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author | Gerald Carter <jerry@samba.org> | 2002-10-25 15:34:10 +0000 |
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committer | Gerald Carter <jerry@samba.org> | 2002-10-25 15:34:10 +0000 |
commit | 092b85b2146518e1bc40633a2de2a24f30161f00 (patch) | |
tree | db7ac1f0091319452d09237f408d263c0fa94e61 /source3 | |
parent | 19889a016d33a5fe80bedfd5e97f0c0bb63151a7 (diff) | |
download | samba-092b85b2146518e1bc40633a2de2a24f30161f00.tar.gz samba-092b85b2146518e1bc40633a2de2a24f30161f00.tar.bz2 samba-092b85b2146518e1bc40633a2de2a24f30161f00.zip |
removing more files...
(This used to be commit c419d4477144058ed9ec475ceb80b995b82511b3)
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-rw-r--r-- | source3/architecture.doc | 134 |
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diff --git a/source3/architecture.doc b/source3/architecture.doc deleted file mode 100644 index eb29792bea..0000000000 --- a/source3/architecture.doc +++ /dev/null @@ -1,134 +0,0 @@ -Samba Architecture ------------------- - -First preliminary version Dan Shearer Nov 97 -Quickly scrabbled together from odd bits of mail and memory. Please update. - -This document gives a general overview of how Samba works -internally. The Samba Team has tried to come up with a model which is -the best possible compromise between elegance, portability, security -and the constraints imposed by the very messy SMB and CIFS -protocol. - -It also tries to answer some of the frequently asked questions such as: - - * Is Samba secure when running on Unix? The xyz platform? - What about the root priveliges issue? - - * Pros and cons of multithreading in various parts of Samba - - * Why not have a separate process for name resolution, WINS, - and browsing? - - -Multithreading and Samba ------------------------- - -People sometimes tout threads as a uniformly good thing. They are very -nice in their place but are quite inappropriate for smbd. nmbd is -another matter, and multi-threading it would be very nice. - -The short version is that smbd is not multithreaded, and alternative -servers that take this approach under Unix (such as Syntax, at the -time of writing) suffer tremendous performance penalties and are less -robust. nmbd is not threaded either, but this is because it is not -possible to do it while keeping code consistent and portable across 35 -or more platforms. (This drawback also applies to threading smbd.) - -The longer versions is that there are very good reasons for not making -smbd multi-threaded. Multi-threading would actually make Samba much -slower, less scalable, less portable and much less robust. The fact -that we use a separate process for each connection is one of Samba's -biggest advantages. - -Threading smbd --------------- - -A few problems that would arise from a threaded smbd are: - -0) It's not only to create threads instead of processes, but you - must care about all variables if they have to be thread specific - (currently they would be global). - -1) if one thread dies (eg. a seg fault) then all threads die. We can -immediately throw robustness out the window. - -2) many of the system calls we make are blocking. Non-blocking -equivalents of many calls are either not available or are awkward (and -slow) to use. So while we block in one thread all clients are -waiting. Imagine if one share is a slow NFS filesystem and the others -are fast, we will end up slowing all clients to the speed of NFS. - -3) you can't run as a different uid in different threads. This means -we would have to switch uid/gid on _every_ SMB packet. It would be -horrendously slow. - -4) the per process file descriptor limit would mean that we could only -support a limited number of clients. - -5) we couldn't use the system locking calls as the locking context of -fcntl() is a process, not a thread. - -Threading nmbd --------------- - -This would be ideal, but gets sunk by portability requirements. - -Andrew tried to write a test threads library for nmbd that used only -ansi-C constructs (using setjmp and longjmp). Unfortunately some OSes -defeat this by restricting longjmp to calling addresses that are -shallower than the current address on the stack (apparently AIX does -this). This makes a truly portable threads library impossible. So to -support all our current platforms we would have to code nmbd both with -and without threads, and as the real aim of threads is to make the -code clearer we would not have gained anything. (it is a myth that -threads make things faster. threading is like recursion, it can make -things clear but the same thing can always be done faster by some -other method) - -Chris tried to spec out a general design that would abstract threading -vs separate processes (vs other methods?) and make them accessible -through some general API. This doesn't work because of the data -sharing requirements of the protocol (packets in the future depending -on packets now, etc.) At least, the code would work but would be very -clumsy, and besides the fork() type model would never work on Unix. (Is there an OS that it would work on, for nmbd?) - -A fork() is cheap, but not nearly cheap enough to do on every UDP -packet that arrives. Having a pool of processes is possible but is -nasty to program cleanly due to the enormous amount of shared data (in -complex structures) between the processes. We can't rely on each -platform having a shared memory system. - -nbmd Design ------------ - -Originally Andrew used recursion to simulate a multi-threaded -environment, which use the stack enormously and made for really -confusing debugging sessions. Luke Leighton rewrote it to use a -queuing system that keeps state information on each packet. The -first version used a single structure which was used by all the -pending states. As the initialisation of this structure was -done by adding arguments, as the functionality developed, it got -pretty messy. So, it was replaced with a higher-order function -and a pointer to a user-defined memory block. This suddenly -made things much simpler: large numbers of functions could be -made static, and modularised. This is the same principle as used -in NT's kernel, and achieves the same effect as threads, but in -a single process. - -Then Jeremy rewrote nmbd. The packet data in nmbd isn't what's on the -wire. It's a nice format that is very amenable to processing but still -keeps the idea of a distinct packet. See "struct packet_struct" in -nameserv.h. It has all the detail but none of the on-the-wire -mess. This makes it ideal for using in disk or memory-based databases -for browsing and WINS support. - -nmbd now consists of a series of modules. It... - - -Samba Design and Security -------------------------- - -Why Isn't nmbd Multiple Daemons? --------------------------------- - |