> Hi,
>
> I've spent some time trying to understand why swapoff is such a slow
> operation.
>
> My experiments show that when there is not much free physical memory,
> swapoff moves pages out of swap at a rate of approximately 5mb/sec. When
> there is a lot of free physical memory, it is faster but still a slow
> CPU-intensive operation, purging swap at about 20mb/sec.
>
> I've read into the swap code and I have some understanding that this is
> an expensive operation (and has to be). This page was very helpful and
> also agrees:
>
http://kernel.org/doc/gorman/html/understand/understand014.html
>
> After reading that, I have an idea for a possible optimization. If we
> were to create a system call to disable ALL swap partitions (or modify
> the existing one to accept NULL for that purpose), could this process be
> signficantly less complex?
>
> I'm thinking we could do something like this:
> 1. Prevent any more pages from being swapped out from this point
> 2. Iterate through all process page tables, paging all swapped
> pages back into physical memory and updating PTEs
> 3. Clear all swap tables and caches
>
> Due to only iterating through process page tables once, does this sound
> like it would increase performance non-trivially? Is it feasible?
>
> I'm happy to spend a few more hours looking into implementing this but
> would greatly appreciate any advice from those in-the-know on if my
> ideas are broken to start with...
