From: ebiederm@xmission.com (Eric W. Biederman)
To: Nadia Derbey <Nadia.Derbey@bull.net>
Cc: Andrew Morton <akpm@osdl.org>, linux-kernel@vger.kernel.org
Subject: Re: [RFC][PATCH 6/6] automatic tuning applied to some kernel components
References: <20070116061516.899460000@bull.net>
	<20070116063030.761795000@bull.net>
	<20070122115638.835b26a1.akpm@osdl.org> <45B61E50.6020607@bull.net>
Date: Wed, 07 Feb 2007 14:18:57 -0700
In-Reply-To: <45B61E50.6020607@bull.net> (Nadia Derbey's message of "Tue, 23
	Jan 2007 15:40:16 +0100")
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Nadia Derbey <Nadia.Derbey@bull.net> writes:

>
> 2) why autotuning:
> There are at least 3 cases where it can be useful
> . for workloads that are known to need a big amount of a given resource type
> (say shared memories), but we don't know what the maximum amount needed will be
> . to solve the case of multiple applications running on a single system, and
> that need the same tunable to be adjusted to feet their needs
> . to make a system correctly react to eventual peak loads for a given resource
> usage, i.e. make it tune up *and down* as needed.

>
> In all these cases, the akt framework will enable the kernel to adapt to
> increasing / decreasing resource consumption:
> 1) avoid allocating "a priori" a big amount of memory that will be used only in
> extreme cases. This is the effect of doing an "echo <huge_value>
>> /proc/sys/kernel/shmmni"
>
> 2) the system will come back to the default values as soon as the peak load is
> over.

At least the ipc ones are supposed to be DOS limits not behavior
modifiers.  I do admit from looking at the code that there are some
consequences of increasing things like shmmni.  However I think we
would be better off with  better data structures and implementations
that remove these consequences than this autotuning of
denial-of-service limits.

i.e. I think you are treating the symptom not the problem.

Does this make sense?

Eric

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