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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