Re: Real-time rw-locks (Re: [patch] Real-Time Preemption, -RT-2.6.10-rc2-mm3-V0.7.32-15)

[William Lee Irwin III]

* Esben Nielsen <simlo@xxxxxxxxxx> wrote:
>> [...] I wanted to start looking at fixing that because it ought to
>> hurt scalability quite a bit - and even on UP create a few unneeded
>> task-switchs. [...]

On Fri, Jan 28, 2005 at 08:38:56AM +0100, Ingo Molnar wrote:
> no, it's not a big scalability problem. rwlocks are really a mistake -
> if you want scalability and spinlocks/semaphores are not enough then one
> should either use per-CPU locks or lockless structures. rwlocks/rwsems
> will very unlikely help much.

I wouldn't be so sure about that. SGI is already implicitly relying on
the parallel holding of rwsems for the lockless pagefaulting, and
Oracle has been pushing on mapping->tree_lock becoming an rwlock for a
while, both for large performance gains.


* Esben Nielsen <simlo@xxxxxxxxxx> wrote:
>> However, the more I think about it the bigger the problem:

On Fri, Jan 28, 2005 at 08:38:56AM +0100, Ingo Molnar wrote:
> yes, that complexity to get it perform in a deterministic manner is why
> i introduced this (major!) simplification of locking. It turns out that
> most of the time the actual use of rwlocks matches this simplified
> 'owner-recursive exclusive lock' semantics, so we are lucky.
> look at what kind of worst-case scenarios there may already be with
> multiple spinlocks (blocker.c). With rwlocks that just gets insane.

tasklist_lock is one large exception; it's meant for concurrency there,
and it even gets sufficient concurrency to starve the write side.

Try test_remap.c on mainline vs. -mm to get a microbenchmark-level
notion of the importance of mapping->tree_lock being an rwlock (IIRC
you were cc:'d in at least some of those threads).

net/ has numerous rwlocks, which appear to frequently be associated
with hashtables, and at least some have some relevance to performance.

Are you suggesting that lockless alternatives to mapping->tree_lock,
mm->mmap_sem, and tasklist_lock should be pursued now?


-- wli
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