Message-ID: <5474C055.8070409@redhat.com>
Date: Tue, 25 Nov 2014 12:45:57 -0500
From: Rik van Riel <riel@redhat.com>
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To: Khalid Aziz <khalid.aziz@oracle.com>, tglx@linutronix.de, corbet@lwn.net,
        mingo@redhat.com, hpa@zytor.com, peterz@infradead.org,
        akpm@linux-foundation.org, rientjes@google.com, ak@linux.intel.com,
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        atomlin@redhat.com, avagin@openvz.org, gorcunov@openvz.org,
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        vdavydov@parallels.com, daeseok.youn@gmail.com, keescook@chromium.org,
        yangds.fnst@cn.fujitsu.com, sbauer@eng.utah.edu, vishnu.ps@samsung.com,
        axboe@fb.com, paulmck@linux.vnet.ibm.com
CC: linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
        linux-api@vger.kernel.org
Subject: Re: [PATCH v3] sched/fair: Add advisory flag for borrowing a timeslice
References: <1416862595-24513-1-git-send-email-khalid.aziz@oracle.com> <5473E388.6000605@redhat.com> <547497A9.1080800@oracle.com> <54749F77.50905@redhat.com> <5474BADC.5030808@oracle.com>
In-Reply-To: <5474BADC.5030808@oracle.com>
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On 11/25/2014 12:22 PM, Khalid Aziz wrote:
> 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.

I am not convinced that permanently adding overhead to the
Linux scheduler in order to deal with a temporary problem in
database and Java software (probably not every JVM, either!)
is a worthwhile trade-off..

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