Re: 4.8.8 kernel trigger OOM killer repeatedly when I have lots of RAM that should be free

[Marc MERLIN]

Thanks for the reply and suggestions.

On Tue, Nov 29, 2016 at 09:07:03AM -0800, Linus Torvalds wrote:
> On Tue, Nov 29, 2016 at 8:34 AM, Marc MERLIN <marc@xxxxxxxxxxx> wrote:
> > Now, to be fair, this is not a new problem, it's just varying degrees of
> > bad and usually only happens when I do a lot of I/O with btrfs.
> 
> One situation where I've seen something like this happen is
> 
>  (a) lots and lots of dirty data queued up
>  (b) horribly slow storage

In my case, it is a 5x 4TB HDD with 
software raid 5 < bcache < dmcrypt < btrfs
bcache is currently half disabled (as in I removed the actual cache) or
too many bcache requests pile up, and the kernel dies when too many
workqueues have piled up.
I'm just kind of worried that since I'm going through 4 subsystems
before my data can hit disk, that's a lot of memory allocations and
places where data can accumulate and cause bottlenecks if the next
subsystem isn't as fast.

But this shouldn't be "horribly slow", should it? (it does copy a few
terabytes per day, not fast, but not horrible, about 30MB/s or so)

> Sadly, our defaults for "how much dirty data do we allow" are somewhat
> buggered. The global defaults are in "percent of memory", and are
> generally _much_ too high for big-memory machines:
> 
>     [torvalds@i7 linux]$ cat /proc/sys/vm/dirty_ratio
>     20
>     [torvalds@i7 linux]$ cat /proc/sys/vm/dirty_background_ratio
>     10

I can confirm I have the same.

> says that it only starts really throttling writes when you hit 20% of
> all memory used. You don't say how much memory you have in that
> machine, but if it's the same one you talked about earlier, it was
> 24GB. So you can have 4GB of dirty data waiting to be flushed out.

Correct, 24GB and 4GB.

> And we *try* to do this per-device backing-dev congestion thing to
> make things work better, but it generally seems to not work very well.
> Possibly because of inconsistent write speeds (ie _sometimes_ the SSD
> does really well, and we want to open up, and then it shuts down).
> 
> One thing you can try is to just make the global limits much lower. As in
> 
>    echo 2 > /proc/sys/vm/dirty_ratio
>    echo 1 > /proc/sys/vm/dirty_background_ratio

I will give that a shot, thank you.

Marc
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