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+BFS - The Brain Fuck Scheduler by Con Kolivas.
+
+Goals.
+
+The goal of the Brain Fuck Scheduler, referred to as BFS from here on, is to
+completely do away with the complex designs of the past for the cpu process
+scheduler and instead implement one that is very simple in basic design.
+The main focus of BFS is to achieve excellent desktop interactivity and
+responsiveness without heuristics and tuning knobs that are difficult to
+understand, impossible to model and predict the effect of, and when tuned to
+one workload cause massive detriment to another.
+
+
+Design summary.
+
+BFS is best described as a single runqueue, O(n) lookup, earliest effective
+virtual deadline first design, loosely based on EEVDF (earliest eligible virtual
+deadline first) and my previous Staircase Deadline scheduler. Each component
+shall be described in order to understand the significance of, and reasoning for
+it. The codebase when the first stable version was released was approximately
+9000 lines less code than the existing mainline linux kernel scheduler (in
+2.6.31). This does not even take into account the removal of documentation and
+the cgroups code that is not used.
+
+Design reasoning.
+
+The single runqueue refers to the queued but not running processes for the
+entire system, regardless of the number of CPUs. The reason for going back to
+a single runqueue design is that once multiple runqueues are introduced,
+per-CPU or otherwise, there will be complex interactions as each runqueue will
+be responsible for the scheduling latency and fairness of the tasks only on its
+own runqueue, and to achieve fairness and low latency across multiple CPUs, any
+advantage in throughput of having CPU local tasks causes other disadvantages.
+This is due to requiring a very complex balancing system to at best achieve some
+semblance of fairness across CPUs and can only maintain relatively low latency
+for tasks bound to the same CPUs, not across them. To increase said fairness
+and latency across CPUs, the advantage of local runqueue locking, which makes
+for better scalability, is lost due to having to grab multiple locks.
+
+A significant feature of BFS is that all accounting is done purely based on CPU
+used and nowhere is sleep time used in any way to determine entitlement or
+interactivity. Interactivity "estimators" that use some kind of sleep/run
+algorithm are doomed to fail to detect all interactive tasks, and to falsely tag
+tasks that aren't interactive as being so. The reason for this is that it is
+close to impossible to determine that when a task is sleeping, whether it is
+doing it voluntarily, as in a userspace application waiting for input in the
+form of a mouse click or otherwise, or involuntarily, because it is waiting for
+another thread, process, I/O, kernel activity or whatever. Thus, such an
+estimator will introduce corner cases, and more heuristics will be required to
+cope with those corner cases, introducing more corner cases and failed
+interactivity detection and so on. Interactivity in BFS is built into the design
+by virtue of the fact that tasks that are waking up have not used up their quota
+of CPU time, and have earlier effective deadlines, thereby making it very likely
+they will preempt any CPU bound task of equivalent nice level. See below for
+more information on the virtual deadline mechanism. Even if they do not preempt
+a running task, because the rr interval is guaranteed to have a bound upper
+limit on how long a task will wait for, it will be scheduled within a timeframe
+that will not cause visible interface jitter.
+
+
+Design details.
+
+Task insertion.
+
+BFS inserts tasks into each relevant queue as an O(1) insertion into a double
+linked list. On insertion, *every* running queue is checked to see if the newly
+queued task can run on any idle queue, or preempt the lowest running task on the
+system. This is how the cross-CPU scheduling of BFS achieves significantly lower
+latency per extra CPU the system has. In this case the lookup is, in the worst
+case scenario, O(n) where n is the number of CPUs on the system.
+
+Data protection.
+
+BFS has one single lock protecting the process local data of every task in the
+global queue. Thus every insertion, removal and modification of task data in the
+global runqueue needs to grab the global lock. However, once a task is taken by
+a CPU, the CPU has its own local data copy of the running process' accounting
+information which only that CPU accesses and modifies (such as during a
+timer tick) thus allowing the accounting data to be updated lockless. Once a
+CPU has taken a task to run, it removes it from the global queue. Thus the
+global queue only ever has, at most,
+
+	(number of tasks requesting cpu time) - (number of logical CPUs) + 1
+
+tasks in the global queue. This value is relevant for the time taken to look up
+tasks during scheduling. This will increase if many tasks with CPU affinity set
+in their policy to limit which CPUs they're allowed to run on if they outnumber
+the number of CPUs. The +1 is because when rescheduling a task, the CPU's
+currently running task is put back on the queue. Lookup will be described after
+the virtual deadline mechanism is explained.
+
+Virtual deadline.
+
+The key to achieving low latency, scheduling fairness, and "nice level"
+distribution in BFS is entirely in the virtual deadline mechanism. The one
+tunable in BFS is the rr_interval, or "round robin interval". This is the
+maximum time two SCHED_OTHER (or SCHED_NORMAL, the common scheduling policy)
+tasks of the same nice level will be running for, or looking at it the other
+way around, the longest duration two tasks of the same nice level will be
+delayed for. When a task requests cpu time, it is given a quota (time_slice)
+equal to the rr_interval and a virtual deadline. The virtual deadline is
+offset from the current time in jiffies by this equation:
+
+	jiffies + (prio_ratio * rr_interval)
+
+The prio_ratio is determined as a ratio compared to the baseline of nice -20
+and increases by 10% per nice level. The deadline is a virtual one only in that
+no guarantee is placed that a task will actually be scheduled by this time, but
+it is used to compare which task should go next. There are three components to
+how a task is next chosen. First is time_slice expiration. If a task runs out
+of its time_slice, it is descheduled, the time_slice is refilled, and the
+deadline reset to that formula above. Second is sleep, where a task no longer
+is requesting CPU for whatever reason. The time_slice and deadline are _not_
+adjusted in this case and are just carried over for when the task is next
+scheduled. Third is preemption, and that is when a newly waking task is deemed
+higher priority than a currently running task on any cpu by virtue of the fact
+that it has an earlier virtual deadline than the currently running task. The
+earlier deadline is the key to which task is next chosen for the first and
+second cases. Once a task is descheduled, it is put back on the queue, and an
+O(n) lookup of all queued-but-not-running tasks is done to determine which has
+the earliest deadline and that task is chosen to receive CPU next.
+
+The CPU proportion of different nice tasks works out to be approximately the
+
+	(prio_ratio difference)^2
+
+The reason it is squared is that a task's deadline does not change while it is
+running unless it runs out of time_slice. Thus, even if the time actually
+passes the deadline of another task that is queued, it will not get CPU time
+unless the current running task deschedules, and the time "base" (jiffies) is
+constantly moving.
+
+Task lookup.
+
+BFS has 103 priority queues. 100 of these are dedicated to the static priority
+of realtime tasks, and the remaining 3 are, in order of best to worst priority,
+SCHED_ISO (isochronous), SCHED_NORMAL, and SCHED_IDLEPRIO (idle priority
+scheduling). When a task of these priorities is queued, a bitmap of running
+priorities is set showing which of these priorities has tasks waiting for CPU
+time. When a CPU is made to reschedule, the lookup for the next task to get
+CPU time is performed in the following way:
+
+First the bitmap is checked to see what static priority tasks are queued. If
+any realtime priorities are found, the corresponding queue is checked and the
+first task listed there is taken (provided CPU affinity is suitable) and lookup
+is complete. If the priority corresponds to a SCHED_ISO task, they are also
+taken in FIFO order (as they behave like SCHED_RR). If the priority corresponds
+to either SCHED_NORMAL or SCHED_IDLEPRIO, then the lookup becomes O(n). At this
+stage, every task in the runlist that corresponds to that priority is checked
+to see which has the earliest set deadline, and (provided it has suitable CPU
+affinity) it is taken off the runqueue and given the CPU. If a task has an
+expired deadline, it is taken and the rest of the lookup aborted (as they are
+chosen in FIFO order).
+
+Thus, the lookup is O(n) in the worst case only, where n is as described
+earlier, as tasks may be chosen before the whole task list is looked over.
+
+
+Scalability.
+
+The major limitations of BFS will be that of scalability, as the separate
+runqueue designs will have less lock contention as the number of CPUs rises.
+However they do not scale linearly even with separate runqueues as multiple
+runqueues will need to be locked concurrently on such designs to be able to
+achieve fair CPU balancing, to try and achieve some sort of nice-level fairness
+across CPUs, and to achieve low enough latency for tasks on a busy CPU when
+other CPUs would be more suited. BFS has the advantage that it requires no
+balancing algorithm whatsoever, as balancing occurs by proxy simply because
+all CPUs draw off the global runqueue, in priority and deadline order. Despite
+the fact that scalability is _not_ the prime concern of BFS, it both shows very
+good scalability to smaller numbers of CPUs and is likely a more scalable design
+at these numbers of CPUs.
+
+It also has some very low overhead scalability features built into the design
+when it has been deemed their overhead is so marginal that they're worth adding.
+The first is the local copy of the running process' data to the CPU it's running
+on to allow that data to be updated lockless where possible. Then there is
+deference paid to the last CPU a task was running on, by trying that CPU first
+when looking for an idle CPU to use the next time it's scheduled. Finally there
+is the notion of "sticky" tasks that are flagged when they are involuntarily
+descheduled, meaning they still want further CPU time. This sticky flag is
+used to bias heavily against those tasks being scheduled on a different CPU
+unless that CPU would be otherwise idle. When a cpu frequency governor is used
+that scales with CPU load, such as ondemand, sticky tasks are not scheduled
+on a different CPU at all, preferring instead to go idle. This means the CPU
+they were bound to is more likely to increase its speed while the other CPU
+will go idle, thus speeding up total task execution time and likely decreasing
+power usage. This is the only scenario where BFS will allow a CPU to go idle
+in preference to scheduling a task on the earliest available spare CPU.
+
+The real cost of migrating a task from one CPU to another is entirely dependant
+on the cache footprint of the task, how cache intensive the task is, how long
+it's been running on that CPU to take up the bulk of its cache, how big the CPU
+cache is, how fast and how layered the CPU cache is, how fast a context switch
+is... and so on. In other words, it's close to random in the real world where we
+do more than just one sole workload. The only thing we can be sure of is that
+it's not free. So BFS uses the principle that an idle CPU is a wasted CPU and
+utilising idle CPUs is more important than cache locality, and cache locality
+only plays a part after that.
+
+When choosing an idle CPU for a waking task, the cache locality is determined
+according to where the task last ran and then idle CPUs are ranked from best
+to worst to choose the most suitable idle CPU based on cache locality, NUMA
+node locality and hyperthread sibling business. They are chosen in the
+following preference (if idle):
+
+* Same core, idle or busy cache, idle threads
+* Other core, same cache, idle or busy cache, idle threads.
+* Same node, other CPU, idle cache, idle threads.
+* Same node, other CPU, busy cache, idle threads.
+* Same core, busy threads.
+* Other core, same cache, busy threads.
+* Same node, other CPU, busy threads.
+* Other node, other CPU, idle cache, idle threads.
+* Other node, other CPU, busy cache, idle threads.
+* Other node, other CPU, busy threads.
+
+This shows the SMT or "hyperthread" awareness in the design as well which will
+choose a real idle core first before a logical SMT sibling which already has
+tasks on the physical CPU.
+
+Early benchmarking of BFS suggested scalability dropped off at the 16 CPU mark.
+However this benchmarking was performed on an earlier design that was far less
+scalable than the current one so it's hard to know how scalable it is in terms
+of both CPUs (due to the global runqueue) and heavily loaded machines (due to
+O(n) lookup) at this stage. Note that in terms of scalability, the number of
+_logical_ CPUs matters, not the number of _physical_ CPUs. Thus, a dual (2x)
+quad core (4X) hyperthreaded (2X) machine is effectively a 16X. Newer benchmark
+results are very promising indeed, without needing to tweak any knobs, features
+or options. Benchmark contributions are most welcome.
+
+
+Features
+
+As the initial prime target audience for BFS was the average desktop user, it
+was designed to not need tweaking, tuning or have features set to obtain benefit
+from it. Thus the number of knobs and features has been kept to an absolute
+minimum and should not require extra user input for the vast majority of cases.
+There are precisely 2 tunables, and 2 extra scheduling policies. The rr_interval
+and iso_cpu tunables, and the SCHED_ISO and SCHED_IDLEPRIO policies. In addition
+to this, BFS also uses sub-tick accounting. What BFS does _not_ now feature is
+support for CGROUPS. The average user should neither need to know what these
+are, nor should they need to be using them to have good desktop behaviour.
+
+rr_interval
+
+There is only one "scheduler" tunable, the round robin interval. This can be
+accessed in
+
+	/proc/sys/kernel/rr_interval
+
+The value is in milliseconds, and the default value is set to 6ms. Valid values
+are from 1 to 1000. Decreasing the value will decrease latencies at the cost of
+decreasing throughput, while increasing it will improve throughput, but at the
+cost of worsening latencies. The accuracy of the rr interval is limited by HZ
+resolution of the kernel configuration. Thus, the worst case latencies are
+usually slightly higher than this actual value. BFS uses "dithering" to try and
+minimise the effect the Hz limitation has. The default value of 6 is not an
+arbitrary one. It is based on the fact that humans can detect jitter at
+approximately 7ms, so aiming for much lower latencies is pointless under most
+circumstances. It is worth noting this fact when comparing the latency
+performance of BFS to other schedulers. Worst case latencies being higher than
+7ms are far worse than average latencies not being in the microsecond range.
+Experimentation has shown that rr intervals being increased up to 300 can
+improve throughput but beyond that, scheduling noise from elsewhere prevents
+further demonstrable throughput.
+
+Isochronous scheduling.
+
+Isochronous scheduling is a unique scheduling policy designed to provide
+near-real-time performance to unprivileged (ie non-root) users without the
+ability to starve the machine indefinitely. Isochronous tasks (which means
+"same time") are set using, for example, the schedtool application like so:
+
+	schedtool -I -e amarok
+
+This will start the audio application "amarok" as SCHED_ISO. How SCHED_ISO works
+is that it has a priority level between true realtime tasks and SCHED_NORMAL
+which would allow them to preempt all normal tasks, in a SCHED_RR fashion (ie,
+if multiple SCHED_ISO tasks are running, they purely round robin at rr_interval
+rate). However if ISO tasks run for more than a tunable finite amount of time,
+they are then demoted back to SCHED_NORMAL scheduling. This finite amount of
+time is the percentage of _total CPU_ available across the machine, configurable
+as a percentage in the following "resource handling" tunable (as opposed to a
+scheduler tunable):
+
+	/proc/sys/kernel/iso_cpu
+
+and is set to 70% by default. It is calculated over a rolling 5 second average
+Because it is the total CPU available, it means that on a multi CPU machine, it
+is possible to have an ISO task running as realtime scheduling indefinitely on
+just one CPU, as the other CPUs will be available. Setting this to 100 is the
+equivalent of giving all users SCHED_RR access and setting it to 0 removes the
+ability to run any pseudo-realtime tasks.
+
+A feature of BFS is that it detects when an application tries to obtain a
+realtime policy (SCHED_RR or SCHED_FIFO) and the caller does not have the
+appropriate privileges to use those policies. When it detects this, it will
+give the task SCHED_ISO policy instead. Thus it is transparent to the user.
+Because some applications constantly set their policy as well as their nice
+level, there is potential for them to undo the override specified by the user
+on the command line of setting the policy to SCHED_ISO. To counter this, once
+a task has been set to SCHED_ISO policy, it needs superuser privileges to set
+it back to SCHED_NORMAL. This will ensure the task remains ISO and all child
+processes and threads will also inherit the ISO policy.
+
+Idleprio scheduling.
+
+Idleprio scheduling is a scheduling policy designed to give out CPU to a task
+_only_ when the CPU would be otherwise idle. The idea behind this is to allow
+ultra low priority tasks to be run in the background that have virtually no
+effect on the foreground tasks. This is ideally suited to distributed computing
+clients (like setiathome, folding, mprime etc) but can also be used to start
+a video encode or so on without any slowdown of other tasks. To avoid this
+policy from grabbing shared resources and holding them indefinitely, if it
+detects a state where the task is waiting on I/O, the machine is about to
+suspend to ram and so on, it will transiently schedule them as SCHED_NORMAL. As
+per the Isochronous task management, once a task has been scheduled as IDLEPRIO,
+it cannot be put back to SCHED_NORMAL without superuser privileges. Tasks can
+be set to start as SCHED_IDLEPRIO with the schedtool command like so:
+
+	schedtool -D -e ./mprime
+
+Subtick accounting.
+
+It is surprisingly difficult to get accurate CPU accounting, and in many cases,
+the accounting is done by simply determining what is happening at the precise
+moment a timer tick fires off. This becomes increasingly inaccurate as the
+timer tick frequency (HZ) is lowered. It is possible to create an application
+which uses almost 100% CPU, yet by being descheduled at the right time, records
+zero CPU usage. While the main problem with this is that there are possible
+security implications, it is also difficult to determine how much CPU a task
+really does use. BFS tries to use the sub-tick accounting from the TSC clock,
+where possible, to determine real CPU usage. This is not entirely reliable, but
+is far more likely to produce accurate CPU usage data than the existing designs
+and will not show tasks as consuming no CPU usage when they actually are. Thus,
+the amount of CPU reported as being used by BFS will more accurately represent
+how much CPU the task itself is using (as is shown for example by the 'time'
+application), so the reported values may be quite different to other schedulers.
+Values reported as the 'load' are more prone to problems with this design, but
+per process values are closer to real usage. When comparing throughput of BFS
+to other designs, it is important to compare the actual completed work in terms
+of total wall clock time taken and total work done, rather than the reported
+"cpu usage".
+
+
+Con Kolivas <kernel@kolivas.org> Tue, 5 Apr 2011