linux-kernel - Re: [PATCH 2/2] mm: add overcommit

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Message-ID: <5214DB1B.6070803@redhat.com>
Date:	Wed, 21 Aug 2013 17:22:03 +0200
From:	Jerome Marchand <jmarchan@...hat.com>
To:	Dave Hansen <dave.hansen@...el.com>
CC:	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] mm: add overcommit_kbytes sysctl variable

On 08/19/2013 06:55 PM, Dave Hansen wrote:
> On 08/19/2013 08:17 AM, Jerome Marchand wrote:
>> Some applications that run on HPC clusters are designed around the
>> availability of RAM and the overcommit ratio is fine tuned to get the
>> maximum usage of memory without swapping. With growing memory, the
>> 1%-of-all-RAM grain provided by overcommit_ratio has become too coarse
>> for these workload (on a 2TB machine it represents no less than
>> 20GB).
>>
>> This patch adds the new overcommit_kbytes sysctl variable that allow a
>> much finer grain.
> 
> Instead of introducing yet another tunable, why don't we just make the
> ratio that comes in from the user more fine-grained?
> 
> 	sysctl overcommit_ratio=0.2
> 
> We change the internal 'sysctl_overcommit_ratio' to store tenths or
> hundreths of a percent (or whatever), then parse the input as two
> integers.  I don't think we need fully correct floating point parsing
> and rounding here, so it shouldn't be too much of a chore.  It'd
> probably end up being less code than you have as it stands.
> 

Now that I think about it, that could break user space. Sure write access
wouldn't be a problem (one can still write a plain integer), but a script
that reads a fractional value when it expects an integer might not be able
to cope with it.

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