Messages in this thread | | | Date | Wed, 05 Mar 2014 17:17:24 -0700 | From | Khalid Aziz <> | Subject | Re: [RFC] [PATCH] Pre-emption control for userspace |
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On 03/05/2014 04:59 PM, David Lang wrote: > what's the cost to setup mmap of this file in /proc. this is sounding > like a lot of work.
That is a one time cost paid when a thread initializes itself.
> >>> is this gain from not giving up the CPU at all? or is it from avoiding >>> all the delays due to the contending thread trying in turn? the >>> yield_to() approach avoids all those other threads trying in turn so it >>> should get fairly close to the same benefits. >>> >> >> The gain is from avoiding contention by giving locking thread a chance >> to complete its critical section which is expected to be very short >> (certainly shorter than timeslice). Pre-emption immunity gives it one >> and only one additional timeslice. > > but the yield_to() does almost the same thing, there is a small bump, > but you don't have to wait for thread B to spin, thread C..ZZZ etc to > spin before thread A can finish it's work. As soon as the second thread > hits the critical section, thread A is going to be able to do more work > (and hopefully finish) > >> Hope this helps clear things up. > > It doesn't sound like you and I are understanding how the yield_to() > approach would work. I hope my comments have helped get us on the same > page. >
I apologize if I am being dense. My understanding of yield_to() is what Oleg had said in his reply earlier, so I will quote the example he gave:
my_lock() { if (!TRY_LOCK()) { yield_to(owner); LOCK(); }
owner = gettid(); }
If thread A had already lost the processor by the time thread B executes above code, wouldn't we have paid the price of two context switches for thread A?
Thanks, Khalid
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