Re: Let's talk about the elephant in the room - the Linux kernel's inability to gracefully handle low memory pressure

[ndrw]

On 08/08/2019 19:59, Michal Hocko wrote:
> Well, I am afraid that implementing anything like that in the kernel
> will lead to many regressions and bug reports. People tend to have very
> different opinions on when it is suitable to kill a potentially
> important part of a workload just because memory gets low.

Are you proposing having a zero memory reserve or not having such option 
at all? I'm fine with the current default (zero reserve/margin).

I strongly prefer forcing OOM killer when the system is still running 
normally. Not just for preventing stalls: in my limited testing I found 
the OOM killer on a stalled system rather inaccurate, occasionally 
killing system services etc. I had much better experience with earlyoom.

> LRU aspect doesn't help much, really. If we are reclaiming the same set
> of pages becuase they are needed for the workload to operate then we are
> effectivelly treshing no matter what kind of replacement policy you are
> going to use.

In my case it would work fine (my system already works well with 
earlyoom, and without it it remains responsive until last couple hundred 
MB of RAM).


> > > PSI is giving you a matric that tells you how much time you
> > > spend on the memory reclaim. So you can start watching the system from
> > > lower utilization already.

I've tested it on a system with 45GB of RAM, SSD, swap disabled (my 
intention was to approximate a worst-case scenario) and it didn't really 
detect stall before it happened. I can see some activity after reaching 
~42GB, the system remains fully responsive until it suddenly freezes and 
requires sysrq-f. PSI appears to increase a bit when the system is about 
to run out of memory but the change is so small it would be difficult to 
set a reliable threshold. I expect the PSI numbers to increase 
significantly after the stall (I wasn't able to capture them) but, as 
mentioned above, I was hoping for a solution that would work before the 
stall.

$ while true; do sleep 1; cat /proc/pressure/memory ; done
[starting a test script and waiting for several minutes to fill up memory]
some avg10=0.00 avg60=0.00 avg300=0.00 total=0
full avg10=0.00 avg60=0.00 avg300=0.00 total=0
some avg10=0.00 avg60=0.00 avg300=0.00 total=10389
full avg10=0.00 avg60=0.00 avg300=0.00 total=6442
some avg10=0.00 avg60=0.00 avg300=0.00 total=18950
full avg10=0.00 avg60=0.00 avg300=0.00 total=11576
some avg10=0.00 avg60=0.00 avg300=0.00 total=25655
full avg10=0.00 avg60=0.00 avg300=0.00 total=16159
some avg10=0.00 avg60=0.00 avg300=0.00 total=31438
full avg10=0.00 avg60=0.00 avg300=0.00 total=19552
some avg10=0.00 avg60=0.00 avg300=0.00 total=44549
full avg10=0.00 avg60=0.00 avg300=0.00 total=27772
some avg10=0.00 avg60=0.00 avg300=0.00 total=52520
full avg10=0.00 avg60=0.00 avg300=0.00 total=32580
some avg10=0.00 avg60=0.00 avg300=0.00 total=60451
full avg10=0.00 avg60=0.00 avg300=0.00 total=37704
some avg10=0.00 avg60=0.00 avg300=0.00 total=68986
full avg10=0.00 avg60=0.00 avg300=0.00 total=42859
some avg10=0.00 avg60=0.00 avg300=0.00 total=76598
full avg10=0.00 avg60=0.00 avg300=0.00 total=48370
some avg10=0.00 avg60=0.00 avg300=0.00 total=83080
full avg10=0.00 avg60=0.00 avg300=0.00 total=52930
some avg10=0.00 avg60=0.00 avg300=0.00 total=89384
full avg10=0.00 avg60=0.00 avg300=0.00 total=56350
some avg10=0.00 avg60=0.00 avg300=0.00 total=95293
full avg10=0.00 avg60=0.00 avg300=0.00 total=60260
some avg10=0.00 avg60=0.00 avg300=0.00 total=101566
full avg10=0.00 avg60=0.00 avg300=0.00 total=64408
some avg10=0.00 avg60=0.00 avg300=0.00 total=108131
full avg10=0.00 avg60=0.00 avg300=0.00 total=68412
some avg10=0.00 avg60=0.00 avg300=0.00 total=121932
full avg10=0.00 avg60=0.00 avg300=0.00 total=77413
some avg10=0.00 avg60=0.00 avg300=0.00 total=140807
full avg10=0.00 avg60=0.00 avg300=0.00 total=91269
some avg10=0.00 avg60=0.00 avg300=0.00 total=170494
full avg10=0.00 avg60=0.00 avg300=0.00 total=110611
[stall, sysrq-f]

Best regards,

ndrw