Messages in this thread | | | Date | Wed, 04 May 2011 16:47:07 -0500 | From | Dave Kleikamp <> | Subject | idle issues running sembench on 128 cpus |
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Thomas, I've been looking at performance running sembench on a 128-cpu system and I'm running into some issues in the idle loop.
Initially, I was seeing a lot of contention on the clockevents_lock in clockevents_notify(). Assuming it is only protecting clockevents_chain, and not the handlers themselves, I changed this to an rwlock (with thoughts of using rcu if successful).
This didn't help, but exposed an underlying problem with high contention on tick_broadcast_lock in tick_broadcast_oneshot_control(). I think with this many cpus, tick_handle_oneshot_broadcast() is holding that lock a long time, causing the idle cpus to spin on the lock.
I am able to avoid this problem with either kernel parameter, "idle=mwait" or "processor.max_cstate=1". Similarly, defining CONFIG_INTEL_IDLE=y and using the kernel parameter intel_idle.max_cstate=1 exposes a different spinlock, pm_qos_lock, but I found this patch which fixes that contention: https://lists.linux-foundation.org/pipermail/linux-pm/2011-February/030266.html https://patchwork.kernel.org/patch/550721/
Of course, we'd like to find a way to reduce the spinlock contention and not resort to prohibiting the cpus from entering C3 state at all. I don't see a simple fix, and want to know if you've seen anything like this before and given it any thought.
I also don't know if it makes sense to be able to tune the cpuidle governors to add more resistance to enter the C3 state, or even being able to switch to a performance governor at runtime, similar to cpufreq.
I'd like to hear your thoughts before I dive any deeper into this.
Thanks, Shaggy
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