"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." -- From Wikipedia
Questions tagged [futex]
107 questions
6
votes
1 answer
Futexes and blocking in the kernel
I was reading some documentation and trying out a few samples of code that issue the futex system call in Linux. I read that if a mutex is acquired by thread_a using FUTEX_LOCK_PI, and say if another thread thread_b tries to acquire the same, the…
user277465
6
votes
2 answers
Python: Process hangs with futex(0x2a5fcc0, FUTEX_WAIT_PRIVATE, 0, NULL in multithreading
So I have a Queue :
q = Queue.Queue()
And I'm putting some items in it.
items = ["First", "Second"]
for val in items:
q.put(val)
And I'm spawning 15 threads.
for i in range(15):
tname = 't-%s' % i
t = my_thread(some_func, q, tname)
…
user1452759
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5
votes
0 answers
Fastest way to wake up a thread of a thread pool
I wrote a thread pool with as many threads as I have (spare cores), to avoid context switching. Whenever a new task needs to be executed, that task is added to a lock-free ring buffer for threads of the thread pool to consume. Each time a new task…
Carlo Wood
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5
votes
2 answers
How are threads/processes parked and woken in Linux, prior to futex?
Before the futex system calls existed in Linux, what underlying system calls were used by threading libraries like pthreads to block/sleep a thread and to subsequently wake those threads from userland?
For example, if a thread tries to acquire a…
BeeOnRope
- 60,350
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5
votes
2 answers
use futex in user space?
I need functionality of do_futex() call in user space outside of lock/unlock context. I.e., I do not need a mutex, but the exact semantics of the kernel call do_futex.
It would seem it should be available in user space, since the intent was to…
n-alexander
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4
votes
1 answer
Profiling synchronization operations in Linux
I want to profile synchronization operations, such as locking and unlocking of mutexes, semaphores etc. in Linux.
I know that deep down they are implemented using futexes, so maybe it is enough to profile locking and unlocking of futexes (please…
MetallicPriest
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4
votes
2 answers
How do I find the line of C++ which locks a Linux futex?
I've got a performance problem with a large application written in C++. The program uses only 150% CPU, while the server is a 24-core hyperthreaded EPYC and other, similar applications can reliably hit the expected 4800% CPU load. iotop shows…
MSalters
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4
votes
0 answers
app process blocked at art::ConditionVariable::WaitHoldingLocks, how to find the reason?
On Android 8.0, developed with kotlin and kotlin coroutine. All the threads will be blocked in the app. I had analyzed using strace, gdbserver to check syscall and get heap.
such heap as:
the first thread's heap from gdb
#0 0x0000007022ab84ac in…
jie zi
- 51
- 1
- 4
4
votes
1 answer
Does a futex wait/wake pair achieve acquire/release semantics?
Given this pseudocode, where there is globally an atomic int a initialized to 0:
Thread 1:
// ... some code here (X) ...
a.store(1, relaxed);
futex_wake(&a);
Thread 2:
if (futex_wait(&a, 1) == woken_up) {
assert(a.load(relaxed) == 1);
// ...…
peppe
- 21,934
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4
votes
1 answer
Is test-and-set (or other atomic RMW operation) a privileged instruction on any architecture?
Hardware provides atomic instructions like test-and-set, compare-and-swap, load-linked-store-conditional. Are these privileged instructions? That is, can only the OS execute them (and thus requiring a system call)?
I thought they are not privileged…
flow2k
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4
votes
1 answer
Why doesn't glib use private futexes?
When investigating some performance issues, I finally ended up in gthread-posix.c.
There I found code such as:
static void __attribute__((noinline))
g_mutex_lock_slowpath (GMutex *mutex)
{
/* Set to 2 to indicate contention. If it was zero before…
Jan Nordén
- 51
- 2
4
votes
1 answer
Measure mutex or futex latency
How could I measure a latency of mutex, semaphore or futex? I mean the latency between two events: unlock previously locked mutex and the locking that mutex. There are two cases: when all threads/processes are on the same CPU (how long will it take…
osgx
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3
votes
2 answers
Play! framework hangs on compile, no errors reported
Just moved my app to a new CentOS server. After fiddling for a long time, I can't play to compile or get any errors. The shell output will show me the last file compiling and then hang. I'm running the Scala 0.9.1 module.
I did a strace on the…
crockpotveggies
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3
votes
2 answers
Apache mod_perl processes hang in futex_wait state
I run a fairly popular browser-based web game, running under Apache (worker) and mod_perl. During peak times, when the server is handling about 4200 requests per minute, once every 3-15 minutes or so an Apache process will hang.
I have established…
parsim
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3
votes
2 answers
Shared pthread_cond_broadcast stuck in futex_wait
I have one "server" process a and potentially multiple "client" processes b. The server creates a shared memory file (shm_open) containing a pthread_mutex_t and a pthread_cond_t that it uses for broadcasting to the clients that something has happned…
Poohl
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