1. CS140 Project Session Notes #1: Threads Haobo Xu, TA Fall 2006-07 Based on Ben Sapp’s slides Download these slides from http://cs140/misc/project1.ppt

2. Overview ● Getting Started ● Alarm Clock ● Priority Scheduling ● Advanced Scheduler (MLFQS) Ask questions anytime

3. Getting Started ● Make sure Pintos is up and running – set path = (/usr/class/cs140/`uname -m`/bin $path) – zcat /usr/class/cs140/pintos/pintos.tar.gz | tar x – cd pintos/src/threads/ – make – cd build/ – pintos run alarm-multiple ● Do this on either the Solaris (elaine) or Linux (vine) machines in Sweet Hall

4. Thread System Overview 4 kB +---------------------------------+ | kernel stack | | | | | V | | grows downward | | | +---------------------------------+ | magic | | : | | status | | tid | 0 kB +---------------------------------+ scheduler timer ready list wait list for keyboard... wait list for hard drive... wait list for some lock... cpu...

5. Alarm Clock ● reimplement timer_sleep() in devices/timer.c to avoid busy waiting: ● instead, take thread off the ready list /* Suspends execution for approximately TICKS timer ticks. */ void timer_sleep (int64_t ticks){ int64_t start = timer_ticks (); ASSERT (intr_get_level () == INTR_ON); while (timer_elapsed (start) < ticks) thread_yield (); }

6. Priority Scheduling ● Implement: (a)If a thread is added to the ready list which has a higher priority than the currently running thread, immediately yield the processor to the new thread. (b)Also, when threads are waiting for a lock, semaphore, or condition variable, the highest priority waiting thread should be woken up first. ● How? (a) Check priority whenever adding to the ready list (as in thread_create() ) and choosing next thread to run ( next_thread_to_run()). (b) Each sema, lock, etc. keeps track of the threads that are associated with it...need to check their priority when the lock is released.

7. Priority Inversion Problem ● But there's a problem – Let's say a high priority thread H needs a lock held by a low priority thread L – H must wait until L runs so L can release its lock. – But L must defer to all higher priorities, so it must wait its turn to run. – In turn, H must wait for all lower priority threads to run before it can run again. – This is bad. H should run first.

8. Priority Donation Solution ● When a high priority thread waits on a lock held by a lower priority thread, donate the priority level of the high level thread to the low level thread. – Be sure to handle multiple donations, in which multiple priorities are donated to a single thread – Be sure to handle nested donations, e.g., H waits on M which waits on L...

9. Advanced Scheduler: Multi-Level Feedback Queue ● 64 ready queues, one for each priority ● scheduler chooses a thread from the highest-priority non-empty queue ● priority calculated using the cpu time used by a thread, and it's “niceness”: – priority = PRI_MAX - recent_cpu/4 - nice*2 ● see manual for formulas to calculate recent_cpu ● need to implement fixed point arithmetic library

10. Synchronization ● keep in mind that threads can be interrupted by other threads – use locks, semas and condition variables to make important updates to shared data ● but, interrupt handlers can't use locks (why?) – this means that you will have to disable interrupts for shared data used by an interrupt handler, i.e., when accessing thread variables, don't want timer interrupt to interfere.

11. Random Tips ● gdb/cvs: useful tools for the rest of your life – Check out the Debugging Tools section. – Check out the Development Tools section. ● use available data structures, especially list – helpful functions: insert, splice, pop/push front/back, end, begin, next, prev, size, list_empty, sort, max, min... ● read the manual (many times!) ● read the design document template!

12. Grading ● 50% automatic tests – no exceptions ● 50% design document – where the real grading happens – data structures, algorithms, synchronization, and rationale – coding standards: don't forget to indent and comment!

13. it's that easy... code to write: devices/timer.c | 42 +++++- threads/fixed-point.h | 120 ++++++++++++++++++ threads/synch.c | 88 ++++++++++++- threads/thread.c | 196 ++++++++++++++++++++++++++---- threads/thread.h | 23 +++ 5 files changed, 440 insertions(+), 29 deletions(-) questions?