On 01.08.2016 13:26, Michal Hocko wrote:

quoted

sdc, sdd and sde each at max speed, with a little bit of garden variety IO
on sda and sdb.

So do I get it right that the majority of the IO is to those slower USB
disks?  If yes then does lowering the dirty_bytes to something smaller
help?

Yes, the vast majority.

I set dirty_bytes to 128MiB and started a fairly IO and memory intensive 
process and the OOM killer kicked in within a few seconds.

Same with 16MiB dirty_bytes and 1MiB.

Some additional IO load from my fast subsystem is enough:

At 1MiB dirty_bytes,

find /btrfs0/ -type f -exec md5sum {} \;

was enough (where /btrfs0 is on a LVM2 LV and the PV is on sda.) It read 
a few dozen files (random stuff with very mixed file sizes, none very 
big) until the OOM killer kicked in.

I'll try 4.6.


Ralf-Peter


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