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Diffstat (limited to 'Documentation/mutex-design.txt')
| -rw-r--r-- | Documentation/mutex-design.txt | 139 |
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diff --git a/Documentation/mutex-design.txt b/Documentation/mutex-design.txt deleted file mode 100644 index 38c10fd7f411..000000000000 --- a/Documentation/mutex-design.txt +++ /dev/null @@ -1,139 +0,0 @@ -Generic Mutex Subsystem - -started by Ingo Molnar <mingo@redhat.com> - - "Why on earth do we need a new mutex subsystem, and what's wrong - with semaphores?" - -firstly, there's nothing wrong with semaphores. But if the simpler -mutex semantics are sufficient for your code, then there are a couple -of advantages of mutexes: - - - 'struct mutex' is smaller on most architectures: E.g. on x86, - 'struct semaphore' is 20 bytes, 'struct mutex' is 16 bytes. - A smaller structure size means less RAM footprint, and better - CPU-cache utilization. - - - tighter code. On x86 i get the following .text sizes when - switching all mutex-alike semaphores in the kernel to the mutex - subsystem: - - text data bss dec hex filename - 3280380 868188 396860 4545428 455b94 vmlinux-semaphore - 3255329 865296 396732 4517357 44eded vmlinux-mutex - - that's 25051 bytes of code saved, or a 0.76% win - off the hottest - codepaths of the kernel. (The .data savings are 2892 bytes, or 0.33%) - Smaller code means better icache footprint, which is one of the - major optimization goals in the Linux kernel currently. - - - the mutex subsystem is slightly faster and has better scalability for - contended workloads. On an 8-way x86 system, running a mutex-based - kernel and testing creat+unlink+close (of separate, per-task files) - in /tmp with 16 parallel tasks, the average number of ops/sec is: - - Semaphores: Mutexes: - - $ ./test-mutex V 16 10 $ ./test-mutex V 16 10 - 8 CPUs, running 16 tasks. 8 CPUs, running 16 tasks. - checking VFS performance. checking VFS performance. - avg loops/sec: 34713 avg loops/sec: 84153 - CPU utilization: 63% CPU utilization: 22% - - i.e. in this workload, the mutex based kernel was 2.4 times faster - than the semaphore based kernel, _and_ it also had 2.8 times less CPU - utilization. (In terms of 'ops per CPU cycle', the semaphore kernel - performed 551 ops/sec per 1% of CPU time used, while the mutex kernel - performed 3825 ops/sec per 1% of CPU time used - it was 6.9 times - more efficient.) - - the scalability difference is visible even on a 2-way P4 HT box: - - Semaphores: Mutexes: - - $ ./test-mutex V 16 10 $ ./test-mutex V 16 10 - 4 CPUs, running 16 tasks. 8 CPUs, running 16 tasks. - checking VFS performance. checking VFS performance. - avg loops/sec: 127659 avg loops/sec: 181082 - CPU utilization: 100% CPU utilization: 34% - - (the straight performance advantage of mutexes is 41%, the per-cycle - efficiency of mutexes is 4.1 times better.) - - - there are no fastpath tradeoffs, the mutex fastpath is just as tight - as the semaphore fastpath. On x86, the locking fastpath is 2 - instructions: - - c0377ccb <mutex_lock>: - c0377ccb: f0 ff 08 lock decl (%eax) - c0377cce: 78 0e js c0377cde <.text..lock.mutex> - c0377cd0: c3 ret - - the unlocking fastpath is equally tight: - - c0377cd1 <mutex_unlock>: - c0377cd1: f0 ff 00 lock incl (%eax) - c0377cd4: 7e 0f jle c0377ce5 <.text..lock.mutex+0x7> - c0377cd6: c3 ret - - - 'struct mutex' semantics are well-defined and are enforced if - CONFIG_DEBUG_MUTEXES is turned on. Semaphores on the other hand have - virtually no debugging code or instrumentation. The mutex subsystem - checks and enforces the following rules: - - * - only one task can hold the mutex at a time - * - only the owner can unlock the mutex - * - multiple unlocks are not permitted - * - recursive locking is not permitted - * - a mutex object must be initialized via the API - * - a mutex object must not be initialized via memset or copying - * - task may not exit with mutex held - * - memory areas where held locks reside must not be freed - * - held mutexes must not be reinitialized - * - mutexes may not be used in hardware or software interrupt - * contexts such as tasklets and timers - - furthermore, there are also convenience features in the debugging - code: - - * - uses symbolic names of mutexes, whenever they are printed in debug output - * - point-of-acquire tracking, symbolic lookup of function names - * - list of all locks held in the system, printout of them - * - owner tracking - * - detects self-recursing locks and prints out all relevant info - * - detects multi-task circular deadlocks and prints out all affected - * locks and tasks (and only those tasks) - -Disadvantages -------------- - -The stricter mutex API means you cannot use mutexes the same way you -can use semaphores: e.g. they cannot be used from an interrupt context, -nor can they be unlocked from a different context that which acquired -it. [ I'm not aware of any other (e.g. performance) disadvantages from -using mutexes at the moment, please let me know if you find any. ] - -Implementation of mutexes -------------------------- - -'struct mutex' is the new mutex type, defined in include/linux/mutex.h -and implemented in kernel/mutex.c. It is a counter-based mutex with a -spinlock and a wait-list. The counter has 3 states: 1 for "unlocked", -0 for "locked" and negative numbers (usually -1) for "locked, potential -waiters queued". - -the APIs of 'struct mutex' have been streamlined: - - DEFINE_MUTEX(name); - - mutex_init(mutex); - - void mutex_lock(struct mutex *lock); - int mutex_lock_interruptible(struct mutex *lock); - int mutex_trylock(struct mutex *lock); - void mutex_unlock(struct mutex *lock); - int mutex_is_locked(struct mutex *lock); - void mutex_lock_nested(struct mutex *lock, unsigned int subclass); - int mutex_lock_interruptible_nested(struct mutex *lock, - unsigned int subclass); - int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock); |