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
