On Wed 25-01-17 11:03:53, hejianet wrote:

On 25/01/2017 12:54 AM, Michal Hocko wrote:

quoted

On Tue 24-01-17 15:49:03, Jia He wrote:

quoted

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.
 

quoted

quoted

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?

quoted

quoted

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

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