In computing , a futex short for "fast userspace mutex " is a kernel system call that programmers can use to implement basic locking , or as a building block for higher-level locking abstractions such as semaphores and POSIX mutexes or condition variables. A futex consists of a kernelspace wait queue that is attached to an atomic integer in userspace. Multiple processes or threads operate on the integer entirely in userspace using atomic operations to avoid interfering with one another , and only resort to relatively expensive system calls to request operations on the wait queue for example to wake up waiting processes, or to put the current process on the wait queue. A properly programmed futex-based lock will not use system calls except when the lock is contended; since most operations do not require arbitration between processes, this will not happen in most cases. Futexes appeared for the first time in version 2. However, Linus Torvalds strongly opposed this idea and rejected any related patches.
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Without subscribers, LWN would simply not exist. Please consider signing up for a subscription and helping to keep LWN publishing November 11, Futexes are the basis of several mutual exclusion constructs commonly used in threaded programming. These include pthread mutexes, condvars, semaphores, rwlocks, and barriers.
They have been through a lot of reconstructive and cosmetic surgery over the last several years, and are now more efficient, more functional, and better documented than ever before. Overview While few application developers will use futexes directly, a cursory knowledge of how to do so is necessary to appreciate the improvements presented a bit later.
By way of a simple example, futexes can be used to store the state of a lock and provide a kernel waitqueue for tasks blocking on the lock. To minimize syscall overhead, this state should allow for atomic lock acquisition when the lock is uncontended.
In this case, the locking thread acquires the lock without involving the kernel and the kernel has no knowledge that this futex exists. When the next thread attempts to acquire the lock, the test for zero will fail and the kernel needs to be involved. This is a an obviously trivial example with lots of room for optimization. Ulrich Drepper's " Futexes are Tricky " [PDF] is still the undisputed reference for using futexes to build locking primitives such as mutexes.
It explores the many race conditions involved with using futexes as well as optimizations to improve on the example given here.
When the user threads call into the kernel with the futex system call, they pass the address of the futex state uaddr , the opcode to perform op , and various other arguments. These relationships are illustrated below: In most cases, there is no policy defining how the user space state variable is to be used despite what the futex man pages may or may not say.
The application or a library such as glibc uses this value to define the state of the locking construct being implemented. This can be as simple as a boolean variable as in the example above , however optimized implementations and other locking mechanisms require more complex state values.
PI and robust futexes are exceptions to the user-defined-policy rule regarding the state variable. Their state depends not only on the locked state of the mutex, but also on the identity of the owner and whether or not there are waiters. As such, the futex value is defined as the thread identifier TID of the owner and a bit to indicate pending owners. This policy still allows for user space atomic operations to avoid calling into the kernel in the uncontended case.
Improvements Futexes have seen numerous improvements from a handful of developers since their debut appearance in 2. Some notable improvements include priority based wake-up for real-time tasks by Pierre Peiffer and robust and PI futexes by Ingo Molnar and Thomas Gleixner. These latter features have been in use for some time and have been adequately covered here on LWN. Your author's foray into futexes picks up here, about two and a half years ago. Aside from several fixes to address rare corner cases and race conditions, the futex code has seen several functional and performance improvements since those earlier contributions.
Significant effort went into reducing futex overhead. Private futexes can only be used by threads of the same process. They are distinguishable from each other simply by their virtual address, while shared futexes have different virtual addresses in each process, requiring the kernel to lookup their physical address for unique identification.
It also eliminates the atomic operations used in reference counting for private futexes, resulting in less cache-line bouncing on SMP machines. Glibc now uses private futexes by default. Due to their dependence on user space addresses, futexes are burdened with several possible fault points. Bitset conditional wakeup was added by Thomas Gleixner February in order to enable optimized rwlock implementations in glibc.
This leads to a wake-up storm as all the waiters race back to user space to contend for the lock. It also fails to ensure that the highest priority task acquires the lock first.
Recent kernels 2. With glibc patches in the works by Dinakar Guniguntala, real-time applications will soon be able to use pthread condition variables with guaranteed wake-up order and fewer overall wake-ups. Now What? However, since no article is complete without a list of next steps, the following items may receive some attention in the future: Man pages: The current man pages do not include some of the new futex operations.
They suggest a policy for the value of the futex which has led to some confusion regarding usage of futexes. Users of futexes must use the syscall interface directly. Adaptive futexes: It is possible that some of the scheduling overhead of futexes can be reduced by some optional amount of spinning prior to going to sleep in the kernel. However, as futexes expose their state to user space, this spinning can also be done in user space, as is done with adaptive mutexes in glibc now, albeit without the knowledge of whether the owner is running, so spinning is reduced to a simple maximum-retries loop.
Interruptible futexes: There is some interest in interruptible blocking lock operations from large proprietary software projects. Futex operations currently restart themselves in the event of a signal, rather than returning -EINTR to user space. Exposing such a feature in the pthread locking primitives would involve non-POSIX compliant changes to the pthread library, but this is not without precedent. The futex hash table is shared across all processes and is protected by spinlocks which can lead to real overhead, especially on large systems.
This overhead is not serving any useful purpose if these systems are partitioned on NUMA nodes, or even for processes that use private futexes exclusively. Futex test suite: Your author has been compiling a list of requirements for an exhaustive test suite to validate futex functionality.
This test suite would serve as a regression suite for future development. The many corner cases and misuse cases possible with futexes complicate the test suite and present a challenge to its design. Acknowledgements I would like to extend my thanks to John Stultz, Will Schmidt, Paul McKenney, Nivedita Singhvi, and, of course, Jon Corbet, whose reviews have made this article far more legible and complete than it would have been otherwise.
I still cannot see the difference in these function names but its been a very long day A futex overview and update. I can't say for sure that this is SMP cach line bouncing, but there's a good chance of this being the case. User: Password:. Please consider signing up for a subscription and helping to keep LWN publishing November 11, This article was contributed by Darren Hart. One puts the task to sleep and sets the bitset by which it can be woken, the other does the wake with a matching bitset.
I believe I had to look at these a couple times myself even while writing the article! A futex overview and update Posted Nov 12, UTC Thu by johnflux guest, [ Link ] I don't know anything about Futexs etc, but given that the last few LWN articles have been along the lines of "The one big locking table was replaced with a per CPU table to increase scalability", do we expect to have an article in a year's time that the Futex hash table is now one-per-cpu?
A futex overview and update Posted Nov 12, UTC Thu by dvhart subscriber, [ Link ] There has been more discussion along the lines of per-numa-node and per- process tables to reduce false-sharing on the futex hash table, but the effect is similar. I'm working on a futex test suite now and I hope some of the perf and stress tests will help here.
A futex overview and update Posted Nov 16, UTC Mon by dlang guest, [ Link ] one real-world example of lock ping-pong with futexes that I ran into recently was rsyslog whould be reduced in recent versions it has threads that receive messages and add them to a lock protected queue, while other threads retrieve messages from the queue to output them.
A futex overview and update Posted Nov 25, UTC Wed by sustrik guest, [ Link ] As far as I understand, using private futex to wait on should be faster by one atomic op than waiting on glibc mutex, right? Any ideas why that may be so? Please consider signing up for a subscription and helping to keep LWN publishing.
futex(2) - Linux man page
The futex short for "Fast userspace mutex" mechanism was proposed by Linux contributors from IBM in  ; it was integrated into the kernel in late The main idea is to enable a more efficient way for userspace code to synchronize multiple threads, with minimal kernel involvement. In this post I want to provide a basic overview of futexes, how they work, and how they're used to implement the more familiar synchronization primitives in higher-level APIs and languages. It is extremely unlikely that you will ever need to use them in application code.
futex() - Unix, Linux System Call
It is typically used to implement the contended case of a lock in shared memory, as described in futex 7. When a futex 7 operation did not finish uncontended in userspace, a call needs to be made to the kernel to arbitrate. Arbitration can either mean putting the calling process to sleep or, conversely, waking a waiting process. Callers of this function are expected to adhere to the semantics as set out in futex 7.
A futex overview and update