diff options
author | GNU Hurd wiki engine <web-hurd@gnu.org> | 2008-03-07 16:59:38 +0000 |
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committer | GNU Hurd wiki engine <web-hurd@gnu.org> | 2008-03-07 16:59:38 +0000 |
commit | b81d5d6f8985dc3fea93ec553ec206fb64af114b (patch) | |
tree | d98b6a43e0ee5e0afe842e2c61d99d61ac3d958e | |
parent | 3c4f994a0d2355035d4182a84e8e295c1008b1ee (diff) |
web commit by antrik: More tasks fleshed out, but still not all
-rw-r--r-- | community/gsoc/project_ideas.mdwn | 285 |
1 files changed, 282 insertions, 3 deletions
diff --git a/community/gsoc/project_ideas.mdwn b/community/gsoc/project_ideas.mdwn index 04c440a5..a2dc13ba 100644 --- a/community/gsoc/project_ideas.mdwn +++ b/community/gsoc/project_ideas.mdwn @@ -263,15 +263,294 @@ part is finding a convenient, flexible, elegant, hurdish method for mapping the special extensions to actual translators... * gnumach code cleanup + +Although there are some attempts to move to a more modern microkernel +alltogether, the current Hurd implementation is based on gnumach, which is only +a slightly modified variant of the original CMU Mach. + +Unfortunately, Mach was created about two decades ago, and is in turn based on +even older BSD code. Parts of the BSD kernel -- file systems, UNIX mechanisms +like processes and signals etc. -- were ripped out (to be implemented in +userspace servers instead); while other mechanisms were added to allow +implementing stuff in userspace. (Pager interface, IPC etc.) + +Also, Mach being a research project, many things were tried, adding lots of +optional features not really needed. + +The result of all this is that the current code base is in a pretty bad shape. +It's rather hard to make modifications -- to make better use of modern hardware +for example, or even to fix bugs. The goal of this project is to improve the +situation. + +The task starts out easy, with fixing compiler warnings. Later it moves on to +more tricky things: Removing dead or unneeded code paths; restructuring code +for readability and maintainability. + +This task requires good knowledge of C, and experience with working on a large +existing code base. Previous kernel hacking experience is an advantage, but not +really necessary. + * fix libdiskfs locking issues + +Nowadays the most often encountered cause of Hurd crashes seems to be lockups +in the ext2fs server. One of these could be traced recently, and turned out to +be a lock inside libdiskfs that was taken and not released in some cases. There +is reason to believe that there are more faulty paths causing these lockups. + +The task is systematically checking the libdiskfs code for this kind of locking +issues. To achieve this, some kind of test harness has to be implemented: For +exmple instrumenting the code to check locking correctness constantly at +runtime. Or implementing a unit testing framework that explicitely checks +locking in various code paths. (The latter could serve as a template for +implementing unit checks in other parts of the Hurd codebase...) + +This task requires experience with debugging locking issues in multithreaded +applications. + * dtrace support -* I/O performance tuning -* VM performance tuning + +One of the main problems of the current Hurd implementation is very poor +performance. While we have a bunch of ideas what could cause the performance +problems, these are mostly just guesses. Better understanding what really +causes bad performance is necessary to improve the situation. + +For that, we need tools for performance measurements. While all kinds of more +or less specific profiling tools could be convieved, the most promising and +generic approach seems to be a framework for logging certain events in the +running system (both in the microkernel and in the Hurd servers). This would +allow checking how much time is spent in certain modules, how often certain +situations occur, how things interact, etc. It could also prove helpful in +debugging some issues that are otherwise hard to find because of complex +interactions. + +The most popular framework for that is Sun's dtrace; but there might be others. +The student has to evaluate the existing options, deciding which makes most +sense for the Hurd; and implement that one. (Apple's implementation of dtrace +in their Mach-based kernel might be helpful here...) + +This project requires ability to evaluate possible solutions, and experience +with integrating existing components as well as low-level programming. + +* disk I/O performance tuning + +The most obvious reason for the Hurd feeling slow compared to mainstream +systems like GNU/Linux, is very slow harddisk access. + +The reason for this slowness is lack and/or bad implementation of common +optimisation techniques, like scheduling reads and writes to minimalize head +movement; effective block caching; effective reads/writes to partial blocks; +reading/writing multiple blocks at once. The ext2 filesystem driver might also +need some optimisation at a higher logical level. (links) + +The goal of this project is to analyze the current situation, and implement/fix +various optimisations, to achieve significantly better disk performance. It +requires understanding the data flow through the various layers involved in +disk acces on the Hurd (filesystem, pager, driver), and general experience with +optimising complex systems. + +* VM tuning + +Hurd/Mach presently make very bad use of the available physical memory in the +system. Some of the problems are inherent to the system design (the kernel +can't distinguish between important application data and discardable disk +buffers for example), and can't be fixed without fundamental changes. Other +problems however are an ordinary lack of optimisation, like extremely crude +heuristics when to start paging. Many parameters are based on assumptions from +a time when typical machines had like 16 MiB of RAM, or simply have been set to +arbitrary values and never tuned for actual use. + +The goal of this project is to bring the virtual memory management in Hurd/Mach +closer to that of modern mainstream kernels (Linux, FreeBSD), by comparing the +implementation to other systems, implementing any worthwhile improvements, and +general optimisation/tuning. It requires very good understanding of the Mach +VM, and virtual memory in general. + +(links) + * improved NFS implementation + +The Hurd has both NFS server and client implementations, which work, but not +very well: File locking doesn't work properly (at least in conjuction with a +GNU/Linux server), and performance is extremely poor. Part of the problems +could be owed to the fact that only NFSv2 is supported so far. + +This project encompasses implementing NFSv3 support, fixing bugs and +performance problems -- the goal is to have good NFS support. The work done in +a previous unfinished GSoC project can serve as a starting point. (link) + +Both client and server parts need work, though the client is probably much more +important for now, and shall be the major focus of this project. + +The task has no special prerequisites besides general programming skills, and +an interest in file systems and network protocols. + * fix file locking -* virtualization based on Hurd mechanisms + +Over the years, UNIX has aquired a host of different file locking mechanisms. +Some of them work on the Hurd, while others are buggy or only partially +implemented. This breaks many applications. + +The goal is to make all file locking mechanisms work properly. This requires +finding all existing shortcomings (through systematic testing and/or checking +for known issues in the bug tracker and mailing list archives), and fixing +them. (links) + +This task will require digging into parts of the code to understand how file +locking works on the Hurd. Only general programming skills are required. + +* virtualization using Hurd mechanisms + +The main idea behind the Hurd design is to allow users to replace almost any +system functionality. Any user can easily create a subenvironment using some +custom servers instead of the default system servers. This can be seen as an +[advanced lightweight +virtualization](http://tri-ceps.blogspot.com/2007/10/advanced-lightweight-virtualization.html) +mechanism, which allows implementing all kinds of standard and nonstandard +virtualization scenarios. + +However, though the basic mechanisms are there, currently it's not easy to make +use of these possibilities, because we lack tools to automatically launch the +desired constellations. + +The goal is to create a set of powerful tools for managing at least one +desirable virtualization scenario. One possible starting point could be the +subhurd/neighbour Hurd mechanism (link), which allows a second almost totally +independant instance of the Hurd in parallel to the main one. The current +implementation has serious limitations though. A subhurd can only be started by +root. There are no communication channels between the subhurd and the main one. +There is no mechanism for safe sharing of hardware devices. Fixing this issues +could turn subhurds into a very powerful solution for lightweight +virtualization using so-called logical partitions. (Similar to Linux-vserver, +OpenVZ etc.) + +While subhurd allow creating a complete second system instance, with an own set +of Hurd servers and UNIX daemons and all, there are also situations where it is +desirable to have a smaller subenvironment, living withing the main system and +using most of its facilities -- similar to a chroot environment. A simple way +to create such a subenvironment with a single command would be very helpful. + +It might be possible to implement (perhaps as a prototype) a wrapper using +existing tools (chroot and unionfs); or it might require more specific tools, +like some kind of unionfs-like filesytem proxy that mirrors other parts of the +filesystem, but allows overriding individual locations, in conjuction with +either chroot or some similar mechanism to create a subenvironment with a +different root filesystem. + +It's also desirable to have a mechanism allowing a user to set up such a custom +environment in a way that it will automatically get launched on login -- +practically allowing the user to run a customized operating system in his own +account. + +Yet another interesting scenario would be a subenvironment -- using some kind +of special filesystem proxy again -- in which the user serves as root, being +able to create local sub-users and/or sub-groups. + +This would allow the user to run "dangerous" applications (webbrowser, chat +client etc.) in a confined fashin, allowing it access to only a subset of the +user's files and other resources. (This could be done either using a lot of +groups for individual resources, and lots of users for individual applications; +adding a user to a group would give the corresponding application access to the +corresponding resource -- an advanced ACL mechanism. Or leave out the groups, +assigning the resources to users instead, and use the Hurd's ability for a +process to have multiple user ID's, to equip individual applications with set's +of user ID's giving them access to the necessary resources -- basically a +capability mechanism.) + +The student will have to pick (at least) one of the described scenarios -- or +come up with some other one in a similar spirit -- and implement all the tools +(scripts, translators) necessary to make it available to users in an +easy-to-use fashion. While the Hurd by default already offers the necessary +mechanisms for that, these are not perfect and could be further refined for +even better virtualization capabilities. Should need or desire for specific +improvements in that regard come up in the course of this project, implementing +these improvements can be considered part of the task. + +Completing this project will require gaining a very good understanding of the +Hurd architecture and spirit. Previous experience with other virtualization +solutions would be very helpful. + * procfs + +Although there is no standard (POSIX or other) for the layout of the /proc +pseudo-filesystem, it turned out a very useful facility in GNU/Linux and other +systems, and many tools concerned with process management use it. (ps, top, +htop, gtop, killall, pkill, ...) + +Instead of porting all these tools to use libps (Hurd's official method for +accessing process information), they could be made to run out of the box, by +implementing a Linux-compatible /proc filesystem for the Hurd. + +The goal is to implement all /proc functionality needed for the various process +management tools to work. (On Linux, the /proc filesystem is used also for +debugging purposes; but this is highly system-specific anyways, so there is +probably no point in trying to duplicate this functionality as well...) + +The existing partially working procfs implementation from the hurdextras +repository (link) can serve as a starting point, but needs to be largely +rewritten. (It should use libnetfs rather than libtrivfs; the data format needs +to change to be more Linux-compatible; and it needs adaptation to newer system +interfaces.) + +This project requires learning translator programming, and understanding some +of the internals of process management in the Hurd. It should not be too hard +coding-wise; and the task is very nicely defined by the exising Linux /proc +interface -- no design considerations necessary. + * mtab + +In traditional monolithic system, the kernel keeps track of all mounts; the +information is available through /proc/mounts (on Linux at least), and in a +very similar form in /etc/mtab. + +The Hurd on the other hand has a totally decentralized file system. There is no +single entity involved in all mounts. Rather, only the parent file system to +which a mountpoint (translator) is attached is involved. As a result, there is +no central place keeping track of mounts. + +As a consequence, there is currently no easy way to obtain a listing of all +mounted file systems. This also means that commands like "df" can only work on +explicitely specified mountpoints, instead of displaying the usual listing. + +One possible solution to this would be for the translator startup mechanism to +update the mtab on any mount/unmount, like in traditional systems. However, +there are same problems with this approach. Most notably: What to do with +passive translators, i.e. translators that are not presently running, but set +up to be started automatically whenever the node is accessed? Probably these +should be counted an among the mounted filesystems; but how to handle the mtab +updates for a translator that is not started yet? Generally, being centralized +and event-based, this is a pretty unelegant, non-hurdish solution. + +A more promising approach is to have mtab exported by a special translator, +which gathers the necessary information on demand. This could work by +traversing the tree of translators, asking each one for mount points attached +to it. (Theoretically, it could also be done by just traversing *all* nodes, +checking each one for attached translators. That would be very inefficient, +though. Thus a special interface is probably required, that allows asking a +translator to list mount points only.) + +There are also some other issues to keep in mind. Traversing arbitrary +translators set by other users can be quite dangerous -- and it's probably not +very interesting anyways what private filesystems some other user has mounted. +But what about the global /etc/mtab? Should it list only root-owned +filesystems? Or should it create different listings depending on what user +contacts it?... + +That leads to a more generic question: Which translators should be actually +listed? There are all kinds of translators: Ranging from traditional +filesystems (disks and other actual stores), but also purely virtual +filesystems like ftpfs or unionfs, and even things that have very little to do +with a traditional filesystem, like gzip translator, mbox translator, xml +translator, or various device file translators... Listing all of these in +/etc/mtab would be pretty pointless, so some kind of classification mechanism +is necessary. By default it probably should list only translators that claim to +be real filesystems, though alternative views with other filtering rules might +be desirable. + +After taking decisions on the outstanding design questions, the student will +implement both the actual mtab translator, and the necessery interface(s) for +gathering the data. It requires getting a good understanding of the translator +mechanism and Hurd interfaces in general. + * xmlfs * fix tmpfs * allow using unionfs early at boot |