Re: swapping and the value of /proc/sys/vm/swappiness

William Lee Irwin III
2004-09-06T22:48:40Z

On Mon, Sep 06, 2004 at 02:11:29PM -0500, Ray Bryant wrote:
> What is unexpected is that the amount of swap space used at a particular
> swappiness setting varies dramatically with the kernel version being
> tested, in spite of the fact that the basic swap_tendency calculation in
> refile_ianctive_zone() is unchanged. (Other, subtle changes in the vm as a
> whole and this routine in particular clearly effect the impact of that
> computation.)
> For example, at a swappiness value of 0, Kernel 2.6.5 swapped out 0 bytes,
> whereas Kernel 2.6.9-rc1-mm3 swapped out 10 GB. Similarly, most kernels
> have a significant change in behavior for swappiness values near 100, but
> for SLES9 the change point occurs at swappness=60.
> A scan of the change logs for swappiness related changes shows nothing that
> might explain these changes. My question is: "Is this change in behavior
> deliberate, or just a side effect of other changes that were made in the
> vm?" and "What kind of swappiness behavior might I expect to find in future
> kernels?".

IIRC no deliberate /proc/sys/vm/swappiness semantic changes were merged.
The policy tweakers have something to answer for here unless some stats
they rely upon have since been flubbed. Logging periodic snapshots of
/proc/vmstat for these benchmarks may be helpful to implicate specific
statistics' bungling or rule out statistic miscalculation as causes.

-- wli