Messages in this thread | | | From | ndrw <> | Subject | Re: Let's talk about the elephant in the room - the Linux kernel's inability to gracefully handle low memory pressure | Date | Thu, 8 Aug 2019 22:59:32 +0100 |
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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
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