• Jakub Jelinek's avatar
    [PATCH] FUTEX_WAKE_OP: pthread_cond_signal() speedup · 4732efbe
    Jakub Jelinek authored
    ATM pthread_cond_signal is unnecessarily slow, because it wakes one waiter
    (which at least on UP usually means an immediate context switch to one of
    the waiter threads).  This waiter wakes up and after a few instructions it
    attempts to acquire the cv internal lock, but that lock is still held by
    the thread calling pthread_cond_signal.  So it goes to sleep and eventually
    the signalling thread is scheduled in, unlocks the internal lock and wakes
    the waiter again.
    
    Now, before 2003-09-21 NPTL was using FUTEX_REQUEUE in pthread_cond_signal
    to avoid this performance issue, but it was removed when locks were
    redesigned to the 3 state scheme (unlocked, locked uncontended, locked
    contended).
    
    Following scenario shows why simply using FUTEX_REQUEUE in
    pthread_cond_signal together with using lll_mutex_unlock_force in place of
    lll_mutex_unlock is not enough and probably why it has been disabled at
    that time:
    
    The number is value in cv->__data.__lock.
            thr1            thr2            thr3
    0       pthread_cond_wait
    1       lll_mutex_lock (cv->__data.__lock)
    0       lll_mutex_unlock (cv->__data.__lock)
    0       lll_futex_wait (&cv->__data.__futex, futexval)
    0                       pthread_cond_signal
    1                       lll_mutex_lock (cv->__data.__lock)
    1                                       pthread_cond_signal
    2                                       lll_mutex_lock (cv->__data.__lock)
    2                                         lll_futex_wait (&cv->__data.__lock, 2)
    2                       lll_futex_requeue (&cv->__data.__futex, 0, 1, &cv->__data.__lock)
                              # FUTEX_REQUEUE, not FUTEX_CMP_REQUEUE
    2                       lll_mutex_unlock_force (cv->__data.__lock)
    0                         cv->__data.__lock = 0
    0                         lll_futex_wake (&cv->__data.__lock, 1)
    1       lll_mutex_lock (cv->__data.__lock)
    0       lll_mutex_unlock (cv->__data.__lock)
              # Here, lll_mutex_unlock doesn't know there are threads waiting
              # on the internal cv's lock
    
    Now, I believe it is possible to use FUTEX_REQUEUE in pthread_cond_signal,
    but it will cost us not one, but 2 extra syscalls and, what's worse, one of
    these extra syscalls will be done for every single waiting loop in
    pthread_cond_*wait.
    
    We would need to use lll_mutex_unlock_force in pthread_cond_signal after
    requeue and lll_mutex_cond_lock in pthread_cond_*wait after lll_futex_wait.
    
    Another alternative is to do the unlocking pthread_cond_signal needs to do
    (the lock can't be unlocked before lll_futex_wake, as that is racy) in the
    kernel.
    
    I have implemented both variants, futex-requeue-glibc.patch is the first
    one and futex-wake_op{,-glibc}.patch is the unlocking inside of the kernel.
     The kernel interface allows userland to specify how exactly an unlocking
    operation should look like (some atomic arithmetic operation with optional
    constant argument and comparison of the previous futex value with another
    constant).
    
    It has been implemented just for ppc*, x86_64 and i?86, for other
    architectures I'm including just a stub header which can be used as a
    starting point by maintainers to write support for their arches and ATM
    will just return -ENOSYS for FUTEX_WAKE_OP.  The requeue patch has been
    (lightly) tested just on x86_64, the wake_op patch on ppc64 kernel running
    32-bit and 64-bit NPTL and x86_64 kernel running 32-bit and 64-bit NPTL.
    
    With the following benchmark on UP x86-64 I get:
    
    for i in nptl-orig nptl-requeue nptl-wake_op; do echo time elf/ld.so --library-path .:$i /tmp/bench; \
    for j in 1 2; do echo ( time elf/ld.so --library-path .:$i /tmp/bench ) 2>&1; done; done
    time elf/ld.so --library-path .:nptl-orig /tmp/bench
    real 0m0.655s user 0m0.253s sys 0m0.403s
    real 0m0.657s user 0m0.269s sys 0m0.388s
    time elf/ld.so --library-path .:nptl-requeue /tmp/bench
    real 0m0.496s user 0m0.225s sys 0m0.271s
    real 0m0.531s user 0m0.242s sys 0m0.288s
    time elf/ld.so --library-path .:nptl-wake_op /tmp/bench
    real 0m0.380s user 0m0.176s sys 0m0.204s
    real 0m0.382s user 0m0.175s sys 0m0.207s
    
    The benchmark is at:
    http://sourceware.org/ml/libc-alpha/2005-03/txt00001.txt
    Older futex-requeue-glibc.patch version is at:
    http://sourceware.org/ml/libc-alpha/2005-03/txt00002.txt
    Older futex-wake_op-glibc.patch version is at:
    http://sourceware.org/ml/libc-alpha/2005-03/txt00003.txt
    Will post a new version (just x86-64 fixes so that the patch
    applies against pthread_cond_signal.S) to libc-hacker ml soon.
    
    Attached is the kernel FUTEX_WAKE_OP patch as well as a simple-minded
    testcase that will not test the atomicity of the operation, but at least
    check if the threads that should have been woken up are woken up and
    whether the arithmetic operation in the kernel gave the expected results.
    Acked-by: default avatarIngo Molnar <mingo@redhat.com>
    Cc: Ulrich Drepper <drepper@redhat.com>
    Cc: Jamie Lokier <jamie@shareable.org>
    Cc: Rusty Russell <rusty@rustcorp.com.au>
    Signed-off-by: default avatarYoichi Yuasa <yuasa@hh.iij4u.or.jp>
    Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
    4732efbe
futex.h 2.36 KB