linux-kernel - Re: [PATCH 1/2] [mmotm] Add notifiers for various swap events

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Date:	Mon, 28 Dec 2009 20:23:25 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Pekka Enberg <penberg@...helsinki.fi>
Cc:	Andi Kleen <andi@...stfloor.org>, Nitin Gupta <ngupta@...are.org>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] [mmotm] Add notifiers for various swap events

>>>> The first question to ask is if compressed swap is worth
>>>> it. Do you have benchmark numbers showing it to be an improvement?
>>>> Are there cases where it is slower than uncompressed swap?
>>>>
>>> http://code.google.com/p/compcache/wiki/Performance
>>
>> That should be included in the changelog of the patches.
>
> Which patches? The driver is already in staging and there's a pointer to 
> the home page in

How can it be in staging if there are no hooks for it yet?

>>> ramzswap is an optional module.
>>
>> I have some doubts on the wisdom of making swap algorithms modular.
>> Better compile them in. Then you don't need messy notifiers either.
>
> What's so messy about them? The whole point of having the notifiers is to 
> avoid CONFIG_RAMZSWAP in core kernel code...

They make the code much harder to read and follow. When you try to follow
the code flow and you find a notifier it's always a complicated operation
to figure out what code will end up being called.

Sometimes they are needed, but they have a high cost in maintainability.

-Andi
-- 
ak@...ux.intel.com -- Speaking for myself only.
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