linux-kernel - Re: [RFC] mm:prototype for the updated swapoff implementation

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Date:	Wed, 19 Feb 2014 16:39:47 -0500
From:	Rik van Riel <riel@...riel.com>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Kelley Nielsen <kelleynnn@...il.com>
CC:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	opw-kernel@...glegroups.com, jamieliu@...gle.com,
	sjenning@...ux.vnet.ibm.com, Hugh Dickins <hughd@...gle.com>
Subject: Re: [RFC] mm:prototype for the updated swapoff implementation

On 02/19/2014 04:27 PM, Andrew Morton wrote:
> On Tue, 18 Feb 2014 16:35:22 -0800 Kelley Nielsen <kelleynnn@...il.com> wrote:
> 
>> The function try_to_unuse() is of quadratic complexity, with a lot of
>> wasted effort. It unuses swap entries one by one, potentially iterating
>> over all the page tables for all the processes in the system for each
>> one.
>>
>> This new proposed implementation of try_to_unuse simplifies its
>> complexity to linear. It iterates over the system's mms once, unusing
>> all the affected entries as it walks each set of page tables. It also
>> makes similar changes to shmem_unuse.
>>
>> Improvement
>>
>> swapoff was called on a swap partition containing about 50M of data,
>> and calls to the function unuse_pte_range were counted.
>>
>> Present implementation....about 22.5M calls.
>> Prototype.................about  7.0K   calls.
> 
> Do you have situations in which swapoff is taking an unacceptable
> amount of time?  If so, please update the changelog to provide full
> details on this, with before-and-after timing measurements.

I have seen plenty of that.  With just a few GB in swap space in
use, on a system with 24GB of RAM, and about a dozen GB in use
by various processes, I have seen swapoff take several hours of
CPU time.

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