+# DragonFly Design Goals
+Table of contents:
+
+* [[Caching|goals#caching]]
+* [[I/O Model|goals#iomodel]]
+* [[Packages|goals#packages]]
+* [[Messaging|goals#messaging]]
+* [[Threads|goals#threads]]
+* [[User API|goals#userapi]]
+* [[VFS Model|goals#vfsmodel]]
+
+
+----
+
+## <a name="intro">What is it?</a>
+
+
+
+DragonFly is going to be a multi-year project. It will take a lot of groundwork
+to even approach the goals we outline here. By checking our various goal
+links, you can bring up position papers on various nitty-gritty aspects
+of kernel and system design which the project hopes to accomplish.
+
+
+First and foremost among all of our goals is a desire to be able to
+implement them in small bite-sized chunks, while at the same time maintaining
+good stability for the system as a whole. While the goals are not listed
+in any particular order, there is a natural order to things which should allow
+us to advance piecemeal without compromising the stability of the
+system as a whole. It's a laudable goal that will sit foremost in our
+minds, even though we know it is probably not 100% achievable. The messaging
+system is going to be key to the effort. If we can get that in-place we
+will have an excellent (and debuggable) API on top of which the remainder of
+the work can be built.
+
+
+## <a name="caching">Caching Infrastructure Overview</a>
+
+
+Our goal is to create a flexible dual-purpose caching infrastructure which
+mimics the well known and mature MESI (Modified Exclusive Shared Invalid)
+model over a broad range of configurations. The primary purpose of this
+infrastructure will be to protect I/O operations and live memory mappings.
+For example, a range-based MESI model would allow multiple processes to
+simultaneously operate both reads and writes on different portions of a single
+file. If we implement the infrastructure properly we can extend it into
+a networked-clustered environment, getting us a long ways towards achieving
+a single-system-image capability.
+
+
+Such a caching infrastructure would, for example, protect a write() from a
+conflicting ftruncate(), and would preserve atomicity between read() and
+write(). The same caching infrastructure would actively invalidate or
+reload memory mappings, effectively replacing most of what VNODE locking
+is used for now.
+
+
+
+The contemplated infrastructure would utilize two-way messaging and focus
+on the VM Object rather than the VNode as the central manager of cached
+data. Some operations, such as a read() or write(), would obtain the
+appropriate range lock on the VM object, issue their I/O, then release the
+lock. Long-term caching operations might collapse ranges together to
+bound the number of range locks being maintained, which allows the
+infrastructure to maintain locks between operations in a scalable fashion.
+In such cases cache operations such as invalidation or, say,
+Exclusive->Shared transitions, would generate a message to the holding
+entity asking it to downgrade or release its range lock.
+In other words, the caching system being contemplated is
+an ***actively managed*** system.
+
+## <a name="iomodel">New I/O Device Model</a>
+
+
+I/O is considerably easier to fix than VFS owing to the fact that
+most devices operate asynchronously already, despite having a semi-synchronous
+API. The I/O model being contemplated consists of three major pieces of work:
+
+<ol>
+<li>I/O Data will be represented by ranges of VM Objects instead of ranges of
+system or user addresses. This allows I/O devices to operate entirely
+independently of the originating user process.</li>
+
+<li>Device I/O will be handled through a port/messaging system.
+(See 'messaging' goal.)</li>
+
+<li>Device I/O will typically be serialized through one or more threads.
+Each device will typically be managed by its own thread but certain high
+performance devices might be managed by multiple threads (up to one per cpu).
+Multithreaded devices would not necessarily compete for resources. For
+example, the TCP stack could be multithreaded with work split up by target
+port number mod N, and thus run on multiple threads (and thus multiple cpus)
+without contention.</li>
+</ol>
+
+
+
+As part of this work I/O messages will utilize a flat 64 bit byte-offset
+rather than block numbers.
+
+
+Note that device messages can be acted upon synchronously by the device.
+Do not make the mistake of assuming that messages are unconditionally
+serialized to the device thread because they aren't. See the messaging
+section for more information.
+
+
+It should also be noted that the device interface is being designed with
+the flexibility to allow devices to operate as user processes rather than
+as kernel-only threads. Though we probably will not achieve this capability
+for some time, the intention is to eventually be able to do it. There are
+innumerable advantages to being able to transparently pull things like
+virtual block devices and even whole filesystems into userspace.
+
+## <a name="packages">Dealing with Package Installation</a>
+
+
+
+Applications are such a god-awful mess these days that it is hard to come
+up with a packaging and installation system that can achieve seamless
+installation and flawless operation. We have come to the conclusion that
+the crux of the problem is that even seemingly minor updates to third
+party libraries (that we have no control over) can screw up an
+already-installed application. A packaging system **CAN** walk the
+dependency tree and upgrade everything that needs upgrading. The
+problem is that the packaging system might not actually have a new
+version of the package or packages that need to be upgraded due to some
+third party library being upgraded.
+
+
+We need to have the luxury and ability to upgrade only the particular
+package we want, without blowing up applications that depend on said package.
+This isn't to say that it is desirable. Instead we say that it is
+necessary because it allows us to do piecemeal upgrades (as well as
+piecemeal updates to the packaging system's database itself)
+without having to worry about blowing up other things in the process.
+***Eventually*** we would synchronize everything up, but there could be
+periods of a few days to a few months where a few packages might not be,
+and certain very large packages could wind up depending on an old version
+of some library for a very long time. We need to be able to support that.
+We also need to be able to support versioned support/configuration directories
+that might be hardwired by a port. Whenever such conflicts occur, the
+packaging system needs to version the supporting directories as well.
+If two incompatible versions of package X both need /usr/local/etc/X
+we would wind up with /usr/local/etc/X:VERSION1 and /usr/local/etc/X:VERSION2.
+
+
+
+It is possible to accomplish this goal by explicitly versioning
+dependencies and tagging the package binary with an 'environment'... A
+filesystem overlay, you could call it, which applies to supporting
+directories like /usr/lib, /usr/local/lib, even /usr/local/etc, which
+makes only the particular version of the particular libraries
+and/or files the package needs visible to it. Everything else would
+be invisible to that package. By enforcing visibility you would
+know very quickly if you specified your
+package dependencies incorrectly, because your package would not
+be able to find incorrectly placed libraries or supporting files,
+because they were not made accessible when the package was installed.
+For example, if the package says a program depends on version 1.5 of the
+ncurses library, then version 1.5 is all that would be visible to the program
+(it would appear as just libncurses.* to the program).
+
+
+With such a system we would be able to install multiple versions of anything
+whether said entities supported fine-grained version control or not, and
+even if (in a normal sytem) there would be conflicts with other entities.
+The packaging system would be responsible for tagging the binaries and
+the operating system would be responsible for the visibility issues. The
+packaging system or possibly even just a cron job would be responsible for
+running through the system and locating all the 'cruft' that is removable
+after you've updated all the packages that used to depend on it.
+
+
+
+Another real advantage of enforced visibility is that it provides us
+with proof-positive that a package does or does not need something. We
+would not have to rely on the packaging system to find out what the
+dependencies were; we could just look at the environment tagged to the
+binary!
+
+## <a name="messaging">The Port/Messaging Model</a>
+
+
+DragonFly will have a lightweight port/messaging API to go along with its
+lightweight kernel threads. The port/messaging API is very simple
+in concept; You construct a message, you send it to a target port, and
+at some point later you wait for a reply on your reply port. On this
+simple abstraction, we intend to build a high level of capability and
+sophistication. To understand the capabilities of the messaging system,
+you must first understand how a message is dispatched. It basically works
+like this:
+<pre>
+ fubar()
+ {
+ FuMsg msg;
+ initFuMsg(&msg, replyPort, ...);
+ error = targetPort->mp_SendMsg(&msg);
+ if (error == EASYNC) {
+ /* now or at some later time, or wait on reply port */
+ error = waitMsg(&msg);
+ }
+ }
+</pre>
+
+
+
+The messaging API will wrap this basic mechanism into synchronous and
+asynchronous messaging functions. For example, lwkt_domsg() will send
+a message synchronously and wait for a reply. It will set a flag to hint
+to the target port that the message will be blocked on synchronously and
+if the target port returns EASYNC, lwkt_domsg() will block. Likewise
+lwkt_sendmsg() would send a message asynchronously, but if the target port
+returns a synchronous error code (i.e. anything not EASYNC) lwkt_sendmsg()
+will manually queue the now complete message on the reply port itself.
+
+
+As you may have guessed, the target port's mp_SendMsg() function has total
+control over how it deals with the message. Regardless of any hints passed
+to it in the messaging flags, the target port can decide to act on the
+message synchronously (in the context of the caller) and return, or it may
+decide to queue the message and return EASYNC. Messaging operations generally
+should not 'block' from the point of view of the initiator. That is, the
+target port should not try to run the message synchronously if doing so would
+cause it to block. Instead, it should queue it to its own thread (or to the
+message queue conveniently embedded in the target port structure itself) and
+return EASYNC.
+
+
+A target port might act on a message synchronously for any
+number of reasons. It is in fact precisely the mp_sendMsg() function for
+the target port which deals with per-cpu caches and opportunistic locking
+such as try_mplock() in order to deal with the request without having to
+resort to more expensive queueing / switching.
+
+
+The key thing to remember here is that our best case optimization is direct
+execution by mp_SendMsg() with virtually no more overhead than a simple
+subroutine call would otherwise entail. No queueing, no messing around
+with the reply port... If a message can be acted upon synchronously, then we
+are talking about an extremely inexpensive operation. It is this key feature
+that allows us to use a messaging interface by design without having to worry
+about performance issues. We are explicitly NOT employing the type of
+sophistication that, say, Mach uses. We are not trying to track memory
+mappings or pointers or anything like that, at least not in the low level
+messaging interface. User<->Kernel messaging interfaces simply employ
+mp_SendMsg() function vectors which do the appropriate translation, so as
+far as the sender and recipient are concerned the message will be local to
+their VM context.
+
+
+## <a name="threads">The Light Weight Kernel Threading Model</a>
+
+
+
+DragonFly employs a light weight kernel threading (LWKT) model at its core.
+Each process in the system has an associated thread, and most kernel-only
+processes are in fact just pure threads. For example, the pageout daemon
+is a pure thread and has no process context.
+
+
+The LWKT model has a number of key features that can be counted on no matter
+the architecture. These features are designed to remove or reduce contention
+between cpus.
+<ol>
+ <li>
+
+ Each cpu in the system has its own self-contained LWKT scheduler.
+ Threads are locked to their cpus by design and can only be moved to other
+ cpus under certain special circumstances. Any LWKT scheduling operation
+ on a particular cpu is only directly executed on that cpu.
+ This means that the core LWKT scheduler can schedule, deschedule,
+ and switch between threads within a cpu's domain without any locking
+ whatsoever. No MP lock, nothing except a simple critical section.</li>
+ <li>
+
+ A thread will never be preemptively moved to another cpu while it is
+ running in the kernel, a thread will never be moved between cpus while
+ it is blocked. The userland scheduler may migrate a thread that is
+ running in usermode. A thread will never be preemptively switched
+ to a non-interrupt thread. If an interrupt thread preempts the current
+ thread, then the moment the interrupt thread completes or blocks, the
+ preempted thread will resume regardless of its scheduling state. For
+ example, a thread might get preempted after calling
+ lwkt_deschedule_self() but before it actually switches out. This is OK
+ because control will be returned to it directly after the interrupt
+ thread completes or blocks.</li>
+
+ <li>
+
+ Due to (2) above, a thread can cache information obtained through the
+ per-cpu globaldata structure without having to obtain any locks and, if
+ the information is known not to be touched by interrupts, without having to
+ enter a critical section. This allows per-cpu caches for various types of
+ information to be implemented with virtually no overhead.</li>
+ <li>
+
+ A cpu which attempts to schedule a thread belonging to another cpu
+ will issue an IPI-based message to the target cpu to execute the operation.
+ These messages are asynchronous by default and while IPIs may entail some
+ latency, they don't necessarily waste cpu cycles due to that fact. Threads
+ can block such operations by entering a critical section and, in fact,
+ that is what the LWKT scheduler does. Entering and exiting a critical
+ section are considered to be cheap operations and require no locking
+ or locked bus instructions to accomplish.</li>
+ <li>
+
+ The IPI messaging subsystem
+ deals with FIFO-full deadlocks by spinning and processing the incoming
+ queue while waiting for its outgoing queue to unstall. The IPI messaging
+ subsystem specifically does not switch threads under these circumstances
+ which allows the software to treat it as a non-blocking API even though
+ some spinning might occasionally occur.</li>
+</ol>
+
+
+
+In addition to these key features, the LWKT model allows for both FAST
+interrupt preemption **AND** threaded interrupt preemption. FAST
+interrupts may preempt the current thread when it is not in a critical section.
+Threaded interrupts may also preempt the current thread. The LWKT system
+will switch to the threaded interrupt and then switch back to the original
+when the threaded interrupt blocks or completes. IPI functions operate
+in a manner very similar to FAST interrupts and have the same trapframe
+capability. This is used heavily by DragonFly's SYSTIMERS API to distribute
+hardclock() and statclock() interrupts to all cpus.
+
+### The IPI Messaging Subsystem
+
+
+The LWKT model implements an asynchronous messaging system for communication
+between cpus. Basically you simply make a call providing the target cpu with
+a function pointer and data argument which the target cpu executes
+asynchronously. Since this is an asynchronous model the caller does not wait
+for a synchronous completion, which greatly improves performance, and the
+overhead on the target cpu is roughly equivalent to an interrupt.
+
+
+IPI messages operate like FAST Interrupts... meaning that they preempt
+whatever is running on the target cpu (subject to a critical section), run,
+and then whatever was running before resumes. For this reason IPI functions
+are not allowed to block in any manner whatsoever. IPI messages are used
+to do things like schedule threads and free memory belonging to other cpus.
+
+
+IPI messaging is used heavily by at least half a dozen major LWKT
+subsystems, including the per-cpu thread scheduler, the slab allocator,
+and messaging subsystems. Since IPI messaging is a DragonFly-native
+subsystem, it does not require and does not use the Big Giant Lock.
+All IPI based functions must therefore be MP-safe (and they are).
+
+### The IPI-based CPU Synchronization Subsystem
+
+
+The LWKT model implements a generalized, machine independent cpu
+synchronization API. The API may be used to place target cpu(s) into a
+known state while one is operating on a sensitive data structure. This
+interface is primarily used to deal with MMU pagetable updates. For
+example, it is not safe to check and clear the modify bit on a page table
+entry and then remove the page table entry, even if holding the proper lock.
+This is because a userland process running on another cpu may be accessing or
+modifying that page, which will create a race between the TLB writeback on the
+target cpu and your attempt to clear the page table entry. The proper
+solution is to place all cpus that might be able to issue a writeback
+on the page table entry (meaning all cpus in the pmap's pm_active mask)
+into a known state first, then make the modification, then release the cpus
+with a request to invalidate their TLB.
+
+
+The API implemented by DragonFly is deadlock-free. Multiple cpu
+synchronization activities are allowed to operate in parallel and this
+includes any threads which are mastering a cpu synchronization event for
+the duration of mastering. Even with this flexibility, since the cpu
+synchronization interface operates in a controlled environment the callback
+functions tend to work just like the callback functions used in the
+IPI messaging subsystem.
+
+### Serializing Tokens
+
+
+A serializing token may be held by any number of threads simultaneously.
+A thread holding a token is guaranteed that no other thread also
+holding that same token will be running at the same time.
+
+
+A thread may hold any number of serializing tokens.
+
+
+A thread may hold serializing tokens through a thread yield or blocking
+condition, but must understand that another thread holding those tokens
+may be allowed to run while the first thread is not running (blocked or
+yielded away).
+
+
+There are theoretically no unresolvable deadlock situations that can
+arise with the serializing token mechanism. However, the initial
+implementation can potentially get into livelock issues with multiply
+held tokens.
+
+
+Serializing tokens may also be used to protect threads from preempting
+interrupts that attempt to obtain the same token. This is a slightly
+different effect from the Big Giant Lock (also known as the MP lock),
+which does not interlock against interrupts on the same cpu. ***It is
+important to note that token atomicity is maintained through preemptive
+conditions, even though preemption involves a temporary switch to another
+thread. It is not necessary to enter a spl() level or critical section
+to preserve token atomicity***.
+
+
+
+Holding a serializing token does <b>not</b> prevent preemptive interrupts
+from occuring, though it might cause some of those interrupts to
+block-reschedule. Unthreaded FAST and IPI messaging interrupts are not
+allowed to use tokens as they have no thread context of their own to operate
+in. These subsystems are instead interlocked through the use of critical
+sections.
+
+## <a name="userapi">Creating a Portable User API</a>
+
+
+Most standard UNIX systems employ a system call table through which many types
+of data, including raw structures, are passed back and forth. The biggest
+obstacle to the ability for user programs to interoperate with kernels
+which are older or newer than themselves is the fact that these raw structures
+often change. The worst offenders are things like network interfaces, route
+table ioctls, ipfw, and raw process structures for ps, vmstat, etc. But even
+nondescript system calls like stat() and readdir() have issues. In more
+general terms the system call list itself can create portability problems.
+
+
+It is a goal of this project to (1) make all actual system calls message-based,
+(2) pass structural information through capability and element lists
+instead of as raw structures, and (3) implement a generic 'middle layer'
+that looks somewhat like an emulation layer, managed by the kernel but loaded
+into userspace. This layer implements all standard system call APIs
+and converts them into the appropriate message(s).
+
+For example, Linux emulation would
+operate in (kernel-protected) userland rather then in kernelland. FreeBSD
+emulation would work the same way. In fact, even 'native' programs will
+run through an emulation layer in order to see the system call we all know
+and love. The only difference is that native programs will know that the
+emulation layer exists and is directly accessible from userland and won't
+waste an extra INT0x80 (or whatever) to enter the kernel just to get spit
+back out again into the emulation layer.
+
+
+Another huge advantage of converting system calls to message-based entities
+is that it completely solves the userland threads issue. One no longer needs
+multiple kernel contexts or stacks to deal with multiple userland threads,
+one needs only **one** kernel context and stack per user process. Userland
+threads would still use rfork() to create a real process for each CPU on the
+system, but all other operations would use a thread-aware emulation layer.
+In fact, nearly all userland upcalls would be issued by the emulation layer
+in userland itself, not directly by the kernel. Here is an example of how
+a thread-aware emulation layer would work:
+
+<pre>
+ ssize_t
+ read(int fd, void *buf, size_t nbytes)
+ {
+ syscall_any_msg_t msg;
+ int error;
+
+ /*
+ * Use a convenient mostly pre-built message stored in
+ * the userthread structure for synchronous requests.
+ */
+ msg = &curthread->td_sysmsg;
+ msg->fd = fd;
+ msg->buf = buf;
+ msg->nbytes = bytes;
+ if ((error = lwkt_domsg(&syscall_port, msg)) != 0) {
+ curthread->td_errno = error;
+ msg->result = -1;
+ }
+ return(msg->result);
+ }
+</pre>
+
+
+And there you have it. The only 'real' system calls DragonFly would implement
+would be message-passing primitives for sending, receiving, and waiting.
+Everything else would go through the emulation layer. Of course, on the
+kernel side the message command will wind up hitting a dispatch table almost
+as big as the one that existed in FreeBSD 4.x. But as more and more
+subsystems become message-based, the syscall messages
+become more integrated with those subsystems
+and the overhead of dealing with a 'message' could actually wind up being
+less than the overhead of dealing with discrete system calls. Portability
+becomes far easier to accomplish because the 'emulation layer' provides a
+black box which separates what a userland program expects from what the
+kernel expects, and the emulation layer can be updated along with the kernel
+(or a backwards compatible version can be created) which makes portability
+issues transparent to a userland binary.
+
+
+Plus, we get all the advantages that a message-passing model provides,
+including a very easy way to wedge into system calls for debugging or other
+purposes, and a very easy way to create a security layer in the kernel
+which could, for example, disable or alter certain classes of system calls
+based on the security environment.
+
+## <a name="vfsmodel">The New VFS Model</a>
+
+
+Fixing the VFS subsystem is probably the single largest piece of work
+we will be doing. VFS has two serious issues which will require a lot
+of reworking. First, the current VFS API uses a massive reentrancy model
+all the way to its core and we want to try to fit it into a threaded
+messaging API. Second, the current VFS API has one of the single most
+complex interfaces in the system... VOP_LOOKUP and friends, which resolve
+file paths. Fixing VFS involves two major pieces of work.
+
+
+
+First, the VOP_LOOKUP interface and VFS cache will be completely redone. All
+file paths will be loaded in an unresolved state into the VFS cache by the
+kernel before **ANY** VFS operation is initiated. The kernel will
+recurse down the VFS cache and when it hits a leaf it will start creating new
+entries to represent the unresolved path elements. The tail of the snake
+will then be handed to VFS_LOOKUP() for resolution. VFS_LOOKUP() will be
+able to return a new VFS pointer if further resolution is required. For
+example, it hits a mount point. The kernel will then no longer pass random
+user supplied strings (and certainly not using user address space!) to the
+VFS subsystem.
+
+
+
+Second, the VOP interface in general will be converted to a messaging
+interface. All direct userspace addresses will be resolved into VM object
+ranges by the kernel. The VOP interface will **NOT** handle direct
+userspace addresses any more. As a messaging interface VOPs can still operate
+synchronously, and initially that is what we will do. But the intention is
+to thread most of the VOP interface (i.e. replace the massive reentrancy
+model with a serialized threaded messaging model). For a high performance
+filesystem running multiple threads (one per cpu) we can theoretically
+achieve the same level of performance that a massively reentrant model can
+achieve. However, blocking points, such as the bread()'s you see all over
+filesystem code, would either have to be asynchronized, which is difficult,
+ or we would have to spawn a lot more threads to handle parallelism.
+Initially we can take the (huge) performance hit and serialize the VOP
+operations into a single thread, then we can optimize the filesystems we
+care about like UFS. It should be noted that a massive reentrancy model
+is not going to perform all that much better than, say, a 16-thread model
+for a filesystem because in both cases the bottleneck is the I/O. As
+long as one thread is free to handle non-blocking (cached) requests we can
+achieve 95% of the performance of a massive reentrancy model.
+
+
+A messaging interface is preferable for many reasons, not the least of
+which being that it makes stacking actually work the way it should work,
+as independent and opaque elements which stack together to form a whole.
+For example, with the new API a capability layer could be slapped onto a
+filesystem that otherwise doesn't implement one of its own, and the
+enduser would not know the difference. Filesytems are almost universally
+self-contained entities. A message-based API would allow these entities
+to run in userspace for debugging or even in a deployment when one
+absolutely cannot afford a crash. Why run msdosfs or cd9660 in the
+kernel and risk a crash when it would operate just as well in userland?
+Debugging and filesystem development are other good reasons for having a
+messaging API rather than a massively reentrant API.
+
+
+
+
+
+
+
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