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OPERATING SYSTEMS
THREADS
TANNENBAUM, SECTION 2-2
THE BASICS
• Sometimes called a lightweight process
• Process
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It’s own memory space
Pgm ctr
Registers
Stack
• Thread
• Sibling threads share memory
• Makes context switching easier
• Each thread is separately scheduled
IMPLICATION
• A program can have 1 or more process
• Each process can 1 or more threads
• Grown important in windowing applications
• One thread could be blocked for i/o
• Another—in the same process—could be doing
other work
• Example: Word programs with three threads
• Handle keyboard input
• Format page
• Save output to disk every n seconds
WORD PROCESSOR WITH THREE
THREADS
Each could be working, while the others are
waiting
A WEB SERVER EXAMPLE
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Some pages are more commonly accessed
than others
More common pages are kept in memory
Less common pages are kept on disk
Requests for pages come in:
1. Dispatcher thread hands off request to worker
thread (ready to running)
2. Worker thread looks in cache. If not available,
worker thread issues read disk request and blocks
3. Dispatcher is scheduled. Go to step 1
THREAD USAGE (2)
A multithreaded Web server.
Tanenbaum & Bo,Modern Operating Systems:4th ed., (c) 2013 Prentice-Hall, Inc. All rights reserved.
MORE PRECISELY
//dispatcher
while(TRUE)
{
get_next_req(&buf);
handoff_work(&buf);
}
//worker
while(TRUE)
{
wait_for_work(&buf);
look_in_cache(&buf, &page);
if (page_not_in_cache(&page))
read_page_from_disk(&buf, &page);
return_page(&page);
}
PROCESSES VS. THREADS
Per Process Items
• Address space
• Global variables
• Open files
• Child processes
• Pending signals
• Accounting information
Per Thread Items (each thread shares all of the
above)
• Program counter
• Registers
• Stack
• State (running, ready, etc.)
PTHREADS
IEEE standard for Unix threads
Pthread_create : creates a new thread
Pthread_exit: terminates the threaad
Pthread_join: waits for a specific thread to exit
Pthread_yield: Release CPU to let another thread
run
See thread, thread1, thread2 on the web site.
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