CS140 Project 1: Threads
Slides by Kiyoshi Shikuma
Overview
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Quick Review
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Threads
Scheduling
Synchronization
Project
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Pintos
Alarm Clock
Priority Scheduling
Advanced Scheduler
Quick Review: Threads
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Threads are an execution stream
Execution Context:
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Current location in program
Values of Variables
All maintained on thread’s stack
Pintos:
Struct thread {
name
ID
stack pointer
page directory
etc.
}
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This struct is defined in thread.h (a very useful file for this project)
Quick Review: Scheduling
New
terminated
admitted
interrupt
Ready
Exit
Running
scheduled
I/O or Event
completion
I/O Event or wait
Waiting
Quick Review: Scheduling cont’d
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Scheduling Policy
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FIFO
Round Robin
Priority
Preemption (Remove the “running” thread
and schedule a new one)
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Timer Interrupt
Device Interrupt (i.e. network, disk, etc)
Quick Review: Thread Scheduling
Wait on I/O
Thread A ___________
________
Thread B
Kernel
Timer Interrupt
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Quick Background: Synchronization
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Will be covered in lecture next week
Synchronization used to prevent race conditions,
avoid concurrency problems One way to avoid
synchronization issues is turn off interrupts (only one
thing running so no issues), but don’t want to do this
Better solution: Use synchronization primitives
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Locks
Semaphores
Condition Variables
You will probably be modifying synch.h/c
Project: Pintos
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Simple OS created by Ben Pfaff
Could run on x86 hardware but we are using emulators
(makes debugging and running easier)
– Bochs
– Qemu
For Threads project you will be (surprise!) working in the
threads directory.
– Alarm Clock will require modification of timer.c in the
devices folder but you shouldn’t need to modify any
other files outside threads
Project: Getting Started
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Follow website instructions to install and set the path
variable (suggest adding to .cshrc so you don’t have
to set every time)
To build run “make” in pintos/src/threads
To run tests run “make check” in
pintos/src/threads/build
– 7/27 tests should pass when you first download
and run
“pintos” script simplifies usage of simulators:
– i.e. “pintos –v -- -q run alarm-single”
– Follow instructions to use two terminals to use
GDB on pintos
– Printf debugging is good too
Project: Alarm Clock
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Function: void timer_sleep (int64_t ticks)
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You will be modifying this function in timer.c
Currently busy waits
Need to instead put thread to sleep and wake it
up when enough time has elapsed
Notice that testing suite already passes (most) of
the alarm clock tests, so make sure your
implementation is correct.
Project: Priority Scheduling
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Each thread is assigned one of 64 priorities.
Current implementation uses a round robin
scheduler
Change this to instead schedule the highest
priority thread that can run
(Useful file is thread.h/c)
Project Priority Scheduling
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Need to consider all cases:
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When scheduling the next thread, choose one
with highest priority
A thread lowers its priority (should yield if there is
a higher-priority thread in the ready list)
A higher priority thread wakes up (such as alarm
clock or after waiting on a lock/semaphore) it
should preempt the lower priority thread.
Etc.
Project: Priority Donation
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Problem: Priority Inversion
– L acquires a lock
– H becomes ready, preempts L and starts running
– M becomes ready
– H waits for the lock held by L
– M starts running
M runs, then L, then H
After H begins waiting on the lock, L should run (until it
releases lock), then H, then M
Project: Priority Donation cont’d
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Solution: Priority Donation
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When H starts waiting on the lock L is holding, it
“donates” its priority to L such that L now runs
with H’s priority
When L releases the lock, it reverts back to its
original priority
Nested Donations: H->M->L
Multiple Donations: H->L <-M
Project: Advanced Scheduler
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BSD Scheduler (Read doc linked on site)
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Attempts to reduce the average response time of
jobs
Calculates statistics during thread execution and
sets priorities based on these
Does NOT do priority donation
Design Document
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Template is on website
Start early (don’t wait until you’re done
coding to write this)
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Thinking through the questions will help you guide
your coding
Do NOT skimp on this, it is worth just as
much as the code
Project: Submission
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Save design doc as DESIGNDOC in threads
directory (no extension)
Run “make grade”
Copy build/grade to GRADE in threads
directory
Run “make clean”
Run “/usr/class/cs140/bin/submit 1”
Submission instructions here
Some Advice:
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Start early
Read through the (relevant) docs and code
first (it’s very well documented)
Think about design before you start coding
Meet often as a group to integrate (don’t wait
until the last second)
Maintain good consistent coding style