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SubjectRe: Let's talk about the elephant in the room - the Linux kernel's inability to gracefully handle low memory pressure
On Mon 05-08-19 14:55:42, Johannes Weiner wrote:
> On Mon, Aug 05, 2019 at 03:31:19PM +0200, Michal Hocko wrote:
> > On Mon 05-08-19 14:13:16, Vlastimil Babka wrote:
> > > On 8/4/19 11:23 AM, Artem S. Tashkinov wrote:
> > > > Hello,
> > > >
> > > > There's this bug which has been bugging many people for many years
> > > > already and which is reproducible in less than a few minutes under the
> > > > latest and greatest kernel, 5.2.6. All the kernel parameters are set to
> > > > defaults.
> > > >
> > > > Steps to reproduce:
> > > >
> > > > 1) Boot with mem=4G
> > > > 2) Disable swap to make everything faster (sudo swapoff -a)
> > > > 3) Launch a web browser, e.g. Chrome/Chromium or/and Firefox
> > > > 4) Start opening tabs in either of them and watch your free RAM decrease
> > > >
> > > > Once you hit a situation when opening a new tab requires more RAM than
> > > > is currently available, the system will stall hard. You will barely be
> > > > able to move the mouse pointer. Your disk LED will be flashing
> > > > incessantly (I'm not entirely sure why). You will not be able to run new
> > > > applications or close currently running ones.
> > >
> > > > This little crisis may continue for minutes or even longer. I think
> > > > that's not how the system should behave in this situation. I believe
> > > > something must be done about that to avoid this stall.
> > >
> > > Yeah that's a known problem, made worse SSD's in fact, as they are able
> > > to keep refaulting the last remaining file pages fast enough, so there
> > > is still apparent progress in reclaim and OOM doesn't kick in.
> > >
> > > At this point, the likely solution will be probably based on pressure
> > > stall monitoring (PSI). I don't know how far we are from a built-in
> > > monitor with reasonable defaults for a desktop workload, so CCing
> > > relevant folks.
> >
> > Another potential approach would be to consider the refault information
> > we have already for file backed pages. Once we start reclaiming only
> > workingset pages then we should be trashing, right? It cannot be as
> > precise as the cost model which can be defined around PSI but it might
> > give us at least a fallback measure.
>
> NAK, this does *not* work. Not even as fallback.
>
> There is no amount of refaults for which you can say whether they are
> a problem or not. It depends on the disk speed (obvious) but also on
> the workload's memory access patterns (somewhat less obvious).
>
> For example, we have workloads whose cache set doesn't quite fit into
> memory, but everything else is pretty much statically allocated and it
> rarely touches any new or one-off filesystem data. So there is always
> a steady rate of mostly uninterrupted refaults, however, most data
> accesses are hitting the cache! And we have fast SSDs that compensate
> for the refaults that do occur. The workload runs *completely fine*.

OK, thanks for this example. I can see how a constant working set
refault can work properly if the rate is slower than the overal IO
plus the allocation demand for other purpose.

Thanks!
--
Michal Hocko
SUSE Labs

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