Re: [RFC] Reproducible OOM with just a few sleeps

Simon Jeons
2013-02-17T09:11:05Z

On 01/14/2013 11:00 PM, Dave Hansen wrote:
> On 01/11/2013 07:31 PM, paul.szabo@sydney.edu.au wrote:
>> Seems that any i386 PAE machine will go OOM just by running a few
>> processes. To reproduce:
>> sh -c 'n=0; while [ $n -lt 19999 ]; do sleep 600 & ((n=n+1)); done'
>> My machine has 64GB RAM. With previous OOM episodes, it seemed that
>> running (booting) it with mem=32G might avoid OOM; but an OOM was
>> obtained just the same, and also with lower memory:
>> Memory sleeps to OOM free shows total
>> (mem=64G) 5300 64447796
>> mem=32G 10200 31155512
>> mem=16G 13400 14509364
>> mem=8G 14200 6186296
>> mem=6G 15200 4105532
>> mem=4G 16400 2041364
>> The machine does not run out of highmem, nor does it use any swap.
> I think what you're seeing here is that, as the amount of total memory
> increases, the amount of lowmem available _decreases_ due to inflation
> of mem_map[] (and a few other more minor things). The number of sleeps

So if he config sparse memory, the issue can be solved I think.

> you can do is bound by the number of processes, as you noticed from
> ulimit. Creating processes that don't use much memory eats a relatively
> large amount of low memory.
>
> This is a sad (and counterintuitive) fact: more RAM actually *CREATES*
> RAM bottlenecks on 32-bit systems.
>
>> On my large machine, 'free' fails to show about 2GB memory, e.g. with
>> mem=16G it shows:
>>
>> root@zeno:~# free -l
>> total used free shared buffers cached
>> Mem: 14509364 435440 14073924 0 4068 111328
>> Low: 769044 120232 648812
>> High: 13740320 315208 13425112
>> -/+ buffers/cache: 320044 14189320
>> Swap: 134217724 0 134217724
> You probably have a memory hole. mem=16G means "give me all the memory
> below the physical address at 16GB". It does *NOT* mean, "give me
> enough memory such that 'free' will show ~16G available." If you have a
> 1.5GB hole below 16GB, and you do mem=16G, you'll end up with ~14.5GB
> available.
>
> The e820 map (during early boot in dmesg) or /proc/iomem will let you
> locate your memory holes.

Dear Dave, two questions here:

1) e820 map is read from BIOS, correct? So if all kinds of ranges dump
from /proc/iomem are setup by BIOS?
2) only "System RAM" range dump from /proc/iomem can be treated as real
memory, all other ranges can be treated as holes, correct?

>
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