Messages in this thread | | | Subject | Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier | From | Steven Rostedt <> | Date | Sat, 09 Jan 2010 19:41:39 -0500 |
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On Sat, 2010-01-09 at 16:03 -0800, Paul E. McKenney wrote: > On Sat, Jan 09, 2010 at 06:16:40PM -0500, Steven Rostedt wrote: > > On Sat, 2010-01-09 at 18:05 -0500, Steven Rostedt wrote: > > > > > Then we should have O(tasks) for spinlocks taken, and > > > O(min(tasks, CPUS)) for IPIs. > > > > And for nr tasks >> CPUS, this may help too: > > > > > cpumask = 0; > > > foreach task { > > > > if (cpumask == online_cpus) > > break; > > > > > spin_lock(task_rq(task)->rq->lock); > > > if (task_rq(task)->curr == task) > > > cpu_set(task_cpu(task), cpumask); > > > spin_unlock(task_rq(task)->rq->lock); > > > } > > > send_ipi(cpumask); > > Good point, erring on the side of sending too many IPIs is safe. One > might even be able to just send the full set if enough of the CPUs were > running the current process and none of the remainder were running > real-time threads. And yes, it would then be necessary to throttle > calls to sys_membarrier(). >
If you need to throttle calls to sys_membarrier(), than why bother optimizing it? Again, this is like calling synchronize_sched() in the kernel, which is a very heavy operation, and should only be called by those that are not performance critical.
Why are we struggling so much with optimizing the slow path?
Here's how I take it. This method is much better that sending signals to all threads. The advantage the sys_membarrier gives us, is also a way to keep user rcu_read_locks barrier free, which means that rcu_read_locks are quick and scale well.
So what if we have a linear decrease in performance with the number of threads on the write side?
-- Steve
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