Class 15
CS 372H
08 March 2011

On the board
------------

1. Last time
2. Scheduling disciplines 
3. Lessons and conclusions

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1. Last time

    Recall three ways the kernel is invoked:

	--exceptions (divide by 0, page fault, etc.)

	--traps (for example, "int 0x30")
	    --confusing because "int" is really a trap here

	--hardware interrupts

    Scheduling intro

    A. When do scheduling decisions happen? [see picture from last time]

    B. What are metrics and criteria?
	--throughput, turnaround time (time for each process to
	complete), response time (time from request to response),
	waiting time (time each process spends in ready queue; this is
	pretty much the same thing as response time), fairness, CPU
	utilization

    C. Context switching costs

	--CPU time in kernel

	    --save and restore registers

	    --switch address spaces 

	--indirect costs

	    --TLB shootdowns, processor cache, OS caches (e.g., buffer
	    caches)

	--result: more frequent context switches will lead to worse
	throughput (higher overhead)

2. Scheduling disciplines

    A. FCFS/FIFO

	--run each job until it's done

	--P1 needs 24 seconds

	--P2 needs 3 seconds

	--P3 needs 3 seconds
	
	--[ P1         P2  P3 ] 

	--throughput: 3 jobs / 30 seconds = .1 jobs/sec

	--average turnaround time?
	    1/3(24 + 27 + 30) = 27

	--observe: scheduling P2,P3,P1 would drastically reduce average
	turnaround time

	--advantages to FCFS:
	    --simple
	    --no starvation
	    --few context switches

	--disadvantage:
	    --short jobs get stuck behind long ones

    ***
    Larger issue: I/O vs computation

	--jobs contain bursts of computation, then must wait for I/O

	--to maximize throughput:

	    --must maximize CPU utilization *and*

	    --must maximize I/O device utilization

	--how?

	    --overlap I/O and computation from multiple jobs

	    --means *response time* very important for I/O intensive jobs:
	    I/O device will be idle until some job gets small bit of CPU
	    to issue next I/O request

    Most CPU bursts are small; a few are very long

    What are implications for FCFS?

	CPU bound jobs will hold CPU until exit or I/O 
	(but I/O rare for CPU-bound process) 

	    --long periods where no I/O requests issued, and CPU held 
	    --Result: poor I/O device utilization 

	Example: one CPU-bound job, many I/O bound 
	    --CPU bound runs (I/O devices idle) 
	    --CPU bound blocks 
	    --I/O bound job(s) run, quickly block on I/O 
	    --CPU bound runs again 
	    --I/O completes 
	    --CPU bound job continues while I/O devices idle 

	Simple hack: run process whose I/O completed? 
	    --What is a potential problem? 
	    (Answer: when the CPU-bound job issues an I/O request, it
	    gets the CPU again and then bursts for a while, leaving us
	    back where we started.)
    ***

    B. Round-robin

	--add timer

	--preempt CPU from long-running jobs. per time slice or quantum

	--after time slice, go to the back of the ready queue

	--most OSes do something of this flavor

	    --JOS does something like this, as you're seeing in lab 4

	--advantages:

	    --fair allocation of CPU across jobs
	    --low average waiting time when job lengths vary
	    --good for responsiveness if small number of jobs

	--disadvantages:

	    --what if jobs are same length?
	    --example: 2 jobs of 50 time units each, quantum of 1
	    --average completion time: 100 (vs. 75 for FCFS)

	--how to choose the quantum size?
	    
	    --want much larger than context switch cost

	    --majority of bursts should be less than quantum

	    --pick too small, and spend too much time context switching

	    --pick too large, and response time suffers (extreme case:
	    system reverts to FCFS)

	    --typical time slice is between 10-100 milliseconds. context
	    switches are usually microseconds or tens of microseconds
	    (maybe hundreds)

    C. SJF (shortest job first)

	--STCF: shortest time to completion first 
	    --Schedule the job whose next CPU burst is the shortest
	    
	--SRTCF: shortest remaining time to completion first
	    --preemptive version of STCF: if job arrives that has a
	    shorter time to completion than the remaining time on the
	    current job, immediately preempt CPU to give to new job

	--idea:
	    --get short jobs out of the system
	    --big (positive) effect on short jobs, small
	    (negative) effect on large jobs
	    --result: minimize waiting time (can prove this)

	--seeks to minimize average waiting time for a given set of
	processes

	--example 1:

	    process     arrival time         burst time
	    P1		0			7
	    P2		2			4
	    P3		4			1
	    P4		5			4

	    preemptive:
	    P1 P1 P2 P2 P3 P2 P2 P4 P4 P4 P4 P1 P1 P1 P1 P1

	--example 2:

	    3 jobs
	    A, B: both CPU bound, run for a week
	    C: I/O bound, loop
		1 ms of CPU
		10 ms of disk I/O

	    by itself, C uses 90% of disk
	    By itself, A or B uses 100% of CPU

	    what happens if we use FIFO?
		--if A or B gets in, keeps CPU for 2 weeks
	    what about RR with 100msec time slice?
		--only get 5% disk utilization
	    what about RR with 1msec time slice?
		--get nearly 90% disk utilization
		--but lots of preemptions
	    with SRTCF:
		--no needless preeemptions
		--get high disk utilization

	--SRTCF advantages:
	    --optimal response time (min waiting time)
	    --low overhead 
	--disadvantages:
	    --not hard to get unfairness or starvation (long-running
	    jobs)
	    --does not optimize turnaround time (only waiting time)
	    --** requires predicting the future **

	so useful as a yardstick for measuring other policies (good way
	to do CS design and development: figure out what the absolute
	best answer is, then figure out how to approximate it)

	however, can attempt to estimate future based on past (another
	thing that people do when designing systems):

	    --Exponentially weighted average a good idea 

	    --t_n: length of proc's nth CPU burst

	    --\tao_{n+1}: estimate for n+1 burst

	    --choose \alpha, 0 < \alpha <= 1

	    --set \tao_{n+1} = \alpha * t_n + (1-\alpha)*\tao_n

	    --this is called an exponential weighted moving average
	    (EWMA)

	    --reacts to changes, but smoothly

	upshot: favor jobs that have been using CPU the least amount of
	time; that ought to approximate SRTCF

    D. Priority schemes
    
	--priority scheme: give every process a number (set by
	administrator), and give the CPU to the process with the highest
	priority (which might be the lowest or highest number, depending
	on the scheme)

	    --can be done preempively or non-preemptively

	--generally a bad idea because of starvation of low priority
	tasks 
	
	    --here's an extreme example:
		--say H at high priority, L at low priority
		--H tries to acquire lock, fails, and spins
		--L never runs

	    --but note: SJF is priority scheduling where priority is the
	    predicted next CPU burst time 

	--solution to this starvation is to increase a process's
	priority as it waits

    E. multilevel feedback queues

	[first used in CTSS; also used in FreeBSD. Linux up until 2.6.23
	did something roughly similar.]
	
	two ideas:
	    --*multiple queues, each with different priority*. OS does RR
	    at each non-empty priority level before moving onto next priority
	    level. 32 levels, for example.
	    --feedback: process's priority changes, for example based on
	    how little or much it's used the CPU
	
	result is to favor interactive jobs that use less CPU but
	without starving all of the jobs

	a process's priority might be set like this:
	    --decreases whenever timer interrupt found the process
	    running
	    --increases while the process is runnable but not running

	advantages:
	    --approximates SRTCF
	disadvantages:
	    --gameable: user puts in meaningless I/O to keep job's
	    priority high
	    --can't donate priority
	    --not very flexible
	    --not good for real-time and multimedia

    F. Real time
	--examples: cars, video playback, robots on assembly lines
	--Soft real time: miss deadline and CD will sound funny 
	--Hard real time: miss deadline and plane will crash 

	--Many strategies. Long literature here. Basically, as long as
	\sum (CPU_needed/period) <= 1, then first-deadline-first works.

    G. What Linux does

	--before Linux 2.4:
	    
	    --O(n) operations for O(n) processes

	    --no affinity (means: which CPU the process wants to, or
	    usually will, run on) on multicore systems (bad for cache)

	    --global run-queue lock (coarse-grained lock)

	--Linux 2.4 to 2.6.23
	
	    goal: O(1) for all operations

	    [see section 10.3.4 in the text]

	    Approach:

	    --140 priority levels

		0-99: for "real-time" tasks
		    --some are FIFO (not preemptible)
		    --some are round-robin (preemptible by clock or higher
		    priority process)
		[none is real "real time" because there are no guarantees or
		deadlines]

		100-140: for "timesharing" tasks
		    --for user tasks (depends on nice and behavior)A

	    --keeps per-process 4-entry "load estimator"

		--how much CPU consumed in each of last 4 seconds

		--adjusts priority by +/- 5 based on behavior

	    --each CPU has a run queue

	    --each run queue is implemented as :
	    
		active array
		expired array
		
		each array has 140 elements, each entry of which is a task
		list of processes

	    --avoids global clock and helps with affinity
		a separate load-balancer can move tasks between CPUs

	    --scheduling algorithm: run highest priority task in active
	    array; after task uses quantum, place it in expired list; swap
	    expired/active pointers when active list empty

		--bitmap cache for empty/non-empty state of each list 

	--post Linux 2.6.23: rough reinvention of ideas from stride
	scheduling

	    --motivation: the above approach is just a fancy multilevel
	    feedback queue with an efficient implementation, so it
	    inherits the disadvantages of multilevel feedback queues

    H. lottery and stride scheduling

	[citation: C. A. Waldsburger and W. E. Weihl. Lottery
	Scheduling: Flexible Proportional-Share Resource Management.
	Proc. Usenix Symposium on Operating Systems Design and
	Implementation, November, 1994.
	http://www.usenix.org/publications/library/proceedings/osdi/full_papers/waldspurger.pdf]

	--lottery scheduling:
	
	    Issue lottery tickets to processes 
	    -- Let p_i have t_i tickets 
	    -- Let T be total # of tickets, T = \sum t_i
	    -- Chance of winning next quantum is t_i / T
	    -- Note lottery tickets not used up, more like season tickets

	    controls long-term average proportion of CPU for each
	    process

	    can also group processes hierarchically for control
		--subdivide lottery tickets
		--can model as currency, so there can be an exchange
		rate between real currencies (money) and lottery tickets


	--lots of nice features

	    --deals with starvation (have one ticket --> will make
	    progress)

	    --don't have to worry that adding one high priority job will
	    starve all others

	    --adding/deleting jobs affects all jobs proportionally (T
	    gets bigger)

	    --can transfer tickets between processes: highly useful if a
	    client is waiting for a server. then client can donate
	    tickets to server so it can run.

		--note difference between donating tix and donating
		priority. with donating tix, recipient amasses enough
		until it runs. with donating priority, no difference
		between one process donating and 1000 processes donating

	--many other details

	    --ticket inflation for processes that don't use their whole
	    quantum

	    --use fraction f of quantum; inflate tix by 1/f until it
	    next gets CPU

	--disadvantages

	    --latency unpredictable

	    --expected error somewhat high 

		--for those comfortable with probability: this winds up
		being a binomial distribution. variance n*p*(1-p) -->
		standard deviation \proportional \sqrt(n),
		    --where:
			p is fraction of tickets owned
			n is number of quanta

	--in reaction to these disadvantages, Waldspurger and Weihl
	proposed *Stride Scheduling*
 
	    [citations:
	    
		C. A. Waldsburger and W. E. Weihl.  Stride Scheduling:
		Deterministic Proportional-Share Resource Management.
		Technical Memorandum MIT/LCS/TM-528, MIT Laboratory for
		Computer Science, June 1995. 
		http://www.psg.lcs.mit.edu/papers/stride-tm528.ps

		Carl A. Waldspurger. Lottery and Stride Scheduling:
		Flexible Proportional-Share Resource Management, Ph.D.
		dissertation, Massachusetts Institute of Technology,
		September 1995. Also appears as Technical Report
		MIT/LCS/TR-667.
		http://waldspurger.org/carl/papers/phd-mit-tr667.pdf]

	    --the current Linux scheduler (post 2.6.23), called CFS
	    (Completely Fair Scheduler), roughly reinvented these ideas

	    --basically, a deterministic version of lottery scheduling.
	    less randomness --> less expected error.
	   


3. Scheduling lessons and conclusions

    --Scheduling comes up all over the place

	--m requests share n resources

	--disk arm: which read/write request to do next?

	--memory: which process to take physical page from?

    --This topic was popular in the days of time sharing, when there was
    a shortage of resources all around, but many scheduling problems
    become not very interesting when you can just buy a faster CPU or a
    faster network.

	--Exception 1: web sites and large-scale networks often cannot
	be made fast enough to handle peak demand (flash crowds,
	attacks) so scheduling becomes important again. For example may
	want to prioritize paying customers, or address
	denial-of-service attacks.

	--Exception 2: some scheduling decisions have non-linear effects on
	overall system behavior, not just different performance for different
	users. For example, livelock scenario.

	--Exception 3: real-time systems:
	    soft real time: miss deadline and CD or MPEG decode will skip
	    hard real time: miss deadline and plane will crash

    --In principle, scheduling decisions shouldn't affect program's
    results

	--This is good because it's rare to be able to calculate the
	best schedule

	--So instead, we build the kernel so that it's correct to do a
	context switch and restore at any time, and then *any* schedule
	will get the right answer for the program

	--This is a case of a concept that comes up a fair bit in
	computer systems: the policy/mechanism split. In this case, the
	idea is that the *mechanism* allows the OS to switch any time
	while the *policy* determines when to switch in order to meet
	whatever goals are desired by the scheduling designer

	    [[--In my view, the notion of "policy/mechanism split" is
	    way overused in computer systems, for two reasons:
	    
		--when someone says they separated policy from mechanism
		in some system, usually what's going on is that they
		separated the hard problem from the easy problem and
		solved the easy problem; or

		--it's simply not the case that the two are separate.
		*every* mechanism encodes a range of possible policies,
		and by choice of mechanism you are usually constraining
		what policies are possible. That point is obvious but
		tends to be overlooked when people advertise that
		they've "fully separated policy from mechanism"]]

    --But there are cases when the schedule *can* affect correctness

	--multimedia: delay too long, and the result looks or sounds
	wrong

	--Web server: delay too long, and users give up

    --Three further lessons (besides policy/mechanism split):

	(i) Know your goals; write them down

	(ii) Compare against optimal, even if optimal can't be built. 

	    --It's a useful benchmark. Don't waste your time improving
	    something if it's already at 99% of optimal.

	    --Provides helpful insight. (For example, we know from the
	    fact that SJF is optimal that it's impossible to be optimal
	    and fair, so don't spend time looking for an optimal
	    algorithm that is also fair.)

	(iii) There are actually many different schedulers in the
	system that interact:

	    --mutexes, etc. are implicitly making scheduling decisions

	    --interrupts: likewise (by invoking handlers)

	    --disk: the disk scheduler doesn't know to favor one
	    process's I/O above another

	    --network: same thing: how does the network code know which
	    process's packets to favor? (it doesn't)

	    --example of multiple interacting schedulers:

		you can optimize the CPU's scheduler and still find it
		does nothing (e.g., if you're getting interrupted
		200,000 times per second, only the interrupt handler is
		going to get the CPU, so you need to solve that problem
		before you worry about how the main CPU scheduler
		allocates the CPU to jobs)


	    --Basically, the _existence_ of interrupts is bad for
	    scheduling.