Re: [PATCH RFC 2/3] mm, vmscan: limit kswapd loop if no progress is made

[Michal Hocko]

On Wed 25-01-17 11:03:53, hejianet wrote:
> 
> 
> On 25/01/2017 12:54 AM, Michal Hocko wrote:
> > On Tue 24-01-17 15:49:03, Jia He wrote:
> > > Currently there is no hard limitation for kswapd retry times if no progress
> > > is made.
> > 
> > Yes, because the main objective of the kswapd is to balance all memory
> > zones. So having a hard limit on retries doesn't make any sense.
> > 
> But do you think even when there is no any process, kswapd still need
> to run and take the cpu usage uselessly?

The question is whether we can get into such a state during reasonable
workloads. So far you haven't explained what you are seeing and on which
kernel version.
 
> > > Then kswapd will take 100% for a long time.
> > 
> > Where it is spending time?
> I've watched kswapd takes 100% cpu for a whole night.

I assume it didn't get to sleep because your request has consumed enough
memory for hugetlb pages to get below watermarks which would keep kswapd
active. Is that correct?

> > > In my test, I tried to allocate 4000 hugepages by:
> > > echo 4000 > /proc/sys/vm/nr_hugepages
> > > 
> > > Then,kswapd will take 100% cpu for a long time.
> > > 
> > > The numa layout is:
> > > available: 7 nodes (0-6)
> > > node 0 cpus: 0 1 2 3 4 5 6 7
> > > node 0 size: 6611 MB
> > > node 0 free: 1103 MB
> > > node 1 cpus:
> > > node 1 size: 12527 MB
> > > node 1 free: 8477 MB
> > > node 2 cpus:
> > > node 2 size: 15087 MB
> > > node 2 free: 11037 MB
> > > node 3 cpus:
> > > node 3 size: 16111 MB
> > > node 3 free: 12060 MB
> > > node 4 cpus: 8 9 10 11 12 13 14 15
> > > node 4 size: 24815 MB
> > > node 4 free: 20704 MB
> > > node 5 cpus:
> > > node 5 size: 4095 MB
> > > node 5 free: 61 MB
> > > node 6 cpus:
> > > node 6 size: 22750 MB
> > > node 6 free: 18716 MB
> > > 
> > > The cause is kswapd will loop for long time even if there is no progress in
> > > balance_pgdat.
> > 
> > How does this solve anything? If the kswapd just backs off then the more
> > work has to be done in the direct reclaim context.
> What if there is still no progress in direct context?

Then we trigger the OOM killer when applicable.
-- 
Michal Hocko
SUSE Labs