From: Charlie Date: Sun, 28 Dec 2008 23:40:29 +0000 (-0500) Subject: Initial recreation X-Git-Url: https://gitweb.dragonflybsd.org/ikiwiki.git/commitdiff_plain/ad830ce66cca4a1d26bb6852fa9b48e225bf3b2f Initial recreation --- diff --git a/goals.mdwn b/goals.mdwn new file mode 100644 index 00000000..dd3a6829 --- /dev/null +++ b/goals.mdwn @@ -0,0 +1,567 @@ +# DragonFly Design Goals + +Table of contents: + +* [[goals#caching|Caching]] +* [[goals#iomodel|I/O Model]] +* [[goals#packages|Packages]] +* [[goals#messaging|Messaging]] +* [[goals#threads|Threads]] +* [[goals#userapi|User API]] +* [[goals#vfsmodel|VFS Model]] + + +---- + +## What is it? + + + +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. + + +## Caching Infrastructure Overview + + +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. + +## New I/O Device Model + + +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: + +
    +
  1. 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.
    2. + +
  2. Device I/O will be handled through a port/messaging system. +(See 'messaging' goal.)
    3. + +
  3. 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.
    4. +
+ + + +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. + +## Dealing with Package Installation + + + +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! + +## The Port/Messaging Model + + +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: +
+    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);    
+        }
+    }
+
+ + + +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. + + +## The Light Weight Kernel Threading Model + + + +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. +
    +
  1. + + 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.
    2. +
  2. + + 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.
    3. + +
  3. + + 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.
    4. +
  4. + + 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.
    5. +
  5. + + 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.
    6. +
+ + + +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 not 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. + +## Creating a Portable User API + + +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: + +
+    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);
+    }
+
+ + +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. + +## The New VFS Model + + +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. + + +