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

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On 12/23/2014 10:13 AM, Khalid Aziz wrote:

On 12/23/2014 03:52 AM, Ingo Molnar wrote:

quoted

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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