Re: Swap defragging

[Jaegeuk Hanse <jaegeuk.hanse@gmail.com>]

Hi Johannes,
On 03/13/2013 01:08 AM, Johannes Weiner wrote:
> On Mon, Mar 11, 2013 at 11:16:29AM +0800, Jaegeuk Hanse wrote:
>> Hi Johannes,
>> On 03/08/2013 10:35 AM, Johannes Weiner wrote:
>>> On Thu, Mar 07, 2013 at 06:07:23PM -0800, Raymond Jennings wrote:
>>>> Just a two cent question, but is there any merit to having the kernel
>>>> defragment swap space?
>>> That is a good question.
>>>
>>> Swap does fragment quite a bit, and there are several reasons for
>>> that.
>>>
>>> We swap pages in our LRU list order, but this list is sorted by first
>>> access, not by access frequency (not quite that cookie cutter, but the
>>> ordering is certainly fairly coarse).  This means that the pages may
>>> already be in suboptimal order for swap in at the time of swap out.
>>>
>>> Once written to disk, the layout tends to stick.  One reason is that
>>> we actually try to not free swap slots unless there is a shortage of
>>> swap space to save future swap out IO (grep for vm_swap_full()).  The
>> Since anonymous page will be swap out if it's dirty and the contents
>> of the page and data store in swap area is not equal now, why can
>> avoid future swap out IO?
> Modified pages get written out freshly, but in a multi-threaded
> application, the original page stays put until all threads have
> modified it or faulted it back in.

Sorry, you didn't resolve my confuse! It seems that this is your second 
reason for why disk layout tends to stick. However, what I confuse is 
your first reason. You said that we actually try to not free swap slots 
unless there is a shortage of swap space to save future swap out IO, 
why? Anonymous pages are swapped out since they are dirty, how can don't 
swap out and swap IO?


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