2 ms·

> Ah OK, this is the part that I feel is maybe always left out of the things I've read on futex(). I guess this is just always implied then that some library im

> Ah OK, this is the part that I feel is maybe always left out of the things I've read on futex(). I guess this is just always implied then that some library implements these semantics correctly? And that library is generally going to glibc? Yup. For example, the pthread_foo functions are futexes under the hood.

tialaramex 5y ago
On Linux. The POSIX threading API accommodates many possible implementations, and so in Linux many of the bits that look expensive (and might be on other systems) are more or less free thanks to futexes, e.g. setting up the mutex in Linux is just allocating an aligned 32-bit value on the heap and setting it to some initialising value, but on some systems it involves an OS system call to get a mutex handle. This is the true benefit of the futex. The typical scheme for using it looks almost exactly like a well known trick for speeding up conventional mutual exclusion features that have low contention, which you'd see in say BeOS as the Benaphore, or described in several books about high performance programming. But those tricks all imply the expense of first obtaining a handle from the operating system for every such mutex whereas with a futex that part is free and you only talk to the operating system at all once there is contention for it to help you manage.
- bogomipz 5y ago
  >"The typical scheme for using it looks almost exactly like a well known trick for speeding up conventional mutual exclusion features that have low contention, which you'd see in say BeOS as the Benaphore, or described in several books about high performance programming." Do you have any links that document this trick in Linux? Is there name for it or search term I could use to find out more?
  - tialaramex 5y ago
    The phrase you'd have used to describe this when it wasn't just how everything works on a good OS is "Lightweight mutex". Here's Be explaining the "Benaphore" for example: https://www.haiku-os.org/legacy-docs/benewsletter/Issue1-26.html https://www.haiku-os.org/legacy-docs/benewsletter/Issue1-26.... Note that when I say "typical scheme for using it" I did not mean, that your program should implement such a scheme itself, instead futex is intended for the sort of people who implement the concurrency features in your language or APIs to use - and this is how they'd go about doing that to deliver the features you just use. So, for most of us this is interesting to know about but unlikely to impact our day-to-day practice.

tialaramex 5y ago

On Linux. The POSIX threading API accommodates many possible implementations, and so in Linux many of the bits that look expensive (and might be on other systems) are more or less free thanks to futexes, e.g. setting up the mutex in Linux is just allocating an aligned 32-bit value on the heap and setting it to some initialising value, but on some systems it involves an OS system call to get a mutex handle. This is the true benefit of the futex. The typical scheme for using it looks almost exactly like a well known trick for speeding up conventional mutual exclusion features that have low contention, which you'd see in say BeOS as the Benaphore, or described in several books about high performance programming. But those tricks all imply the expense of first obtaining a handle from the operating system for every such mutex whereas with a futex that part is free and you only talk to the operating system at all once there is contention for it to help you manage.

bogomipz 5y ago
>"The typical scheme for using it looks almost exactly like a well known trick for speeding up conventional mutual exclusion features that have low contention, which you'd see in say BeOS as the Benaphore, or described in several books about high performance programming." Do you have any links that document this trick in Linux? Is there name for it or search term I could use to find out more?
- tialaramex 5y ago
  The phrase you'd have used to describe this when it wasn't just how everything works on a good OS is "Lightweight mutex". Here's Be explaining the "Benaphore" for example: https://www.haiku-os.org/legacy-docs/benewsletter/Issue1-26.html https://www.haiku-os.org/legacy-docs/benewsletter/Issue1-26.... Note that when I say "typical scheme for using it" I did not mean, that your program should implement such a scheme itself, instead futex is intended for the sort of people who implement the concurrency features in your language or APIs to use - and this is how they'd go about doing that to deliver the features you just use. So, for most of us this is interesting to know about but unlikely to impact our day-to-day practice.