Re: [PATCH] remove throttle_vm_writeout()

Peter Zijlstra
2007-10-04T18:15:45Z

On Thu, 2007-10-04 at 10:46 -0700, Andrew Morton wrote:
> On Thu, 04 Oct 2007 18:47:07 +0200 Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:

> > static int may_write_to_queue(struct backing_dev_info *bdi)
> > {
> > if (current->flags & PF_SWAPWRITE)
> > return 1;
> > if (!bdi_write_congested(bdi))
> > return 1;
> > if (bdi == current->backing_dev_info)
> > return 1;
> > return 0;
> > }
> >
> > Which will write to congested queues. Anybody know why?

OK, I guess I could have found that :-/

> commit c4e2d7ddde9693a4c05da7afd485db02c27a7a09
> Author: akpm <akpm>
> Date: Sun Dec 22 01:07:33 2002 +0000
>
> [PATCH] Give kswapd writeback higher priority than pdflush
>
> The `low latency page reclaim' design works by preventing page
> allocators from blocking on request queues (and by preventing them from
> blocking against writeback of individual pages, but that is immaterial
> here).
>
> This has a problem under some situations. pdflush (or a write(2)
> caller) could be saturating the queue with highmem pages. This
> prevents anyone from writing back ZONE_NORMAL pages. We end up doing
> enormous amounts of scenning.
>
> A test case is to mmap(MAP_SHARED) almost all of a 4G machine's memory,
> then kill the mmapping applications. The machine instantly goes from
> 0% of memory dirty to 95% or more.

With dirty page tracking this is not supposed to happen anymore.

> pdflush kicks in and starts writing
> the least-recently-dirtied pages, which are all highmem.

with highmem >> normal, and user pages preferring highmem, this will
likely still be true.

> The queue is
> congested so nobody will write back ZONE_NORMAL pages. kswapd chews
> 50% of the CPU scanning past dirty ZONE_NORMAL pages and page reclaim
> efficiency (pages_reclaimed/pages_scanned) falls to 2%.

So, the problem is a heavy writer vs swap. Which is still possible.

> So this patch changes the policy for kswapd. kswapd may use all of a
> request queue, and is prepared to block on request queues.

So request queue's have a limit above the congestion level on which they
will block?

NFS doesn't have that AFAIK

> What will now happen in the above scenario is:
>
> 1: The page alloctor scans some pages, fails to reclaim enough
> memory and takes a nap in blk_congetion_wait().
>
> 2: kswapd() will scan the ZONE_NORMAL LRU and will start writing
> back pages. (These pages will be rotated to the tail of the
> inactive list at IO-completion interrupt time).
>
> This writeback will saturate the queue with ZONE_NORMAL pages.
> Conveniently, pdflush will avoid the congested queues. So we end up
> writing the correct pages.
>
> In this test, kswapd CPU utilisation falls from 50% to 2%, page reclaim
> efficiency rises from 2% to 40% and things are generally a lot happier.
>
>
> The downside is that kswapd may now do a lot less page reclaim,
> increasing page allocation latency, causing more direct reclaim,
> increasing lock contention in the VM, etc. But I have not been able to
> demonstrate that in testing.
>
>
> The other problem is that there is only one kswapd, and there are lots
> of disks. That is a generic problem - without being able to co-opt
> user processes we don't have enough threads to keep lots of disks saturated.
>
> One fix for this would be to add an additional "really congested"
> threshold in the request queues, so kswapd can still perform
> nonblocking writeout. This gives kswapd priority over pdflush while
> allowing kswapd to feed many disk queues. I doubt if this will be
> called for.

I could do that.