Forking & Process SchedulingMechanicsA Quick ReviewHow To Launch a New Process?Can We Generalize?But We Want a Parent and a ChildWhat Gets CopiedShared MemoryCopy-On-WriteIssues of Copy-On-WriteProcess Creation & TerminationSignalsA Signaling SidebarScheduling PrimitivesOur Friend, The Transition DiagramProcess State Transition of Non-Preemptive SchedulingWho’s Happy Right NowSchedulerMore on SchedulerWhere Should PCB Be Saved?Physical Memory & MultiprogrammingJob SwappingAdd Job Swapping to State Transition DiagramThink About SwappingForking & Process SchedulingVivek Pai / Kai LiPrinceton University2MechanicsExams not graded–Hopefully, this weekFilesystem project–Extension + bonus?–This wasn’t supposed to happenToday: forking + scheduling3A Quick ReviewWhat have we covered–How to store data via files–How to virtualize memory•Every process gets uniform address space•Lots of tricks can be played to share memoryWhat’s left–How to share/virtualize the processor–Having processes communicate/cooperate4How To Launch a New Process?Obvious choice: “start process” system callBut not all processes start the same–“testprogram” versus “testprogram >& outfile” versus “testprogram arg1 arg2 >& outfile”The “parent” process wants to specify various aspects of the child’s “environment”–Next step: add more parameters to specify environment5Can We Generalize?What happens as more information gets added to the process’s “environment” – more parameters? New system calls? This gets uglyWhat’s the most general way of setting up all of the environment?So, why not allow process setup at any point?–This is the exec( ) system call (and its variants)6But We Want a Parent and a ChildThe exec call “destroys” the current processSo, instead, destroy a copy of the process–The fork( ) call duplicates the current process–Better yet, don’t tightly couple fork and exec•This way, you can customize the child’s environmentSo what does fork( ) entail?–Making a copy of everything about the process–Ouch!7What Gets CopiedSo far, we’ve covered the following:–VM system–File system–SignalsHow do we go about copying this information?What parts are easy to copy, and what’s hard?What’s the common case with fork/exec?–What needs to get preserved in this scenario?8Shared MemoryHow to destroy a virtual address space?–Link all PTEs–Reference countHow to swap out/in?–Link all PTEs–Operation on all entriesHow to pin/unpin?–Link all PTEs–Reference count............Process 1Process 2w...wPage tablePage tablePhysicalpages9............Copy-On-WriteChild’s virtual address space uses the same page mapping as parent’sMake all pages read-onlyMake child process readyOn a read, nothing happensOn a write, generates an access fault–map to a new page frame–copy the page over–restart the instructionParent processChild processrr...rrPage tablePage tablePhysicalpages10Issues of Copy-On-WriteHow to destroy an address space–Same as shared memory case?How to swap in/out?–Same as shared memoryHow to pin/unpin–Same as shared memory11Process Creation & TerminationFour primitives:Fork – create a copy of this processExec – replace this process with this programWait – wait for child process to finishKill – (potentially) end a running processProcesses form a tree – what happens when parent disappears?12SignalsAsynchronous event delivery mechanismExamples – FPE, segv, ctrl-c, hang up, resumeDefault actions – ignore, abort, core dumpHandler – program-specified routine for signal13A Signaling SidebarWhat’s wrong with this program:int randVal;void SigHand(void){printf(“your rand val is %d\n”, randVal);}int main(int argc, char *argv[]){set up ctrl-c handler;while (1) {randVal = 0;randVal = 1;…randVal = 9;}}14Scheduling PrimitivesBlock – wait on some event/resource–Network packet arrival–Keyboard, mouse input–Disk activity completionYield – give up running for now–Directed (let my friend run)–Undirected (let any process run)Synchronization–We will talk about this later15Our Friend, The Transition DiagramRunningBlockedReadyScheduler dispatchWait forresourceResource becomesavailableCreatea processterminate16Process State Transition ofNon-Preemptive SchedulingRunningBlockedReadyResource becomes available(move to ready queue)Createa processTerminate(call scheduler)Yield(call scheduler)Block for resource(call scheduler)Schedulerdispatch17Who’s Happy Right NowSchedulerA non-preemptive scheduler invoked by explicit block or yield callsThe simplest formScheduler:save current process state (into PCB)choose next process to rundispatch (load PCB and run)Does this work?More on SchedulerShould the scheduler use a special stack?–Yes, because a user process can overflow and it would require another stack to deal with stack overflowShould the scheduler simply be a kernel process?–You can view it that way because it has a stack, code and its data structure–This process always runs when there is no user process20Where Should PCB Be Saved?Save the PCB on its user stack–Many processors have a special instruction to do it “efficiently”–But, need to deal with the overflow problem–When the process terminates, the PCB vanishesSave the PCB on the kernel heap data structure–May not be as efficient as saving it on stack–But, it is very flexible and no other problems21Physical Memory & MultiprogrammingMemory is a scarce resourceWant to run many programsPrograms need memory to runWhat happens whenM(a) + M(b) + M(c) > physical mem?Answer: paging. But what if no paging?22Job SwappingPartially executedswapped-out processesReady QueueCPUI/O WaitingqueuesI/OTerminateSwap outSwap in23Add Job Swapping toState Transition DiagramRunningBlockedReadyResource becomes available(move to ready queue)Createa processTerminate(call scheduler)Yield(call scheduler)Block for resource(call scheduler)SchedulerdispatchSwap outSwap inSwap24Think About SwappingIs swappingNecessaryDesirableGoodIdealThings to considerPerformanceComplexityEfficiencyMoreover, what decides swapping versus paging?Is each appropriate