Re: [RFC] [PATCH] Pre-emption control for userspace

[Davidlohr Bueso]

On Mon, 2014-03-03 at 11:07 -0700, Khalid Aziz wrote:
> I am working on a feature that has been requested by database folks that
> helps with performance. Some of the oft executed database code uses
> mutexes to lock other threads out of a critical section. They often see
> a situation where a thread grabs the mutex, runs out of its timeslice
> and gets switched out which then causes another thread to run which
> tries to grab the same mutex, spins for a while and finally gives up.

This strikes me more of a feature for a real-time kernel. It is
definitely an interesting concept but wonder about it being abused.
Also, what about just using a voluntary preemption model instead? I'd
think that systems where this is really a problem would opt for that.

> This can happen with multiple threads until original lock owner gets the
> CPU again and can complete executing its critical section. This queueing
> and subsequent CPU cycle wastage can be avoided if the locking thread
> could request to be granted an additional timeslice if its current
> timeslice runs out before it gives up the lock. Other operating systems
> have implemented this functionality and is used by databases as well as
> JVM. This functionality has been shown to improve performance by 3%-5%.

Could you elaborate more on those performance numbers? What
benchmark/workload?

Thanks,
Davidlohr

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