
instruction sets of older machines.
Success of Microsoft due to the compatibility with
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products.
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Functionality ( Making it
works)
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Plan to throw one away; you will anyhow
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It costs a lot less if you plan to have a prototype. Rethink
about old and new features.
Keep secrets of the implementation. Assumptions
about an implementation that client programs are not
allowed to make. They are things that can change; the
interface defines the things that cannot change.
Divide and conquer.
Use a good idea again instead of generalizing it.
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A specialized implementation of the idea may be much more
effective than a general one.
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Functionality ( Handling
cases)
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Handle normal and worst cases separately : The
normal case must be fast. The worst case must make
some progress. In scheduling we might face deadlock.
Checking safety of status are extremely costly. Treat
deadlock differently.
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Speed ( 1)
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Split resources in a fixed way if in doubt, rather than
sharing them.
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It is usually faster to allocate dedicated resources, access
them, and the behavior of the allocator is more predictable.
Disadvantage is that more total resources are needed,
ignoring multiplexing overheads, than if all come from a
common pool.
Ex. Use registers for local variable since they fast.
Dynamic translation (compact, easily modified or easily
displayed) representation to one that can be quickly
interpreted.
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Smalltalk
use byte codes
and translate them on invoking18
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Speed ( 2)
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Cache answers to expensive computations, rather than
doing them over or precomputing. Prefetching
instructions.
Use hints to speed up normal execution. Using link or
references for allocated/free spaces. Try to avoid
lengthy searches
When in doubt, use brute force. Cost of hardware
declines, straightforward and easily analyzed solution
requiring computing cycles is better than a complex,
poorly characterized one that may work well under
certain assumptions. Ex. Ken Thompson’s chess
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machine,
personal computers
vs.
time-sharing
Speed ( 3)
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Compute in background when possible. Ex. garbage
collection algorithm, backups on the middle of night.
Use batch processing if possible.
Safety first. In allocating resources, strive to avoid
disaster rather than to attain an optimum. a generalpurpose system cannot optimize the use of resources.
It is easy enough to overload a system and drastically
degrade the service if it exceeds two-thirds of the
capacity. Ex. Page prediction algorithm have no use,
the memory is cheep
The nicest thing about the Alto is that it doesn’t run faster at night. (J. Morris)
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Speed ( 4)
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Shed load to control demand, rather than
allowing the system to become overloaded.
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An interactive system can refuse new users, or
even deny service to existing ones. A memory
manager can limit the jobs being served so that all
their working sets fit in the available memory.
Bob Morris suggested that a shared interactive
system should have a large red button on each
terminal.
Ethernet, IP are best effort delivery.
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Fault-tolerance( 1)
The unavoidable price of reliability is simplicity. (C. Hoare)
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End-to-end. Ex. Error recovery at the reliable data
transmission.
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Requires a cheap test for success.
Lead to working systems with severe performance defects
that may not appear until the system becomes operational
and is placed under heavy load.
Log updates to record the truth about the state of an
object.
Make actions atomic or restartable.
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Final word of Lampson
Most humbly do I take my leave, my lord.

Still we are left with building big and working
systems.
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Worse is Better
Two style of design
 MIT/Stanford style: Get it right. Ex. Common
Lisp.
 ATT style or New Jersey approach: Get it work.
Ex. Early Unix and C
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MIT/Stanford style
 Simplicity-the design must be simple, both in
implementation and interface. Interface is more
important.
 Correctness.
 Consistency-the design must not be inconsistent. A
design is allowed to be slightly less simple and less
complete to avoid inconsistency. Consistency is as
important as correctness.
 Completeness-the design must cover as many
important situations as is practical. All reasonably
expected cases must be covered. Simplicity is not
allowed to overly reduce completeness.
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New Jersey style
 Simplicity-the design must be simple, both in implementation
and interface. It is more important for the implementation to be
simple than the interface. Simplicity is the most important
consideration in a design.
 Correctness-the design must be correct. It is slightly better to
be simple than correct.
 Consistency-the design must not be overly inconsistent.
Consistency can be sacrificed for simplicity in some cases, but it
is better to drop those parts of the design that deal with less
common circumstances than to introduce either
implementational complexity or inconsistency.
 Completeness-the design must cover as many important
situations as is practical. Completeness can be sacrificed in
favor of any other quality. In fact, completeness must sacrificed
whenever implementation simplicity is jeopardized. Consistency
can be sacrificed to achieve completeness if simplicity is
retained;
especially worthless
consistency
of interface. 26
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Operating
Systems
Which one is better?
New Jersey Style has better survival
characteristics than the-right-thing.
The PC loser-ing problem (in calling a routine)
 MIT philosophy: Back out and restore the user
program PC to the instruction that invoked the
system routine.
 New Jersey Style: The system routine to always
finish, but sometimes an error code would be
returned that signaled that the system routine had
failed to complete its action. A correct user
program, then, had to check the error code to
determine whether to simply try the system routine
again. The MIT guy did not like this solution
because
the Operating
right Systems
thing.
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Which one is better?
 Unix and C are the ultimate computer viruses. Why?
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easy to write a decent compiler.
writing programs that is easy for the compiler to interpret.
simple structures, easy to port, require few machine resources to run.
provide about 50%--80% of functionality of an operating system and
programming language.
 Implementation simplicity has highest priority!
 Programmer is conditioned to sacrifice some safety,
convenience, and hassle to get good performance and modest
resource use.
 Initial virus has to be basically good. Then, the viral spread is
assured as long as it is portable. Then, there will be pressure to
improve it, possibly by increasing its functionality closer to 90%.
 gain acceptance
 will condition its users to expect less.
 will be improved to a point that is almost the right thing.
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 Compare
Lisp with C.
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Which one is better?
 How does the right thing stack up?
 ``big complex system scenario'‘
 ``diamond-like jewel'' scenario
 big complex system'' scenario: First, designed. Then its
implementation designed. Finally, its implemented.
 it has nearly 100% of desired functionality, implementation
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simplicity is not a concern.
long time to implement.
large and complex.
requires complex tools to use properly.
The last 20% takes 80% of the effort,
the right thing takes a long time to get out, and it only runs
satisfactorily on the most sophisticated hardware
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Which one is better?
 The ``diamond-like jewel'' scenario:
 takes forever to design.
 but it is quite small at every point along the way.
 To implement it to run fast is either impossible or beyond
the capabilities of most implementers.
 The first scenario is classic artificial intelligence
software.
 Both scenarios correspond to Common Lisp.
 Lesson: Not go for the right thing first. Get half of the
right thing available so that it spreads like a virus.
Once people are hooked on it, take the time to
improve it to 90% of the right thing.
 Wrong Lesson: C is not the right vehicle for AI
software
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History of OS: Change!
1980
Speed
2000
Factor
CPU
1 MIPS
1000 MIPS
1000
Memory
500 ns
2 ns
250
Disk
18 ms
2 ms
9
Modem
300 bits/sec
56 Kbits/sec
200
Memory
64 Kbytes
128 Mbytes
2000
Disk
1 Mbytes
6 Gbytes
6000
Cost
Per MIP
$100K
<= $1
100000
Other
Address bits
8
64
8
Users/machine
10s
<=1
.01
Capacity
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Interaction of OS & Arch.
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New Trend in Computer Architecture
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Simple, directly-executed instruction sets.
Open Arch., Make implementation visible to higher
level.
Migration of functions from HW to SW
Arch. Features are removed unless can be justified
by cost/performance
OS new requirements,
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Fast local communication,
Distributed programming
Parallel programming
Virtual Memory
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Interaction of OS & Arch.
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improved extensibility,
maintainability,
fault tolerance,
New system like Mach are moving toward more
modular organization.
Unfortunately, modern OS and Arch. have evolved
somewhat independently:
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Studies for performance overlook OS. In VAX %50 of
reference are for OS. Application result does not apply to
OS.
Modern Arch. Studies traditional OS not new requirement.
OS is focused on Performance, until HW limits. Then, HW is
bottleneck. But still it takes into account old Arch.
OS might be optimized not Software on top of it.
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Some tests
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Compared performance of several modern
microprocessors based on:
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Null system call — time to enter a null C procedure in the
kernel, with interrupt (re-)enabled, and then return.
Trap — time to take a data access fault (e.g., on a
protection violation), vector to a null C procedure in the
kernel, and return. This requires saving and restoring any
registers that are not preserved across procedure calls.
Page table entry change — the time, once in the kernel, to
convert a virtual address into its corresponding page table
entry, update that entry to change protection information,
and then update any hardware (e.g., the translation buffer)
that
caches this information.
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Some tests (Cont)

Results for a CISC, VAXstation 3200 (11.1 MHz CVAX and four RISC
Tektronix XD88/01 (2o MHz Motorola 88000, DECstation 3100 (16.6 MHz
MIPS R2000 [Kane 87]), DECstation 5000/200 (25 MHz MIPS R30001 ) and
SPARCstation 1+ (25 MHz Sun SPARC).
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Some tests (Cont)
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Objective: isolate architectural impact on operating
system functions. Restructured drivers to remove
operating system dependencies and measure only the
operating system independent parts. This often
reduced the execution times by a factor of two.
Further optimized (e.g., removing extraneous
procedure calls). Handlers were almost entirely written
in assembler.
Result: OS performance is not as good as application
according to the table Why?

More microcode for OS Primitives in RISC.
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Some tests (Cont)
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Major OS components
(IPC)
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IPC is central to modern operating system
structure and performance.
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From monolithic, centralized kernels to a more
decentralized structure. Independent address
spaces communicating through messages. Using
message, instead of shared memory simplifies the
move to a physically distributed topology or
distributed computing
Modularity, fault tolerance, extensibility of design.
Small kernel. Faster?
Resource sharing
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IPC: Cross-Machine
Communication

RPC has become the preferred method to communicate
between address spaces, both on the same machine and
across a network.
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Encapsulates message oriented communication in procedure call
semantics.
OS overhead dominates network Latency.
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IPC: Cross-Machine
Communication
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Cross-machine RPC involves communication between two
remote address spaces, the operating system must be involved
to transfer control and data between the client and server.
Tripling CPU speed would reduce SRC RPC latency by about
%50, on the expectation that the 83% of the time not spent on
the wire will decrease by a factor of3.
System calls and traps do not scale well.
Large register sets and pipelines, present on most modern
RISCS, are not likely to benefit interrupt processing and thread
management because of the additional state to examine, save,
and restore. checksum processing is memory intensive and not
compute intensive; each checksum addition is paired with a
load (which on some
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IPC: Local Communication
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How effectively the operating system can be decomposed,
as well as how rapidly clients can communicate with other
servers.
Lightweight remote procedure call (LRPC).
For the simplest local calls, LRPC achieves a 3-fold
performance
Each LRPC
enter Kernel
twice
Kernel is
bottleneck.
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IPC: Sys call & interrupt
Handling
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VAX performs functions in hardware as part of the system call and
return from exception instructions, the time to enter and exit the kernel
is longer, but the cost once in the kernel is much less.
SPARC use Register window to improve application, but %30 of the null
sys. Call is on register window processing.
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IPC: Sys call & interrupt
Handling
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Motorola 88000 loses much of its performance
advantage because of the complexity of managing
its pipelines in software when a trap occurs. The
88000 includes a large number of registers
containing pipeline state, and these must be
examined and manipulated on a trap to check for
and service any outstanding faults.
Low level trap handling code on the R2000 makes
relatively poor use of load and branch delay slots.
Nearly %50 of the delay slots in this code path are
unfilled, accounting for approximately %13 of the
null system call time.
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IPC: Data Copying
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
Data copying is another area in which modern
processors have not scaled proportionally to their
integer performance, yet it is an important aspect of
local and remote communication.
arguments may have to be copied as many as 4
times: from client to kernel, from kernel to server,
from server to kernel, and from kernel to client.
Various optimizations can be applied, but even in
LRPC which uses a shared client/server buffer, two
copies are necessary: one to copy arguments from
the invocation stack on the call, and one to copy
results on the return.
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IPC: Data Copying

Problems: Mismatch between memory speed
and processor speed.


On Chip cache? One level, two level? But it is
relatively small.
Data copying for message passing and cache
interface is there too.
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Virtual Memory
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The most basic use of virtual memory is to support
address spaces larger than the physical memory.
Enhance the system performance. Ex. Mach uses
copy on write to speed program startup and crossaddress space communication
transparently support parallel programming across
networks. Important for an illusion of a shared
memory multiprocessors.
Other supports with virtual memory like transaction
locking, garbage collection
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Virtual Memory

performance of a virtual memory

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The ratio of physical to virtual memory size.
The size and organization of the TLB
The cost of servicing a fault
Page replacement algorithm.
For OS more important factors



Flexibility of the addressing mechanism,
The information provided by that mechanism, and
The ease of handling faults and changing hardware VM
addressing state.
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V. M. (Fault Handling)

New Arch. Are not good in exception handling, besides


Motorola 88000 has 5 internal pipelines, including an instruction fetch
pipeline, each of which must be restarted after a fault. Associated
with these pipelined execution units are nearly 30 internal registers.
During an exception, many of these registers must be read, saved,
and restored, adding to the complexity and latency of fault handling.
When a page fault occurs on the 88000, several instructions may be
in the process of execution, and instructions following the faulting
instruction may have already completed.
Reduction of the information provided to handle faults. Intel i860
provides no information on the faulting address; in fact, it provides
little information about why the fault occurred. Trap handler knows
only that a data access fault of some type occurred, and the address
of the faulting instruction. The fault handler must interpret the
faulting instruction to determine the type of fault and the offending
address. This requirement adds 26 instructions to our trap handler
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V. M. (TLB and page tables)


Formerly, TLB miss handling was hidden from the operating
system, but some new RISC architectures have moved TLB
management into software. Adv. Flexibility in page table
structure.
MIPS virtual address space is divided into two halves: user
and system spaces. User space addresses are always
translated through the TLB. System space, however, is again
subdivided into four regions: unmapped and cached,
unmapped and uncached, mapped and cached, and mapped
and uncached. Unfortunately, TLB is used poorly by OS, in
VAX 11/780 two thirds of all TLB misses.

Unmapped region is accessed directly through a physical base
register, there is no indirection and therefore no ability to specify
page-level protection or access control, except to the entire region.
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Threads and Multiproccessing




Threads, or “lightweight processes,” have become a
common and necessary component of new
languages and operating systems
System level and user level thread.
User-level thread systems can provide highperformance parallel programming primitives that
approach minimal hardware costs due to low cost of
communication within a single address space.
The importance of threads will continue to increase
in the future.
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Arch & User-level Threads
While fine-grained user-level threads require no
special architectural support, the architecture can
have a large impact on thread performance.
 Most crucial is the amount of state that must be
managed to context switch from one thread to
another thread in the same address space.
 Most of the newer RISC processors, such as the
Sun SPARC, the MIPS R2000 and the IBM RS6000,
have more than 64 registers, compared to 16 in
most earlier 32-bit CISC architectures.

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Arch & User-level Threads
On SPARC # of registers to be saved depends on the number
of register windows in use at the time of the context switch. It
shown that with 8 windows, on average three need to be
saved/restored on each context switch. It spend %70 of its time
To save and restore windows. Window pointer is privileged
Register, then, user level context is impossible.
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Arch & User-level Threads

Larger register sets can reduce memory accesses.
Register windows greatly reduce the cost of
parameter passing and register saving on procedure
call. The assumption was that procedure calls are
much more frequent than context switches. Since
context switching was expensive, it was avoided if
possible. But for user level thread parallelism we can
avoid it

In Synapse, OO, ratio of procedure call to C. S. is 21:1.
But, it spend more time on C.S since it is 50 times costly.
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Arch & Kernel-level Threads


Kernel-level threads can be problematic due to less
TLB effectiveness (thread context switches between
separate address spaces), specially, in architectures
with small numbers of TLB entries and when threads
are scheduled independently of the address space
with which they are associated.
MIPS R2000/R3000 has no atomic semaphore; this
hurts performance since threads often are used for
program structure as well as for parallel
programming. Then, system have to use trap into
the kernel or locking. Both are expensive.
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OS & Applications

Performance of primitive Os functions has not
scaled with overall processor performance,



Low-level primitives such as trap and context switch are
frequently used
These primitives are becoming more frequently used as
operating system structure evolves.
Applications: spellcheck- 1; latex-150; andrew-local
(a script of file system intensive programs such as copy,
compile and search, run using an entirely local file system);
andrew-remote (the same script run using a remote file
system); link- vmunix (the final link phase of a Mach
kernel build) and parthenon (a resolution-based theorem
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prover
that rises multiple
threads)
OS & Applications
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Advanced Operating Systems
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OS & Applications
All applications were run on a MIPS R3000baaed DECstation 5000/200 with 24 megabytes
of memory. We ran each program three times
to smooth out the effects of paging and file
caching
 Application were running on Mach OS.

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OS & Appl. (Results)
1.
2.
3.
4.
The most important, operating system primitives occur
frequently, particularly in a small-kernel operating system
such as Mach 3.o, and their performance has a clear effect
on application
Decomposed system will execute more low-level system
functions than a monolithic system.
The number of kernel-level TLB misses is significantly larger
for all applications running under Mach 3.o than it is under
Mach 2.5.
The emulated instruction have significant performance due to
lack of an atomic Test-and-Set instruction. Critical sections
execute in kernel mode in Mach 2.5 and can simply disable
interrupts. But in Mach 3.0 the operating system’s critical
sections execute at user-level; a trap to the kernel is needed
to provide mutual exclusion.
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Next Lecture
Kernel
 References




"Exokernel: An Operating System Architecture for
Application-Level Resource Management",
"The x-Kernel: An Architecture for Implementing
Network Protocols"
TinyOS: An Operating System for Sensor Networks
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