linux-kernel - Re: [PATCH] swap: send callback when swap slot is freed

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Date:	Thu, 13 Aug 2009 08:11:30 +0530
From:	Nitin Gupta <ngupta@...are.org>
To:	Hugh Dickins <hugh.dickins@...cali.co.uk>
CC:	Matthew Wilcox <willy@...ux.intel.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] swap: send callback when swap slot is freed

On 08/13/2009 04:18 AM, Hugh Dickins wrote:
> On Wed, 12 Aug 2009, Nitin Gupta wrote:
>
>> Currently, we have "swap discard" mechanism which sends a discard bio request
>> when we find a free cluster during scan_swap_map(). This callback can come a
>> long time after swap slots are actually freed.
>>
>> This delay in callback is a great problem when (compressed) RAM [1] is used
>> as a swap device. So, this change adds a callback which is called as
>> soon as a swap slot becomes free. For above mentioned case of swapping
>> over compressed RAM device, this is very useful since we can immediately
>> free memory allocated for this swap page.
>>
>> This callback does not replace swap discard support. It is called with
>> swap_lock held, so it is meant to trigger action that finishes quickly.
>> However, swap discard is an I/O request and can be used for taking longer
>> actions.
>>
>> Links:
>> [1] http://code.google.com/p/compcache/
>
> Please keep this with compcache for the moment (it has no other users).
>

Oh, I missed this one.

This small patch can be considered as first step for merging compcache 
to mainline :)   Actually, it requires callbacks for swapon, swapoff too 
but that, I think, should be done in a separate patches.

BTW, last time compcache was not accepted due to lack of performance 
numbers. Now the project has lot more data for various cases:
http://code.google.com/p/compcache/wiki/Performance
Still need to collect data for worst-case behaviors and such...


Thanks,
Nitin
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