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

[Vlastimil Babka <vbabka@suse.cz>]

On 8/6/19 3:08 AM, Suren Baghdasaryan wrote:
>> @@ -1280,3 +1285,50 @@ static int __init psi_proc_init(void)
>>         return 0;
>>  }
>>  module_init(psi_proc_init);
>> +
>> +#define OOM_PRESSURE_LEVEL     80
>> +#define OOM_PRESSURE_PERIOD    (10 * NSEC_PER_SEC)
> 
> 80% of the last 10 seconds spent in full stall would definitely be a
> problem. If the system was already low on memory (which it probably
> is, or we would not be reclaiming so hard and registering such a big
> stall) then oom-killer would probably kill something before 8 seconds
> are passed.

If oom killer can act faster, than great! On small embedded systems you probably
don't enable PSI anyway?

> If my line of thinking is correct, then do we really
> benefit from such additional protection mechanism? I might be wrong
> here because my experience is limited to embedded systems with
> relatively small amounts of memory.

Well, Artem in his original mail describes a minutes long stall. Things are
really different on a fast desktop/laptop with SSD. I have experienced this as
well, ending up performing manual OOM by alt-sysrq-f (then I put more RAM than
8GB in the laptop). IMHO the default limit should be set so that the user
doesn't do that manual OOM (or hard reboot) before the mechanism kicks in. 10
seconds should be fine.