CMPT 401 Summer 2007

Dr. Alexandra Fedorova

Lecture III: OS Support

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The Role of the OS

• The operating system needs to provide support for implementation of distributed systems

• We will look at how distributed systems services interact with the operating systems

• We will discuss the support that the operating system needs to provide

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Direct Interaction with the OS

OS

Process:a DS

component

system calls

• A process directly interacts with the OS via system calls

• Example: a web browser, a web server

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Interaction via Middleware Layer

OS

Process:a DS

component

system calls

Middleware

Function calls or IPC

• A process directly interacts with the OS via a middleware layer

• A middleware layer directly interacts with the OS

• Example: a peer-to-peer file system implemented over a distributed hash table

5CMPT 401 Summer 2007 © A. Fedorova

Interaction via Inclusion

OS DS component

• A DS component is a part of the operating system, i.e., an operating system daemon

• Example: Network File System (NFS) daemon• Runs as a separate kernel thread, shares address space with the

kernel, interacts with the rest of the OS via function calls• Why would one want to build a DS component that interacts with

the OS via inclusion?

6CMPT 401 Summer 2007 © A. Fedorova

Infrastructure Provided by the OS

• Networking– Interface to network devices– Implementation of common protocols: TPC, UDP, IP

• Processes and threads– Efficient scheduling, load balancing and thread

switching– Efficient thread synchronization– Efficient inter-process communication (IPC)

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The Need for Good Process/Thread Support

• Many distributed applications are implemented using multiple threads or processes

• Why?

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Motivation for Multithreaded Designs

• Servers provide access to large data sets (web servers, e-commerce servers)

• Even in the presence of caching, they often need to do I/O (to access files on disk or a network FS)

• I/O takes much longer than computation

• Overlapping I/O with computation to improve response time

• Threads make it easy to overlap I/O with computation

• While one thread blocks on I/O another can perform computation

Single thread

Multiple threads

1 request 1.6 requests

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Process or Thread Scheduling

• Will use “process” and “thread” interchangeably– A single-threaded process maps to a kernel thread– Each thread in a multithreaded process (usually) maps to a kernel

thread• A scheduler decides which thread runs next on the CPU• To ensure good support for DS components, a scheduler

must:– Be scalable– Balance the load well– Ensure good interactive response– Keep context switches to a minimum

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Case Study: Solaris™ 10 OS

• Solaris is often used on server systems• Known for its good scalability properties, good

load balancing and interactive performance• We will look at Solaris runqueues and how they

are managed– A runqueue is a scheduling queue– A structure containing pointers to runnable threads –

i.e., threads that are waiting for CPU

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Runqueues in SolarisGlobal kernel priority

queue kpqueueUser priority queues for CPU0 disp_qs

User priority queues for CPU1 disp_qs

… …

Pri 0 Pri 1 Pri N Pri 0 Pri 1 Pri N

• There is a user-level queue for each priority level• A dispatcher runs the thread from the highest-priority non-empty queue

12CMPT 401 Summer 2007 © A. Fedorova

Processor Load Balancing

• Load balancing ensures that the load is evenly distributed among the CPUs on a multiprocessor

• This improves the overall response time• Solaris kernel ensures that queues are well balanced when it

enqueues a thread into a runqueue

/* * setbackdq() keeps runqs balanced such that the difference in length * between the chosen runq and the next one is no more than RUNQ_MAX_DIFF. * (…) */

A comment from Solaris source code. Source: http://cvs.opensolaris.org/source/xref/onnv/onnv-gate/usr/src/uts/common/disp/disp.c, line 1200

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Tuning Thread Priorities For Improved Response Time

• If a thread has waited too long for a processor, its priority is elevated, so no thread is starved

• Threads holding critical resources are put to the front of the queue so that they release those resources as quickly as possible

/* * Put the specified thread on the front of the dispatcher * queue corresponding to its current priority. * * Called with the thread in transition, onproc or stopped state * and locked (transition implies locked) and at high spl. * Returns with the thread in TS_RUN state and still locked. */

A comment on setfrontdq from Solaris source code. Source: http://cvs.opensolaris.org/source/xref/onnv/onnv-gate/usr/src/uts/common/disp/disp.c, line 1381

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Ensuring Good Responsiveness in Time-Sharing Scheduler

• Solaris’s time-sharing scheduler (the default scheduler) assigns priorities so as to ensure good interactive performance

• If thread T used up it’s entire timeslice on CPU:– priority(T)↓, timeslice(T)↑

• If thread T has given up CPU before using up its timeslice:– priority(T) ↑, timeslice (T) ↓

• Why is this done?

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Time-Sharing Scheduler: Answers

• Minimizing context switch costs:– CPU-bound threads stay on CPU longer without a context switch– In compensation, they are scheduled less often, due to decreased

priority– Reducing the number of context switches improves performance

• Ensuring good response for interactive applications– Interactive applications usually don’t use up their entire timeslice– They process a network message and release the CPU before the

timeslice expires– Those applications will have their priority elevated, so they will

respond quickly when the next message arrives

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What Limits Performance of MP/MT Applications?

• The cost of context switching – depends on the hardware; the OS cannot fix it alone– Save/restore the registers– Flush the CPU pipeline– If switching address spaces

• May need to flush the TLB (depends on the processor)• May need to flush the cache (depends on the processor)

• The cost of IPC (IPC requires context switching)• The cost of inter-thread synchronization – by and large

depends on the program structure; OS can fix some of it, but not all