linux-kernel - Re: [PATCH] sysctl: Add a feature to drop caches selectively

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Date:	Thu, 26 Jun 2014 14:10:28 +0200
From:	Bernd Schubert <bernd.schubert@...m.fraunhofer.de>
To:	Lukáš Czerner <lczerner@...hat.com>,
	Artem Bityutskiy <dedekind1@...il.com>
CC:	Dave Chinner <david@...morbit.com>,
	Thomas Knauth <thomas.knauth@....de>,
	David Rientjes <rientjes@...gle.com>,
	Maksym Planeta <mcsim.planeta@...il.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sysctl: Add a feature to drop caches selectively

On 06/26/2014 01:57 PM, Lukáš Czerner wrote:
> On Thu, 26 Jun 2014, Artem Bityutskiy wrote:
>> On Thu, 2014-06-26 at 12:36 +0200, Bernd Schubert wrote:
>>> On 06/26/2014 08:13 AM, Artem Bityutskiy wrote:
>>>> On Thu, 2014-06-26 at 11:06 +1000, Dave Chinner wrote:
>>>>> Your particular use case can be handled by directing your benchmark
>>>>> at a filesystem mount point and unmounting the filesystem in between
>>>>> benchmark runs. There is no ned to adding kernel functionality for
>>>>> somethign that can be so easily acheived by other means, especially
>>>>> in benchmark environments where *everything* is tightly controlled.
>>>>
>>>> If I was a benchmark writer, I would not be willing running it as root
>>>> to be able to mount/unmount, I would not be willing to require the
>>>> customer creating special dedicated partitions for the benchmark,
>>>> because this is too user-unfriendly. Or do I make incorrect assumptions?
>>>
>>> But why a sysctl then? And also don't see a point for that at all, why
>>> can't the benchmark use posix_fadvise(POSIX_FADV_DONTNEED)?
>>
>> The latter question was answered - people want a way to drop caches for
>> a file. They need a method which guarantees that the caches are dropped.
>> They do not need an advisory method which does not give any guarantees.

I'm not sure if a benchmark really needs that so much that FADV_DONTNEED 
isn't sufficient.
Personally I would also like to know if FADV_DONTNEED succeeded. I.e. 
'ql-fstest' is to check if the written pattern went to the block device 
and currently it does not know if data really had been dropped from the 
page cache. As it reads files several times this is not critical, but 
only would be a nice to have - nothing worth to add a new syscall.


Cheers,
Bernd
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