Re: [PATCH v3] sched/fair: Add advisory flag for borrowing a timeslice

[Khalid Aziz <khalid.aziz@oracle.com>]

On 11/25/2014 08:25 AM, Rik van Riel wrote:
> On 11/25/2014 09:52 AM, Khalid Aziz wrote:
>> On 11/24/2014 07:03 PM, Rik van Riel wrote:
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>>> On 11/24/2014 03:56 PM, Khalid Aziz wrote:
>>>> sched/fair: Add advisory flag for borrowing a timeslice
>>>>
>>>> This patch adds a way for a task to request to borrow one
>>>> timeslice from future if it is about to be preempted, so it could
>>>> delay preemption and complete any critical task it is in the middle
>>>> of.
>>>>
>>>> This feature helps with performance on databases and has been used
>>>> for many years on other OSs by the databases. This feature helps in
>>>> situation where a task acquires a lock before performing a critical
>>>> operation on the database and happens to get preempted
>>>
>>> Why don't the other tasks that want the lock sleep on the
>>> lock?
>>>
>>> I can see this "solution" help mostly with userspace spinlocks,
>>> which are relics of a past era that need to die. There is no
>>> way userspace spinlocks will not fail miserably on virtual
>>> machines, and it is time to get rid of them.
>>
>> This solution indeed is for userspace spinlocks. Database code has been
>> written with all critical locking implemented in userspace (as I have
>> been told by database folks. I am not a database guy).
>
> They should fix that.

Agreed, but that is a very large task for databases for sure and 
potentially for java as well (think exhaustive testing as well besides 
code re-write). I do not see this happening except as part of major 
re-architecture. In the mean time, if we can give them help without 
impacting kernel significantly for other users, it only makes Linux a 
more usable platform for them.

Thanks,
Khalid