linux-kernel - Re: [PATCH v7 2/2] mm: replace hardcoded 3% with admin_reserve

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Date:	Mon, 8 Apr 2013 12:49:25 -0600
From:	Andrew Shewmaker <agshew@...il.com>
To:	Simon Jeons <simon.jeons@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, alan@...rguk.ukuu.org.uk,
	ric.masonn@...il.com
Subject: Re: [PATCH v7 2/2] mm: replace hardcoded 3% with admin_reserve_pages knob

On Sat, Apr 6, 2013 at 6:48 PM, Simon Jeons <simon.jeons@...il.com> wrote:
> Hi Andrew,
>
> On 04/05/2013 11:02 PM, Andrew Shewmaker wrote:
>>
>> On Wed, Apr 3, 2013 at 9:50 PM, Simon Jeons <simon.jeons@...il.com> wrote:
>>>>
>>>> FAQ
>>>>
>> ...
>>>>
>>>>    * How do you calculate a minimum useful reserve?
>>>>
>>>>      A user or the admin needs enough memory to login and perform
>>>>      recovery operations, which includes, at a minimum:
>>>>
>>>>      sshd or login + bash (or some other shell) + top (or ps, kill,
>>>> etc.)
>>>>
>>>>      For overcommit 'guess', we can sum resident set sizes (RSS).
>>>>      On x86_64 this is about 8MB.
>>>>
>>>>      For overcommit 'never', we can take the max of their virtual sizes
>>>> (VSZ)
>>>>      and add the sum of their RSS.
>>>>      On x86_64 this is about 128MB.
>>>
>>>
>>> 1.Why has this different between guess and never?
>>
>> The default, overcommit 'guess' mode, only needs a reserve for
>> what the recovery programs will typically use. Overcommit 'never'
>> mode will only successfully launch an app when it can fulfill all of
>> its requested memory allocations--even if the app only uses a
>> fraction of what it asks for.
>
>
> VSZ has already cover RSS, is it? why account RSS again?

Right. Technically, I could leave out the RSS of the process that
I'm taking the VSZ of. Leaving it in makes the estimate 2-4 MB
larger, but it is just an estimate.

Choosing a good minimum is difficult because the behavior with
and without swap is different. With swap, only about 8MB reserves
are needed whether overcommit is disabled or not. Without swap,
I was always able to recover when I set the reserves to over 230MB
each. However, I was often able to recover with much less.

128MB seemed to me like a decent compromise between swap
and noswap modes, but it isn't totally safe for the case when
both swap and memory overcommit are disabled. In that case,
I think that an admin will have to tune the reserves for their specific
situation.

--
Andrew Shewmaker
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