Nick Piggin wrote:

OK, this is where I start to worry. Swap prefetch AFAIKS doesn't fix
the updatedb problem very well, because if updatedb has caused swapout
then it has filled memory, and swap prefetch doesn't run unless there
is free memory (not to mention that updatedb would have paged out other
files as well).

It is *not* about updatedb. That is just a trivial case which people 
notice. Therefore fixing updatedb to be nicer, as was discussed at 
various points in this thread, is *not* the solution.
Most users are also *not*at*all* interested in kernel builds as a metric 
of system performance.

When I'm at work, I run a large, commercial, engineering application. 
While running, it takes most of the system memory (4GB and up), and it 
reads and writes very large files. Swap prefetch noticeably helps my 
desktop too. Can I measure it? Not sure. Can people on lkml fix the 
application? Certainly not.

Frank