Re: Swap defragging

[Simon Jeons <simon.jeons@gmail.com>]

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.

One question here:

The comments of setup_swap_extents:
An ordered list of swap extents is built at swapon time and is then used 
at swap_writepage/swap_readpage tiem for locating where on disk a page 
belongs.
But I didn't see any handle of swap extents in 
swap_writepage/swap_readpage, why?

>
> 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
> other reason is that if a page shared among multiple threads is
> swapped out, it can not be removed from swap until all threads have
> faulted the page back in because of page table entries still referring
> to the swap slot on disk.  In a multi-threaded application, this is
> rather unlikely.
>
> So even though the referencing order of the application might change,
> the disk layout won't.  But adjusting the disk layout speculatively
> increases disk IO, so it could be hard to prove that you came up with
> a net improvement.
>
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