Re: git: 636592343c3e - main - tmpfs: increase memory reserve to a percent of available memory + swap

On 21 Jan 2024, at 21:48, Alexey Dokuchaev wrote:

> On Mon, Jan 22, 2024 at 03:27:57AM +0000, Jessica Clarke wrote:
>> On 19 Dec 2023, at 15:34, Mike Karels <karels@FreeBSD.org> wrote:
>>> commit 636592343c3ec0feb61a4d8043676381384420dd
>>>
>>>    tmpfs: increase memory reserve to a percent of available memory + swap
>>>
>>>    [...]
>>>
>>>    Add sysctl for vfs.tmpfs.memory_percent and the pre-existing
>>>    vfs.tmpfs.memory_reserved to tmpfs(5).
>>>
>>>    PR:             275436
>>>    MFC after:      1 month
>>>    Reviewed by:    rgrimes
>>>    Differential Revision:  https://reviews.freebsd.org/D43011
>>
>> I just got:
>>
>>   make[6]: Could not create temporary file /tmp/makeDl4i9S:
>>   No space left on device
>>
>> after doing a buildworld -j4 on a 4 core, 16 GiB system where /tmp is a
>> tmpfs, with the prior buildworld having taken me past this commit,
>> which I strongly suspect to be the cause. Whilst I understand the
>> motivation behind your commits, this is a sad regression to have; it's
>> not exactly a particularly strange situation.

Bug 276402 is about the interaction of this change with ZFS ARC, which
seems to be problematical.  I spent some time investigating yesterday.
Depending on the dynamics, I get different results.  If tmpfs grows,
the ARC will be reduced if needed.  But if the ARC grows to the point
that tmpfs sets free space to 0 or quite low, attempts to write to
tmpfs fail without any reduction in ARC.

Jess, two quesions:

1. Are you using ZFS on this system?

2. Can you try with vfs.tmpfs.memory_percent set to 100?

> FWIW, I've seen two people complain about this last week, apparently
> this kernel OOM protection doesn't work very well. :-/

So far, I have only seen OOM kills when memory + swap are quite short,
with a tmpfs memory_percent above 95.  But with ZFS, it can happen
when the ARC could have been reduced.

I will investigate more today.  If I don't find a way to resolve this,
I'll probably commit a change to set vfs.tmpfs.memory_percent to 100
by default, at least for now, and if that works around the problem.

		Mike