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SubjectRe: [patch 0/9] mutex subsystem, -V4

* Andrew Morton <akpm@osdl.org> wrote:

> One side point on semaphores and mutexes: the so-called "fast path" is
> generally not performance-critical, because we just don't take them at
> high frequencies. Any workload which involves taking a semaphore at
> more than 50,000-100,000 times/second tends to have ghastly
> overscheduling failure scenarios on SMP. So people hit those
> scenarios and the code gets converted to a lockless algorithm or to
> use spinlocking.
>
> For example, for a while ext3/JBD was doing 200,000 context-switches
> per second due to taking lock_super() at high frequencies. When I
> converted the whole fs to use spin locking throughout the performance
> in some workloads went up by 1000%.

actually, i'm 99.9% certain [ ;-) ] that all that ext3 spinlock
conversion pain could have been avoided by converting ext3 to the mutex
code. Mutexes definitely do not overschedule, even in very high
frequency lock/unlock scenarios. They behave and perform quite close to
spinlocks. (which property is obviously a must for the -rt kernel, where
all spinlocks, rwlocks, seqlocks, rwsems and semaphores are mutexes -
providing a big playground for locking constructs)

hm, can you see any easy way for me to test my bold assertion on ext3,
by somehow moving/hacking it back to semaphores? I could then apply the
MUTEX_DEBUG_FULL mechanism to it and redo those ext3 semaphore vs.
spinlock benchmarks on a couple of boxes, and add a mutex column to the
table of numbers.

also, often it's simpler to use a sleeping lock than to use a spinlock
for something, so we want to have that option open.

Ingo
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