On Sat 13-08-16 13:34:29, Mikulas Patocka wrote:

On Fri, 12 Aug 2016, Michal Hocko wrote:

quoted

On Thu 04-08-16 14:49:41, Mikulas Patocka wrote:

quoted

On Wed, 3 Aug 2016, Michal Hocko wrote:

quoted

But the device congestion is not the only condition required for the
throttling. The pgdat has also be marked congested which means that the
LRU page scanner bumped into dirty/writeback/pg_reclaim pages at the
tail of the LRU. That should only happen if we are rotating LRUs too
quickly. AFAIU the reclaim shouldn't allow free ticket scanning in that
situation.

The obvious problem here is that mempool allocations should sleep in 
mempool_alloc() on &pool->wait (until someone returns some entries into 
the mempool), they should not sleep inside the page allocator.

I agree that mempool_alloc should _primarily_ sleep on their own
throttling mechanism. I am not questioning that. I am just saying that
the page allocator has its own throttling which it relies on and that
cannot be just ignored because that might have other undesirable side
effects. So if the right approach is really to never throttle certain
requests then we have to bail out from a congested nodes/zones as soon
as the congestion is detected.

Now, I would like to see that something like that is _really_ necessary.

Currently, it is not a problem - device mapper reports the device as 
congested only if the underlying physical disks are congested.

But once we change it so that device mapper reports congested state on its 
own (when it has too many bios in progress), this starts being a problem.

OK, can we wait until it starts becoming a real problem and solve it
appropriately then?

I will repost the patch which removes thottle_vm_pageout in the meantime
as it doesn't seem to be needed anymore.

-- 
Michal Hocko
SUSE Labs

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