Re: [PATCH RESEND v4] sched/fair: Add advisory flag for borrowing a timeslice

[Rik van Riel]

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On 12/23/2014 10:13 AM, Khalid Aziz wrote:
> On 12/23/2014 03:52 AM, Ingo Molnar wrote:
>> 
>> 
>> to implement what Thomas suggested in the discussion: a proper 
>> futex like spin mechanism? That looks like a totally acceptable 
>> solution to me, without the disadvantages of your proposed 
>> solution.
> 
> Hi Ingo,
> 
> Thank you for taking the time to respond. It is indeed possible to 
> implement a futex like spin mechanism. Futex like mechanism will
> be clean and elegant. That is where I had started when I was given
> this problem to solve. Trouble I run into is the primary
> application I am looking at to help with this solution is Database
> which implements its own locking mechanism without using POSIX
> semaphore or futex. Since the locking is entirely in userspace,
> kernel has no clue when the userspace has acquired one of these
> locks. So I can see only two ways to solve this - find a solution
> in userspace entirely, or have userspace tell the kernel when it
> acquires one of these locks. I will spend more time on finding a
> way to solve it in userspace and see if I can find a way to 
> leverage futex mechanism without causing significant change to
> database code. There may be a way to use priority inheritance to
> avoid contention. Database performance people tell me that their
> testing has shown the cost of making any system calls in this code
> easily offsets any gains from optimizing for contention avoidance,
> so that is one big challenge. Database rewriting their locking code
> is extremely unlikely scenario. Am I missing a third option here?

An uncontended futex is taken without ever going into kernel
space. Adaptive spinning allows short duration futexes to be
taken without going into kernel space.

Only long held locks cause a thread to go into kernel space,
where it goes to sleep, freeing up the cpu, and increasing
the chance that the lock holder will run.

- -- 
All rights reversed
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