Re: Kernel 2.6.8.1: swap storm of death - nr_requests > 1024 on swap partition

Andrew Morton
2004-08-29T20:59:17Z

Jens Axboe <axboe@suse.de> wrote:
>
> > That was my point.
>
> I didn't understand your message at all, maybe that wasn't clear enough
> in my email :-). You state that the main effect of that particular patch
> is to bump nr_requests to 8192, which is definitely not true. The main
> effect of the patch is to make sure that ->nr_requests was valid, so
> that cfqd->max_queued is valid. ->nr_requests was always overwritten
> with 8192 for quite some time, irregardless of that patch. So this
> particular change has nothing to do with that, and other io schedulers
> will experience exactly this very problem with 8192 requests.
>
> Why you do see a difference is that when ->max_queued isn't valid, you
> end up block a lot more in get_request_wait() because cfq_may_queue will
> disallow you to queue a lot more than with the patch. Since other io
> schedulers don't have these sort of checks, they behave like CFQ does
> with the bug in blk_init_queue() fixed.

The changlog wasn't that detailed ;)

But yes, it's the large nr_requests which is tripping up swapout. I'm
assuming that when a process exits with its anonymous memory still under
swap I/O we're forgetting to actually free the pages when the I/O
completes. So we end up with a ton of zero-ref swapcache pages on the LRU.

I assume. Something odd's happening, that's for sure.