Is maximum swap usage tunable?

[bob prohaska]

On Fri, Mar 02, 2018 at 10:04:56AM -0700, Warner Losh wrote:
> On Fri, Mar 2, 2018 at 9:19 AM, Mike <the.lists at mgm51.com> wrote:
> 
> >
> > buildworld is still running.  I started it around 3PM (EST) on March 1.
> >
> > I'm seeing a lot of swap messages on the console.  The text is:
> >
> >   swap_pager: indefinite wait buffer: bufobj: 0,
> >      blkno: [varies], size: [varies]
> 
> 
> This message comes from the swap pager when the I/O doesn't complete
> quickly enough. It has nothing to do with sizes of the swap space.
> 
> I've seen three causes for this: (1) The driver going crazy and not
> completing I/Os it gets notified as being done; (2) The driver not having a
> backstop timer on I/Os to fail them with timeout; (3) timeouts not firing
> for the driver to fail the I/O up the stack. (4) when a drive suddenly slow
> way way down (~100x or more slower), the I/O queues balloons and you can
> get these.
> 

Is there some sort of experiment which can distinguish hardware delays
from software delays? For example, would logging gstat output shed any
light? 

Thanks for reading,

bob prohaska