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| ▲ | adrian_b 10 hours ago | parent | next [-] | | The number of Linux syscalls depends on the architecture and on x86-64 there are over 450 Linux syscalls. Nonetheless, about one third of those syscalls are obsolete (being replaced by newer improved syscalls), even if a large number of programs are still using them, and about one other third of them are rarely used in the environments familiar to most Linux users, like personal computers and small servers. The obsolete syscalls inflate a lot the number of Linux syscalls, because there are a lot of syscalls that were originally defined with an inadequate set of parameters, then they were fixed by adding parameters or changing the meaning of the existing parameters. There are many syscalls were even 3 or 4 attempts were needed until reaching an acceptable definition for their parameters. New programs should use only the newest improved syscalls, but the documentation of the Linux syscalls in various places is usually not up-to-date itself, so a lot of people are still using older syscalls, without being aware that they are suboptimal. So the obsolete syscalls will have to be supported forever by the Linux kernels (though in most cases when an obsolete syscall is invoked it just fills the parameters with adjusted values and then it jumps to the current syscall). | | |
| ▲ | rocqua 9 hours ago | parent [-] | | I'd love to learn more about the syscalls that took 3 or 4 attempts. |
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| ▲ | Joker_vD 10 hours ago | parent | prev | next [-] | | Well, there really is about 400 syscall numbers allocated, and each has its own entrypoint in the kernel. That's an entirely objective criterion as opposed to arguing "well, sbrk and mmap (or recv and read) are kinda similar, maybe we should count them as just one syscall instead of two", because in the end we can might as well say "well, technically there is only one syscall, it just has an additional seventh argument that serves as the 'function denominator', like the ioctl's request parameter, you know". | | |
| ▲ | ReDress 9 hours ago | parent [-] | | I'm obviously not going to discuss this argument further. Trying to counter a logical argument with a technical argument is pretty lame and doesn't make for a healthy conversation. | | |
| ▲ | Joker_vD 9 hours ago | parent [-] | | Uh, sorry, what? Trying to measure similarity between syscalls is a) impossible, b) pointless, so obviously I'd really like to understand why someone would want to do something like this — but I'd rather not speculate about what and why you think and believe, and instead to just straight hear from you. If you're interested in why I think what I think — there's a sibling comment where I tried to articulate my position. I'd like to hear about yours, if you don't mind. P.S. You didn't actually offer any argument, logical (?) or technical (?!), in your original comment, and I think you're actually have the meaning of those words reversed: "logical argument" appeals to logical relations between the abstract concepts, while "technical argument" appeals to the underlying technology (the kernel implementation in our case) and tries to deal with particularities of reality. So if anything, my argument was the logical (semantical?) one, not that it really matters. | | |
| ▲ | ReDress 8 hours ago | parent [-] | | Yeah, sure, but the obvious fact that is a lot of syscalls are 'family' relatives. And they wouldn't ever count as distinct syscalls. | | |
| ▲ | asalahli 8 hours ago | parent [-] | | > they wouldn't ever count as distinct syscalls. According to whom, exactly? | | |
| ▲ | ReDress 7 hours ago | parent [-] | | If a syscall morphs into several slightly variant syscalls, then there's no way these are distinct syscalls. If a syscall is designed to counter the flaws have been observed in another syscall, then there's no way this is a distinct syscall. If a syscall is designed to add feature to an another syscall, then there's no way this is distinct syscall. And so on and so forth. If you actually look at Linux kernel code, you will quickly note that a lot of these related syscall even happen to share a large amount of implementation code. So, yeah |
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| ▲ | trashb 7 hours ago | parent | prev | next [-] | | > 400 distinct syscalls is a very large number of syscalls
Compared to what?
It seems windows has 2000+ syscalls all combined (nt+win32).
For just ntos there are 490.https://github.com/hfiref0x/SyscallTables https://github.com/j00ru/windows-syscalls And does the number matter? Why can't it be 1000, 2000, 4000. In my opinion the interface is free to be defined as long as it is documented properly. | |
| ▲ | PhilipRoman 10 hours ago | parent | prev [-] | | I think the number is mostly meaningless even if we ignore whether they're distinct or not. Consider ioctl() - it's a single syscall but if I remember correctly the number of subcommands is in the thousands. | | |
| ▲ | Joker_vD 10 hours ago | parent | next [-] | | Yes, the ioctl has been notorious for sneaking in arbitrary amount of functionality into the kernel space, and serving generally as a kitchen sink: "You can rarely convince Linus to make a new syscall, but making a new ioctl is much easier". That's still one syscall though. And how do you even compare how the distinct the syscalls are (e.g., pipe() and open() are about 70% similar)? And after that, what do you do with this metrics? How do you sum it up? Again, pipe() and open() are 70% similar — does that mean we have 1.3 syscalls instead of 2? | | |
| ▲ | adrian_b 10 hours ago | parent [-] | | I would count "pipe" and "open" together as 0 (useful) syscalls, because both have been obsolete for a very long time (even if they occupy 2 slots in the syscall table and those 2 slots will remain occupied forever). These 2 syscalls have a history typical for Linux syscalls, where frequently new syscall variants make completely obsolete their predecessors, but the old syscalls must still remain supported forever by the newer Linux kernels, to allow running legacy programs. "pipe" has been obsoleted by "pipe2" (on 2008-10-09), which in turn has been obsoleted by "IORING_OP_PIPE". "open" has been obsoleted by "openat", which in turn has been obsoleted by "IORING_OP_OPENAT". Most users do not invoke Linux syscalls directly, but they use the standard C library, which has frequently shielded the end user from the evolution of the Linux syscalls. For instance, you might use the functions "open()" and "pipe()" in your C program, but the C library might invoke instead of those the more modern syscalls "openat" and "pipe2", after adjusting their parameters. | | |
| ▲ | Joker_vD 9 hours ago | parent [-] | | I think you're missing the point I'm trying to make. Both pipe2 and openat has about 70% similar functionality (and so do the IORING_OP_PIPE and IORING_OP_OPENAT operations) — really, the only difference is that pipe2 returns two new fds instead of just one, and that the file opened is not a regular file. So, are those 2 syscalls or 1.3? What about triples or quadruples of syscalls with overlapping functionality, do we go full inclusion-exclusion formula on them? What about that 70% similarity estimation — how do establish that? Do you count the similarity/overlap of the underlying implementations? Only of the user-facing functionality? Both, somehow? Something else entirely? Or, you know, we can instead not engage in this subjective quagmire and look at the actual, factual number that undeniably exists. It may not be very useful for any practical purpose except for pointless dick-measuring between OSes with radically different APIs and internal architectures — but neither is this trying to measure syscall similarity, really. | | |
| ▲ | smj-edison 6 hours ago | parent [-] | | I think it's a bit unfair to mainly focus on the API surface while ignoring a lot of internal details. Linus has talked about how Linux will often have massive amounts of internal code change between releases, while the API surface stays the same. So yeah, the API might be old and crufty, but it's also been around for 30 years. And as long as the API is sufficient, it's the internals that are a lot more interesting. Radically redesigning the API every few years would just accelerate towards XKCD 927, because of backwards compatibility. |
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| ▲ | adrian_b 10 hours ago | parent | prev [-] | | That's right. Moreover, for a large number of traditional Linux syscalls (including the many variants of read/write/send/recv) there are now better alternatives that are invoked as subcommands of a single syscall: io_uring_enter. | | |
| ▲ | ReDress 10 hours ago | parent [-] | | io-uring is great for this. However, I am bit skeptical as to whether io-uring will ever support all Linux syscalls. At this point I am convinced that io-uring primarily supports I/O API / syscalls. That would mean syscalls relating to disk and network I/O. While this covers a lot of syscalls it most definitely doesn't cover all. Though, to give some credit her, it does also cover to syscalls relating to kernel features like futex but yeah, for the most part it doesn't cover much that doesn't have to do with I/O. Which is obviously not a problem but raises the question of whether it ever will support other kernel features. | | |
| ▲ | adrian_b 5 hours ago | parent [-] | | The io_uring interface is preferable to classic syscalls for any kind of syscall that requires the kernel to perform a significant amount of work or to wait for some external events, so a long time could pass until the completion of the work or until the occurrence of the external events, and possibly for some auxiliary syscalls that modify settings relevant to the previous syscalls. Many of the most important syscalls belong to this class, but most of them already have io_uring alternatives. Many other syscalls are not of this kind, so for them there is no reason to replace the traditional syscall interface. For the syscalls appropriate for io_uring the essential characteristic is that their completions can occur in a different order than the submissions of their requests, which is why all previous methods of handling them were inefficient, because they required the user program to waste time with superfluous activities, instead of just picking the completion messages from a queue. Where this out-of-order execution is not needed, invoking the syscalls in the same way as you invoke any user function is fine. The initial implementation of io_uring lacked the alternatives for many important syscalls, but since then most of the holes have been filled. Besides more obvious syscalls of the read/write type, which are used for files accessed in the classic way, in order to reach maximum performance with memory-mapped files the old "madvise" syscall must be replaced with IORING_OP_MADVISE, used with Linux-specific advices (MADV_COLD, MADV_PAGEOUT, MADV_POPULATE_READ, MADV_POPULATE_WRITE). |
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