Re: Please test: workaround to help swapoff behaviour

Marcelo Tosatti
2001-06-08T22:47:37Z

On Thu, 7 Jun 2001, Bulent Abali wrote:

>
>
>
>
> >This is for the people who has been experiencing the lockups while running
> >swapoff.
> >
> >Please test. (against 2.4.6-pre1)
> >
> >
> >--- linux.orig/mm/swapfile.c Wed Jun 6 18:16:45 2001
> >+++ linux/mm/swapfile.c Thu Jun 7 16:06:11 2001
> >@@ -345,6 +345,8 @@
> > /*
> > * Find a swap page in use and read it in.
> > */
> >+ if (current->need_resched)
> >+ schedule();
> > swap_device_lock(si);
> > for (i = 1; i < si->max ; i++) {
> > if (si->swap_map[i] > 0 && si->swap_map[i] != SWAP_MAP_BAD)
> {
>
>
> I tested your patch against 2.4.5. It works. No more lockups. Without
> the
> patch it took 14 minutes 51 seconds to complete swapoff (this is to recover
> 1.5GB of
> swap space). During this time the system was frozen. No keyboard, no
> screen, etc. Practically locked-up.
>
> With the patch there are no more lockups. Swapoff kept running in the
> background.
> This is a winner.
>
> But here is the caveat: swapoff keeps burning 100% of the cycles until it
> completes.
> This is not going to be a big deal during shutdowns. Only when you enter
> swapoff from
> the command line it is going to be a problem.
>
> I looked at try_to_unuse in swapfile.c. I believe that the algorithm is
> broken.
> For each and every swap entry it is walking the entire process list
> (for_each_task(p)). It is also grabbing a whole bunch of locks
> for each swap entry. It might be worthwhile processing swap entries in
> batches instead of one entry at a time.
>
> In any case, I think having this patch is worthwhile as a quick and dirty
> remedy.

Bulent,

Could you please check if 2.4.6-pre2+the schedule patch has better
swapoff behaviour for you?

Thanks