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

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Message-ID: <5474BADC.5030808@oracle.com>
Date:	Tue, 25 Nov 2014 10:22:36 -0700
From:	Khalid Aziz <khalid.aziz@...cle.com>
To:	Rik van Riel <riel@...hat.com>, tglx@...utronix.de, corbet@....net,
	mingo@...hat.com, hpa@...or.com, peterz@...radead.org,
	akpm@...ux-foundation.org, rientjes@...gle.com, ak@...ux.intel.com,
	mgorman@...e.de, raistlin@...ux.it,
	kirill.shutemov@...ux.intel.com, atomlin@...hat.com,
	avagin@...nvz.org, gorcunov@...nvz.org, serge.hallyn@...onical.com,
	athorlton@....com, oleg@...hat.com, vdavydov@...allels.com,
	daeseok.youn@...il.com, keescook@...omium.org,
	yangds.fnst@...fujitsu.com, sbauer@....utah.edu,
	vishnu.ps@...sung.com, axboe@...com, paulmck@...ux.vnet.ibm.com
CC:	linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
	linux-api@...r.kernel.org
Subject: Re: [PATCH v3] sched/fair: Add advisory flag for borrowing a timeslice

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:
>>> -----BEGIN PGP SIGNED MESSAGE-----
>>> Hash: SHA1
>>>
>>> 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
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