Hello!
Midway through the recent "RT patch acceptance" thread, someone mentioned
that it might be good to summarize the various approaches. The following
is an attempt to do just this, with an eye to providing a reasonable
framework for future discussion.
Thoughts? Errors? Omissions?
[Quickly donning the asbestos suit with tungsten pinstripes...]
Thanx, Paul
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CONTENTS
A. INTRODUCTION
B. DESIRABLE PROPERTIES
C. LINUX REALTIME APPROACHES
D. SUMMARY
Search for a line beginning with the corresponding capital letter followed
by a period to jump to the corresponding section.
A. INTRODUCTION
Common wisdom dictates that realtime operating systems, particularly
hard-realtime operating systems, must be designed from ground up; that
serious realtime support cannot be simply grafted onto an existing
general-purpose operating system. Although this common wisdom was
not arrived at lightly, it is often worthwhile to look for important
exceptions to this sort of general rule of thumb. Candidate exceptions
include:
1. Many realtime applications use a very restricted subset of
the services provided by a general-purpose OS like Linux.
Some applications require realtime support only for scheduling
user-mode code, for example, an application that directly accesses
MMIO registers mapped into its address space. This observation
leads to the possibility of providing very limited realtime
support.
2. Computer performance and capacity has increased dramatically
over the past few decades, quite literally by multiple orders
of magnitude. A small embedded system can easily be much more
capable than a mid-70s supercomputer, for example, the vaunted
Cray-1, introduced in 1976, ran at 160MFLOPs and sported 8MB of
main memory. In today's terms, this would be a modest embedded
system -- and just you try running Linux on an 8MB system!
This dramatic increase in performance permits some applications
that would have required heavy-duty RTOS support in the 70s to
run reasonably well on unmodified general-purpose OSes.
There are still limits to the degree of realtime support that one can
expect from a general-purpose OS -- there are some extremely demanding
applications that can be satisfied only by hand-coded assembly running
on bare metal. In fact, there are applications that can be satisfied
only by custom hardware implementations.
Nevertheless, it is clear that Linux can support significant realtime
requirements, as it is already being used heavily in the realtime arena.
But how far should Linux extend its realtime support, and what is the
best way to extend Linux in this direction? Can one approach to realtime
satisfy all reasonable requirements, or would it be better to support
multiple approaches, each with its area of applicability?
The answers to these questions are not yet clear, and have been the
subject of much spirited discussion, for example, see the more than
300 messages in the following LKML thread:
http://lkml.org/lkml/2005/5/23/156
http://marc.theaimsgroup.com/?l=linux-kernel&m=111689227213061&w=2
This document looks at some strategies that have been proposed for
realtime Linux, comparing and contrasting their capabilities. But, to
evaluate these strategies, it is first necessary to determine what exactly
one might want in a realtime Linux. If you would rather skip straight to
the comparing and contrasting, search for "LINUX REALTIME APPROACHES".
B. DESIRABLE PROPERTIES
As usual, there are conflicting desires, at least they conflict given
the current state of the art. These desires fall into the following
categories:
1. Quality of service
2. Amount of code that must be inspected to assure quality of service
3. API provided
4. Relative complexity of OS and applications
5. Fault isolation: what non-RT failures endanger RT code?
6. What hardware and software configurations are supported?
Each of these categories is expanded upon below, and later used to compare
a number of proposed realtime approaches for Linux. The discussion does
go for some time, which is not surprising given that it is summarizing
many hundreds of email messages. ;-) Search for the corresponding
number at the beginning of a line to skip directly to the discussion of
a given category.
1. Quality of Service
The traditional view is that the entire operating system is either
hard realtime, soft realtime, or non-realtime, but this viewpoint
is too coarse grained. Different workloads have different needs,
and there is disagreement over the exact definitions of these three
categories of realtime. For example, (at least) the following two
definitions of "hard realtime" are in use:
a. In absence of hardware failures, software provably meets
the specified deadlines. This is fine and good, but many
applications simply do not need this "diamond hard" realtime.
b. Failure to meet the specified deadline results in application
failure. This is OK, but -only- if there is a corresponding
required probability of success. Otherwise, one could claim
"hard realtime" by simply failing the application every time it
tries to do anything, which is clearly not useful.
A better approach is to simply specified the required probability
of meeting the specified deadline in absence of hardware failure.
A probability of 1.0 is consistent with definition (a). Other
applications will be satisfied with a probability such as 0.999999,
which might be sufficiently high that the probability of software
scheduling failure is "in the noise" compared with the probability
of hardware failure. A recent LKML thread called this "metal hard"
realtime. Or was it "ruby hard"? ;-)
Of course, one can increase the reliability of hardware through
redundancy, but no hardware configuration provides perfect reliability.
For example, clusters can increase reliability, so that the probability
of failure of the cluster is p^n, where "p" is the probability of a
single node failing and "n" is the number of nodes. Note that this
expression never reaches a probability of 1, no matter how large "n" is.
In addition, this mathematical expression assumes that the failover
software is perfectly reliable and perfectly configured. This assumption
conflicts sharply with my own experience, in which there has always been
a point beyond which adding nodes -decreased- cluster reliability.
The timeframe is also critically important. Any system can provide
hard realtime guarantees if the deadline is an infinite amount of time
in the future. No computer system that I am aware of at this writing
is capable of meeting a 1-picosecond scheduling deadline for any task
of non-zero duration, but then neither can dedicated digital hardware.
Some applications have definite response-time goals, for example,
industrial process-control applications tend to have response-time
goals ranging from 100s of microseconds to small numbers of seconds.
Other applications can benefit from any improvement in response-time
goals -- faster is better, think in terms of Doom players -- but even
in these cases there is normally a point of diminishing returns.
The services used by the realtime application also figure in. Given
current disk technology, it is not possible to meet a 100-microsecond
deadline for a 1MB synchronous write to disk. Not even if you cheat and
supply the disk with a battery-backed-up DRAM. However, many realtime
applications need only a few of the services that an operating system
might provide. This list might include interrupt handling, process
scheduling, disk I/O, network I/O, process creation/destruction, VM
operations, and so on. Keep in mind that many popular RTOSes provide
very little in the way of services! They frequently leave the
complex stuff (e.g., web serving) to general-purpose operating systems.
Note that each service can have an associated deadline that it can
meet. The interrupt system might be able to meet a 1-microsecond
deadline, the real-time process scheduler a 10-microsecond deadline,
the disk I/O system a 10-millisecond deadline for moderate-sized
I/Os, and so on. The deadline that a service can meet might also
depend on the parameters, so that the disk-I/O system would be
expected to take longer for larger I/Os.
Furthermore, the probability might vary from service to service or with
the parameters to that service. For example, the probability of network
I/O completing successfully in minimal time might well be a function
of the number of packets transmitted (to account for the probability of
packet loss) as well as of packet size (to account for bit-error rate).
To make things even more complicated, the probability of meeting the
deadline will vary depending on the length of time allowed. Considering
the networking example, a very short deadline might not allow the data
transmission to complete, even if it proceeds at wire speed. A longer
deadline might allow transmission to complete, but only if there are
no transmission errors. An even longer deadline might allow time for
a limited number of retransmissions, in order to recover from packet
loss due to transmission errors. Of course, a deadline infinitely far
into the future would allow guaranteed completion, but I for one am not
that patient.
Finally, the performance and scalability of both realtime and non-realtime
applications running on the system can be important. Given the current
state of the art, one must pay a performance penalty for realtime support,
but the smaller the penalty, the better.
So, to sum up, here are the components of a quality-of-service metric
for realtime OSes:
a. List of services for which realtime response is supported.
b. For each service:
i. Probability of missing a deadline due to software,
ranging from 0 to 1, with the value of 1 corresponding
to the hardest possible hard realtime.
ii. Allowable deadline, measured from the time that
the request is initiated to the time by which the
response must be received.
c. Performance and scalability provided to both realtime and
non-realtime applications.
2. Amount of Code Inspection Required
So you add a new feature to a realtime operating system. How much of
the rest of the system must you inspect and understand in order to be
able to guarantee that your new feature provides the required level
of realtime response? The smaller this amount of code, the easier it
is to add new features and fix bugs, and the greater the number of
people who will be able to contribute to the project. In addition,
the smaller the amount of such code, the smaller the probability that
some well-intentioned bug fix will break realtime response.
Each of the following categories of code might need to be inspected:
a. The low-level interrupt-handing code.
b. The realtime process scheduler.
c. Any code that disables interrupts.
d. Any code that disables preemption.
e. Any code that holds a lock, mutex, semaphore, or other resource
that is needed by the code implementing your new feature.
Of course, use of automated tools could make such inspection much
more reliable and less onerous, but such tools would need to deal with
the very large number of CPU architectures and configuration options
that Linux supports. The smaller the amount of code that must be
inspected, the less chance there is that such a tool will fall victim
to configuration-architecture combinatorial explosion.
Each of Linux realtime approaches uses a different strategy to
minimize the amount of code in these categories. These differences
are surprisingly important, and will be discussed in more detail
when going over the various approaches to Linux realtime.
3. API Provided
I never have learned to -really- like the POSIX API, with the gets()
primitive being a particular cause of heartburn, but given the huge
amount of software out there that relies on it and the equally huge
number of developers who are familiar with it, one should certainly
strive to provide it, or at least a sizeable subset of it.
Other popular APIs include the various Java runtime environments,
and of course the feared and loathed, but quite ubiquitous, Windows
API.
There are a lot of developers and a lot of software out there. The
more of these existing developers and software your API supports,
the more successful your realtime facility is likely to be.
4. Relative Complexity
How much realtime capability should be added to the operating system?
How much of this burden should the applications take on? Is it better
to push some of the complexity into a nanokernel, hypervisor, or other
software or firmware layer? Let's first look at the tradeoff between
OS and application.
For example, although it is certainly possible to program for separate
realtime and non-realtime operating-system instances, doing so adds
complexity to the application. Complexity is particularly deadly in the
hard realtime arena, and can be literally so if human lives are at risk.
Balancing this consideration is the need for simplicity in the
operating-system kernel. This balancing act must be carefully considered,
taking both the relative complexities and the number of uses into
account. Some would argue that it is worthwhile adding 1,000 lines
to the OS if that saves 100 lines in each of 1,000 applications.
Others would disagree, perhaps citing the greater fault isolation
that might be provided by the separation.
But this balance clearly must be struck somewhere between writing the
application to bare metal on the one hand (but achieving a perfectly
simple zero-size operating system) and bloating the operating system
beyond the limits of maintainability on the other hand.
Similar arguments can be made for moving some functionality into a
hypervisor or nanokernel layer, though fault isolation also comes
into play here.
Many of the most vociferous arguments seem to revolve around this
complexity issue.
5. Fault Isolation
Can a programming error in a non-realtime application or in a non-realtime
portion of the OS harm a realtime application?
Some applications do not care: in these cases, a failure anywhere
causes a user-visible failure, so it is not important to isolate
faults. Of course, even in these cases, it may be valuable to isolate
faults in order to aid debugging, but, other than that, the fault
isolation does not help overall application reliability.
In other cases, the realtime portion of the application is protecting
someone's life and limb, but the non-realtime portion is only compiling
statistics and reports. In this case, fault isolation can be of the
utmost importance.
What sorts of faults need isolating?
o Excessive disabling of interrupts.
o Excessive disabling of preemption.
o Holding a lock, mutex, or semaphore for too long, when that
resource must be acquired by realtime code.
o Memory corruption, either via wild pointers or via wild DMA.
These faults might occur in the main kernel, in a loadable module, or in
some debugging tool, such as a kprobe procedure or a kernel-debugger
breakpoint script. Though in the latter case, perhaps realtime
deadlines should not be guaranteed when actively debugging. After all,
straightforward debugging techniques, such as use of kprint(), can cause
response-time problems even in non-realtime environments.
6. Hardware and Software Configurations
Is SMP required? If so, how many CPUs? How many tasks? How many
disks? How many HBAs?
If all the code in the kernel were O(1), it might not matter, but the
Linux kernel has not yet reached this goal. Therefore, some applications
may choose to restrict the software or the hardware configuration of
the platform in order to meet the realtime deadlines. This approach is
consistent with traditional RTOS methodology -- RTOS vendors have been
known to restrict the configurations in which they will support hard
realtime guarantees.
C. LINUX REALTIME APPROACHES
The following general approaches to Linux realtime have been proposed,
along with many variations on each of these themes:
1. non-CONFIG_PREEMPT
2. CONFIG_PREEMPT
3. CONFIG_PREEMPT_RT
4. Nested OS
5. Dual-OS/Dual-Core
6. Migration Between OSes
7. Migration Within OS
Each of these general approaches is discussed in the following sections.
Each section ends with a brief (but perhaps controversial) summary of
the corresponding approach's strengths and weaknesses. I do not address
"strength of community", even though this may well be the decisive factor.
After all, the technical comparision will provide sufficient flame-bait.
This document does not present measured comparisons among all of the
approaches, despite the fact that such comparisons would be extremely
useful. The reason for this, aside from gross laziness, is that it is
wise to agree on the metrics beforehand. Therefore, the comparisons
in this document are for the most part qualitative. In some cases,
they are based on actual measurements, but these measurements were
taken by different people on different configurations using different
benchmarks. This is a prime area for future improvement.
1. non-CONFIG_PREEMPT
This is the stock kernel, not even using preemption. Why would -anyone-
think of using stock 2.6 for a realtime task? Because some realtime
applications have very forgiving scheduling deadlines. One project
I worked on in the early 1980s had 2-second response-time deadlines.
This was quite a challenge, given that it was running on a 4MHz Z80 CPU --
though, to be fair, the Z80 was accompanied by a hardware floating-point
processor that was able to compute a 32-bit floating-point multiply in
well under a millisecond. Modern hardware running a stock Linux 2.6
kernel would have no problem with this application. Hey, just having
32 address bits rather than only 16 would help a lot!
a. Quality of service: soft realtime, with timeframe of 10s of
milliseconds for most services. Some I/O requests can take
longer. Provides full performance and scalability to both
realtime and non-realtime applications.
b. Amount of code that must be inspected to assure quality of service
for a new feature: the entire kernel, every little bit of it,
since the entire kernel runs with preemption disabled.
c. API provided: POSIX with limited realtime extensions.
Realtime and non-realtime applications can interact using
the normal POSIX services.
d. Relative complexity of OS and applications: everything is
stock, and all the normal system calls operate as expected.
e. Fault isolation: none.
f. Hardware and software configurations supported: all of them.
Larger hardware configurations and some device drivers can
result in degraded response time.
Strengths: Simplicity and robustness. "Good enough" realtime support
for undemanding realtime applications. Excellent performance
and scalability for both realtime and non-realtime applications.
Applications and administrators see a single OS instance.
Weaknesses: Poor realtime response, need to inspect the entire kernel
to find issues that degrade realtime response.
2. CONFIG_PREEMPT
The CONFIG_PREEMPT option renders much of the kernel code preemptible,
with the exception of spinlock critical sections, RCU read-side critical
sections, code with interrupts disabled, code that accesses per-CPU
variables, and other code that explicitly disables preemption.
a. Quality of service: soft realtime, with timeframe of 100s of
microseconds for task scheduling and interrupt handling.
System services providing I/O, networking, task creation, and VM
manipulation can take much longer. A very small performance
penalty is exacted, since spinlocks and RCU must suppress
preemption.
b. Amount of code that must be inspected to assure quality of service
for a new feature:
i. The low-level interrupt-handing code.
ii. The process scheduler.
iii. Any code that disables interrupts, which includes
all interrupt handlers, both hardware and softirq.
iv. Any code that disables preemption, including spinlock
critical sections, RCU read-side critical sections,
code with interrupts disabled, code that accesses
per-CPU variables, and other code that explicitly
disables preemption.
v. Any code that holds a lock, mutex, semaphore, or other
resource that is needed by the code implementing the
new feature.
c. API provided: POSIX with limited realtime extensions.
d. Relative complexity of OS and applications: all the normal system
calls operate as expected, so realtime and non-realtime processes
can interact normally.
e. Fault isolation: none.
f. Hardware and software configurations supported: all of them.
Larger hardware configurations and some device drivers can
result in degraded response time.
Strengths: Simplicity. Available now, even from distributions.
Provides "good enough" realtime support for a large number
of applications. Applications and administrators see a
single OS instance.
Weaknesses: Limited testing, so that some robustness issues remain.
Need to inspect large portions of the kernel in order
to find issues that degrade realtime response.
3. CONFIG_PREEMPT_RT
The CONFIG_PREEMPT_RT patch by Ingo Molnar introduces additional
preemption, allowing most spinlock (now "mutexes") critical sections,
RCU read-side critical sections, and interrupt handlers to be preempted.
Preemption of spinlock critical sections requires that priority
inheritance be added to prevent the "priority inversion" problem where
a low-priority task holding a lock is preempted by a medium-priority
task, while a high-priority task is blocked waiting on the lock.
The CONFIG_PREEMPT_RT patch addresses this via "priority inheritance",
where a task waiting on a lock "donates" its priority to the task holding
that lock, but only until it releases the lock. In the example above,
the low-priority task would run at high priority until it released the
lock, preempting the medium-priority task, so that the high-priority
task gets the lock in a timely fashion. Priority inheritance has been
used in a number of realtime OS environments over the past few decades,
so it is a well-tested concept.
One problem with priority inheritance is that it is difficult to implement
for reader-writer locks, where a high-priority writer might wish to
donate its high priority to a large number of low-priority readers.
The CONFIG_PREEMPT_RT patch addresses this by allowing only one task at
a time to read-acquire a reader-writer lock, although it is permitted
to do so recursively.
Note that a few critical spinlocks remain non-preemptible, using the
"raw spinlock" implementation.
a. Quality of service: soft realtime, with timeframe of about 10
microseconds for task scheduling and interrupt-handler entry.
System services providing I/O, networking, task creation,
and VM manipulation can take much longer. Since spinlocks are
replaced by blocking mutexes, some believe that the performance
penalty is significant. There is likely to be some performance
penalty exacted from RCU, but, with luck, this penalty will be
minimal.
b. Amount of code that must be inspected to assure quality of service
by a new feature:
i. The low-level interrupt-handing code.
ii. The process scheduler.
iii. Any code that disables interrupts, but -not- including
interrupt handlers, which now run in process context.
iv. Any code that disables preemption, including raw-spinlock
critical sections, code with interrupts disabled, code
that accesses per-CPU variables, and other code that
explicitly disables preemption.
v. Any code that holds a lock, mutex, semaphore, or other resource
that is needed by the code implementing the new feature.
c. API provided: POSIX with limited realtime extensions.
d. Relative complexity of OS and applications: all the normal system
calls operate as expected, so realtime and non-realtime processes
can interact normally.
e. Fault isolation: none.
f. Hardware and software configurations supported: most of them.
SMP support is a bit rough, and a number of drivers have not yet
been upgraded to work properly in the CONFIG_PREEMPT_RT environment.
It is likely that larger hardware configurations and some device
drivers can result in degraded scheduling latency, but given that
normal spinlocks are now preemptible, this effect should be much
less of an issue than for CONFIG_PREEMPT.
Strengths: Excellent scheduling latencies, potential for hard
realtime for some services (e.g., user-mode execution) in
some configurations. A number of aspects of this approach
might be incrementally added to Linux (e.g., priority
inheritance for semaphores to prevent semaphore priority
inversion). Applications and administrators see a single
OS instance.
Weaknesses: Limited testing, so that robustness issues remain.
Large patch to Linux (~18K lines of context diff).
Both realtime and non-realtime applications pay performance
and scalability penalties for the realtime service.
4. Nested OS
The Linux instance runs as a user process in an enclosing RTOS. Realtime
service is provided by the RTOS, and a richer set of non-realtime services
is provided by the Linux instance.
a. Quality of service: hard realtime, with timeframe of about 10
microseconds for services provided by the underlying RTOS.
More complex services (I/O, task creation, and so on) will
likely take longer to execute, which may impose a significant
performance and scalability penalty.
b. Amount of code that must be inspected to assure quality of service
by a new feature:
i. All of the RTOS. One would strive to keep the RTOS
quite small, the greater the number of realtime services
provided, the larger the RTOS must be.
ii. Any Linux-kernel code that disables interrupts.
Note that in many implementations, the Linux kernel
will be prevented from disabling interrupts, since
any attempt to disable interrupts will trap into
the RTOS.
If the Linux kernel runs in privileged mode, however,
all bets are off. In this case, special care must be
used to avoid disabling the real hardware interrupts,
including such disabling within any kernel modules
that might be loaded.
c. API provided: Whatever the RTOS wants to provide, often a
subset of POSIX with realtime extensions.
d. Relative complexity of OS and applications: there are now two
operating systems, both of which must be configured and
administered. Applications that contain both realtime and
non-realtime components must be explicitly aware of both OS
instances, and of their respective APIs.
e. Fault isolation: the following faults may propagate from the
Linux OS to the underlying RTOS, or not, depending on the
implementation:
i. Excessive disabling of interrupts, if the Linux instance
is permitted to disable them (hopefully not).
ii. Memory corruption, if the Linux instance is given direct
access to the hardware MMU or to DMA-capable I/O devices.
f. Hardware and software configurations supported: depends on
the implementation, however, there are products with this
architecture that support SMP and a reasonable variety of devices.
Note that supporting a large variety of devices either requires
that this support be present in the RTOS, or that Linux be
granted access to the devices. In the latter case, Linux will
likely have the ability to DMA over the top of the RTOS.
Strengths: Excellent scheduling latencies. Hard-realtime support for
some services in some configurations. Reasonable fault isolation
for some implementations. Well-tested and robust implementations
are available.
Weaknesses: Realtime application software must deal with two separate
OS instances and their respective APIs, with explicit
communication. Administrators must deal with two OS instances.
Non-realtime applications are likely to suffer significant
performance and scalability penalties. The pair of cores
will be more expensive than a single core, though one might
use virtualization to emulate the two CPUs.
5. Dual-OS/Dual-Core
Linux and RTOS instances run side-by-side on different CPUs in the
same system. The CPUs might be different physical CPUs, different
hardware threads in the same CPU, or different virtual CPUs provided by
a virtualizing layer, such as Xen. The two instances might or might not share
memory, and, if they do share memory, there might or might not be hardware
protection to prevent one OS from overwriting the other OS's memory.
a. Quality of service: hard realtime, with timeframe of about 10
microseconds for services provided by the RTOS. More complex
services (I/O, task creation, and so on) will likely take longer
to execute. Since the Linux instance runs on a separate core,
there need not be any performance or scalability penalty for
non-realtime tasks.
b. Amount of code that must be inspected to assure quality of service
by a new feature: all of the RTOS, but only the RTOS. One would
strive to keep the RTOS quite small, but the greater the number
of realtime services provided, the larger the RTOS must be.
c. API provided: Whatever the RTOS wants to provide, often a
subset of POSIX with realtime extensions.
d. Relative complexity of OS and applications: there are now two
operating systems, both of which must be configured and
administered. Applications that contain both realtime and
non-realtime components must be explicitly aware of both
OS instances and APIs, and must also be aware of whatever
hardware facility is used to communicate between the realtime
and non-realtime CPUs.
e. Fault isolation: the following faults may propagate from the
Linux OS to the underlying RTOS, or not, depending on the
implementation:
i. Memory corruption, but only if the Linux instance is given
direct access to the RTOS's memory or to DMA-capable
I/O devices that can access the RTOS's memory.
f. Hardware and software configurations supported: depends on
the implementation, however, there are products based on this
approach that support SMP and a reasonable variety of devices.
Strengths: Best possible scheduling latencies with the hardest reasonable
realtime -- just as good as bare metal in some implementations.
Best possible fault isolation for some implementations.
Well-tested and robust implementations are available.
Linux can be used as is, so full performance and scalability
can be provided to non-realtime tasks.
Weaknesses: Realtime application software must deal with two separate
OS instances, with explicit communication. Administrators must
deal with two OS instances. "RTOSes" that provide the best
latencies offer the least services -- in extreme cases, the only
service is execution of raw code on bare metal.
6. Migration Between OSes
A Linux and RTOS instance run side-by-side in the same system. The two
OSes might run on different physical CPUs, different hardware threads
in the same CPU, different virtual CPUs provided by a virtualizing
layer like Xen, or alternatively, the two OSes might use some sort
of interrupt-pipeline scheme (such as Adeos) to share a single CPU.
However, applications see a single unified environment. Applications
run on the RTOS, but the RTOS provides Linux-compatible system calls and
memory layout. If the application invokes a non-realtime system call,
the task is transparently migrated to the Linux OS instance for the
duration of that system call. This differs from the other dual-OS
approaches, where the applications must be explicitly aware of the
different OSes.
At this writing, it appears that the two instances need to share memory,
since tasks can migrate from one OS to the other.
a. Quality of service: hard realtime, with timeframe of about 10
microseconds for services provided by the RTOS. More complex
services (I/O, task creation, and so on) will likely take longer
to execute. It is also possible for tasks to be "trapped"
in the Linux instance, for example, if they are sleeping, but
have not yet been given a chance to respond to some event that
should wake them up. The performance and scalability penalties
to non-realtime tasks can be expected to depend on the amount
of protection provided for realtime tasks against non-realtime
misbehavior -- the greater the protection, the greater the
expected penalty. It may be possible to provide hardware
support to improve this tradeoff.
b. Amount of code that must be inspected to assure quality of service
by a new feature:
i. All of the RTOS. One would strive to keep the RTOS
quite small, but the greater the number of realtime
services provided, the larger the RTOS must be.
ii. Any Linux-kernel code that disables interrupts.
Note that in many implementations, the Linux kernel
will be prevented from disabling interrupts, since
any attempt to disable interrupts will trap into
the RTOS or into the underlying software/firmware
layer (e.g., Xen or Adeos).
If the Linux kernel runs in privileged mode, however,
all bets are off. In this case, special care must be
used to avoid disabling the real hardware interrupts,
including such disabling within any kernel modules
that might be loaded.
c. API provided: Full POSIX with realtime extensions. Anytime
a task running in the context of the RTOS attempts to execute
a non-realtime system call, it is migrated to the Linux instance.
d. Relative complexity of OS and applications: there are now two
operating systems, both of which must be configured and
administered. However, applications can be written as if
there was only one OS instance that provided the full set
of services, some realtime and some not.
e. Fault isolation: the following faults may propagate from the
Linux OS to the underlying RTOS, or not, depending on the
implementation:
i Excessive disabling of interrupts, if the Linux OS
is permitted to disable hardware interrupts (hopefully
not, though preventing this may require special hardware).
ii. Memory corruption, either due to wild pointer or
via wild DMA.
f. Hardware and software configurations supported: depends on
the implementation, however, it is reasonable to believe that
SMP and a reasonable variety of devices could be supported.
Note that supporting a large variety of devices either requires
that this support be present in the RTOS, or that Linux be
granted access to the devices. In the latter case, Linux will
likely have the ability to DMA over the RTOS.
Strengths: Excellent scheduling latencies. Hard-realtime support for
some services in some configurations. Applications see a
single OS.
Weaknesses: Administrators must deal with two OS instances.
The two OSes will be extremely sensitive to each other's
version and patch level, since they access each other's
data structures.
7. Migration Within OS
A Linux instance runs on multiple CPUs, either different physical CPUs,
different hardware threads in the same CPU, or different virtual CPUs
provided by a virtualizing layer such as Xen. Some (but not all!) of
the CPUs are designated as realtime CPUs. If a task running on a
realtime CPU executes a trap or system call that contains non-deterministic
code sequences, the task is migrated to a non-realtime CPU to complete
execution of the trap or system call, then migrated back. This prevents
any non-realtime execution of a given realtime task from interfering
with that of other realtime tasks.
Interrupts can be directed away from realtime CPUs. Such interrupt
redirection is supported on a few architectures, and has in fact been
used for realtime support since at least the 2.4 kernel.
a. Quality of service: unknown at present. This approach is
expected to provide hard realtime with timeframe of about 10
microseconds for services provided on the realtime CPUs. More
complex services (I/O, task creation, and so on) will likely take
longer to execute. It is also possible for tasks to be "trapped"
on the non-realtime CPUs, for example, if they are sleeping,
but have not yet been given a chance to respond to some event
that should wake them up. Since a stock non-CONFIG_PREEMPT Linux
may be used, there need be no performance or scalability penalty
for non-realtime tasks, nor for realtime tasks that execute only
realtime operations. There can be a significant migration penalty
when realtime tasks frequently execute non-realtime operations.
b. Amount of code that must be inspected to assure quality of service
by a new feature:
i. Any part of the Linux kernel that is permitted to execute
on the realtime CPUs. This would normally be only the
realtime portions of the scheduler and the low-level
interrupt and trap handling code (the actual interrupts
and traps would be migrated, if necessary).
ii. Any critical section of any lock acquired by the portion
of the Linux kernel that is permitted to execute on the
realtime CPUs.
c. API provided: Full POSIX with realtime extensions.
d. Relative complexity of OS and applications: There is but
one OS, though it has a bit of added complexity due to the
migration capability. Applications see only one OS.
e. Fault isolation: the following faults may propagate from the
non-realtime CPUs to the realtime CPUs:
i Holding a lock, mutex, or semaphore for too long, when
that resource must be acquired by code that is permitted
to run on the realtime CPUs.
ii. Memory corruption, either due to wild pointer or
via wild DMA.
f. Hardware and software configurations supported: all configurations,
though single-CPU systems must have some sort of virtualizing
facility so that the OS sees at least two virtual CPUs.
Strengths: Excellent scheduling latencies. Hard-realtime support for
some services in some configurations. Applications and
administrators see a single OS and API. Full performance and
scalability for non-realtime and for pure-realtime tasks.
Weaknesses: Migration overhead. Strictly theoretical, as no implementations
exist. Requires multiple CPUs, either real or virtual.
D. SUMMARY
At this point, it does not appear that any one approach can be all things
to all realtime applications. It is therefore too early to pick a winner.
Advocates of a given approach are therefore advised to concentrate their
energy on implementations of their favorite approach, rather than engaging
in flamewars with advocates of other approaches. ;-)
After all, in the end, the approaches that best meet the needs of the
user community will win out. In fact, given that the Linux community
has come up with no fewer than seven classes of solutions to a problem
that is commonly thought to be unsolvable, it seems quite reasonable
to expect that yet more classes of solutions will yet appear.
So, which of these approaches can be combined? The first three can
be thought of as elaborations on the general preemption theme, and
can be combined with each of the remaining four. The nested-OS and
dual-OS/dual-core ideas can be combined by having one of the OSes
on one of the cores have another OS nested within it. The
dual-core/dual-OS approach can be combined with either of the
migration approaches, simply by having one of the cores implement
the migration approach. It should be possible to combine the two
migration approaches, though it is not clear that this is useful.
Regardless of whether Linux's direction ends up being a single one
of these approaches, a yet-as-unknown approach, or some combination,
realtime Linux looks to remain an exciting area.
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