On Sat, Mar 12, 2016 at 09:38:35AM +0100, Gary Jennejohn wrote:
> In the course of the last year or so the behavior of the vm system
> has changed in regard to how aggressively Inact memory is recycled.
> My box has 8GB of memory.  At the moment I'm copying 100s of gigabytes
> from one file system to another one.

How exactly are you copying them? How large are the files you're
copying? Which filesystems are in use?

> Looking at top I observe that there are about 6GB of Inact memory.
> This value hardly changes.  Instead of aggressively recycling the
> Inact memory the vm now seems to prefer to swap.

The VM will swap a small number of dirty pages as it encounters them
during inactive queue scans. If the system is swapping more than it used
to, it's presumably because the pagedaemon is encountering more dirty
pages in the inactive queue than it used to. This could simply be the
result of external factors (e.g., the applications you're running are
generating more dirty pages than they were a year ago for some reason).
On the other hand, some of the changes to remove object page cache uses
could cause this: cache pages would be reused in preference to inactive
pages, so if pages that were previously cached are now being enqueued at
the end of the inactive queue, I'd expect to see more swapping than
before. For example, with r281079+r286255, we deactivate pages that
precede a faulted page rather than caching them. I think this would
result in more churn of the inactive queue, which could lead to
increased swap usage. cp(1), for instance, will mmap small source files,
so the above-mentioned changes might be relevant if you're copying many
small files that aren't already resident in memory. But I think you
need to be more specific about your setup.

> Last year, can't rmember excatly when, the behavior was totally
> different.  The vm very aggessively recycled Inact memory and,
> even when copying 100s of GB of files, the system hardly swapped.
> It seems rather strange to me that the vm happily allows gigbytes
> of Inact memory to be present and prefers swapping to recyclincg.
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