1. Operating Systems CMPSC 473 Processes (5) September 22 2008 - Lecture 11 Instructor: Bhuvan Urgaonkar

2. Announcements Quiz 1 will be out tonight and due in a week Suggested reading: Chapter 4 of SGG If you want to do more work/learn more things –Get in touch with us –We can provide more work in your projects Honors credits? Honors thesis? Just for fun?! Impress me and get good letters when you apply to grad school?

3. Overview of Process- related Topics How a process is born –Parent/child relationship –fork, clone, … How it leads its life –Loaded: Later in the course –Executed CPU scheduling Context switching Where a process “lives”: Address space –OS maintains some info. for each process: PCB –Process = Address Space + PCB How processes request services from the OS –System calls How processes communicate Some variants of processes: LWPs and threads How processes die Done Partially done Done Start today

4. codedatafiles registersstack heap The notion of a thread Roughly, a flow of execution that is a basic unit of CPU utilization –E.g., a process, a KCP Note: this is not showing an address space (Fig. 4.1 from GGN) thread A single-threaded process

5. Multi-process Applications Many applications need to do multiple activities simultaneously –E.g., Web browser Parse requested URL and find IP address from DNS server Use system calls to send request to some Web server Receive response Assemble response and display it –Can you give another example? Solution #1: Write multi-process application as follows: –forks off multiple processes, each responsible for a certain “flow of execution” Programmer’s choice/decision –Employ IPC mechanisms for these processes to communicate (coming up soon) We already know about signals, how many have used pipes? Shared memory? –Employ synchronization (ccoming up in a few lectures) We would like these “flows of execution” (and not just the initiating process or the entire application) to be the basic unit across which CPU is partitioned (schedulable entity) –Why? –What about resources other than the CPU? (Will discuss this in a little while) –The OS design we have studied so far already achieves this

6. Multi-process Applications Many applications need to do multiple activities simultaneously –E.g., Web browser Parse requested URL and find IP address from DNS server Use system calls to send request to some Web server Receive response Assemble response and display it –Can you give another example? Approach #1: Write multi-process application as follows: –forks off multiple processes, each responsible for a certain “flow of execution” Programmer’s choice/decision –Employ IPC mechanisms for these processes to communicate (coming up soon) We already know about signals, how many have used pipes? Shared memory? –Employ synchronization (coming up in a few lectures) We would like these “flows of execution” (and not just the initiating process or the entire application) to be the basic unit across which CPU is partitioned (schedulable entity) –Why? –What about resources other than the CPU? (Will discuss this in a little while) –The OS design we have studied so far already achieves this

7. Multi-process Applications Many applications need to do multiple activities simultaneously –E.g., Web browser Parse requested URL and find IP address from DNS server Use system calls to send request to some Web server Receive response Assemble response and display it –Can you give another example? Approach #1: Write multi-process application as follows: –forks off multiple processes, each responsible for a certain “flow of execution” Programmer’s choice/decision –Employ IPC mechanisms for these processes to communicate (coming up soon) We already know about signals, how many have used pipes? Shared memory? –Employ synchronization (coming up in a few lectures) We would like these “flows of execution” (and not just the initiating process or the entire application) to be the basic unit across which CPU is partitioned (schedulable entity) –Why? –What about resources other than the CPU? (Will discuss this again for VMM and IO) –The OS design we have studied so far already achieves this

8. Approach #1: Writing a multi-process Application E.g., a Web browser What’s wrong with (or lacking in) this approach to programming? –Hint: Approach #1 has performance problems, although it is great for the programmer (why?) Potentially lot of redundancy in code and data segments! –Virtual memory wastage => More contention for precious RAM codedatafiles registersstack heap URL parsing process codedatafiles registersstack heap codedatafiles registersstack heap codedatafiles registersstack heap Network sending process Network reception process Interprets response, composes media together and displays on browser screen In virtual memory

9. Approach #1: Writing a multi-process Application E.g., a Web browser What’s wrong with (or lacking in) this approach to programming? –Hint: Approach #1 has performance problems, although it is great for the programmer (why?) Potentially, lot of redundancy in code and data segments! –Virtual memory wastage => More contention for precious RAM => More work for the memory manager => Reduction in computer’s throughput URL parsing processNetwork sending process Network reception process Interprets response, composes media together and displays on browser screen In virtual memory codedatafiles registersstack heap codedatafiles registersstack heap codedatafiles registersstack heap codedatafiles registersstack heap

10. Approach #2: Share code, data, files! E.g., a Web browser Share code, data, files (mmaped), via shared memory mechanisms (coming up) –Burden on the programmer Better yet, let kernel or a user-library handle sharing of these parts of the address spaces and let the programmer deal with synchronization issues –User-level and kernel-level threads URL parsing processNetwork sending process Network reception process Interprets response, composes media together and displays on browser screen In virtual memory codedata files registersstack heap registersstackregistersstackregistersstack

11. Approach #3: User or kernel support to automatically share code, data, files! E.g., a Web browser Share code, data, files (mmaped), via shared memory mechanisms (coming up) –Burden on the programmer Better yet, let kernel or a user-library handle sharing of these parts of the address spaces and let the programmer deal only with synchronization issues URL parsing processNetwork sending process Network reception process Interprets response, composes media together and displays on browser screen In virtual memory codedata files registersstack heap registersstackregistersstackregistersstack threads

12. Approach #3: User or kernel support to automatically share code, data, files! E.g., a Web browser Share code, data, files (mmaped), via shared memory mechanisms (coming up) –Burden on the programmer Better yet, let kernel or a user-library handle sharing of these parts of the address spaces and let the programmer deal with synchronization issues –User-level and kernel-level threads URL parsing processNetwork sending process Network reception process Interprets response, composes media together and displays on browser screen In virtual memory codedata files registersstackregistersstackregistersstackregistersstack threads heap

13. Multi-threading Models User-level thread libraries –E.g., the one provided with Project 1 –Implementation: You are expected to gain this understanding as you work on Project 1 –Pop quiz: Context switch overhead smaller. Why? –What other overheads are reduced? Creation? Removal? Kernel-level threads There must exist some relationship between user threads and kernel threads –Why? Which is better?

14. Multi-threading Models: Many-to-one Model Thread management done by user library Context switching, creation, removal, etc. efficient (if designed well) Blocking call blocks the entire process No parallelism on uPs? Why? Green threads library on Solaris k Kernel thread User thread

15. Multi-threading Models: One-to-many Model Each u-l thread mapped to one k-l thread Allows more concurrency –If one thread blocks, another ready thread can run –Can exploit parallelism on uPs Popular: Linux, several Windows (NT, 2000, XP) kkkk Kernel thread User thread

16. Multi-threading Models: Many-to-many Model # u-l threads >= #k-l threads Best of both previous approaches? kkk Kernel thread User thread