Hi Michal, Thank you for spending your time on this. On Tue, Aug 27, 2019 at 08:16:06AM +0200, Michal Hocko wrote: > On Tue 27-08-19 02:14:20, Bharath Vedartham wrote: > > Hi Michal, > > > > Here are some of my thoughts, > > On Wed, Aug 21, 2019 at 04:06:32PM +0200, Michal Hocko wrote: > > > On Thu 15-08-19 14:51:04, Khalid Aziz wrote: > > > > Hi Michal, > > > > > > > > The smarts for tuning these knobs can be implemented in userspace and > > > > more knobs added to allow for what is missing today, but we get back to > > > > the same issue as before. That does nothing to make kernel self-tuning > > > > and adds possibly even more knobs to userspace. Something so fundamental > > > > to kernel memory management as making free pages available when they are > > > > needed really should be taken care of in the kernel itself. Moving it to > > > > userspace just means the kernel is hobbled unless one installs and tunes > > > > a userspace package correctly. > > > > > > From my past experience the existing autotunig works mostly ok for a > > > vast variety of workloads. A more clever tuning is possible and people > > > are doing that already. Especially for cases when the machine is heavily > > > overcommited. There are different ways to achieve that. Your new > > > in-kernel auto tuning would have to be tested on a large variety of > > > workloads to be proven and riskless. So I am quite skeptical to be > > > honest. > > Could you give some references to such works regarding tuning the kernel? > > Talk to Facebook guys and their usage of PSI to control the memory > distribution and OOM situations. Yup. Thanks for the pointer. > > Essentially, Our idea here is to foresee potential memory exhaustion. > > This foreseeing is done by observing the workload, observing the memory > > usage of the workload. Based on this observations, we make a prediction > > whether or not memory exhaustion could occur. > > I understand that and I am not disputing this can be useful. All I do > argue here is that there is unlikely a good "crystall ball" for most/all > workloads that would justify its inclusion into the kernel and that this > is something better done in the userspace where you can experiment and > tune the behavior for a particular workload of your interest. > > Therefore I would like to shift the discussion towards existing APIs and > whether they are suitable for such an advance auto-tuning. I haven't > heard any arguments about missing pieces. I understand your concern here. Just confirming, by APIs you are referring to sysctls, sysfs files and stuff like that right? > > If memory exhaustion > > occurs, we reclaim some more memory. kswapd stops reclaim when > > hwmark is reached. hwmark is usually set to a fairly low percentage of > > total memory, in my system for zone Normal hwmark is 13% of total pages. > > So there is scope for reclaiming more pages to make sure system does not > > suffer from a lack of pages. > > Yes and we have ways to control those watermarks that your monitoring > tool can use to alter the reclaim behavior. Just to confirm here, I am aware of one way which is to alter min_kfree_bytes values. What other ways are there to alter watermarks from user space? > [...] > > > Therefore I would really focus on discussing whether we have sufficient > > > APIs to tune the kernel to do the right thing when needed. That requires > > > to identify gaps in that area. > > One thing that comes to my mind is based on the issue Khalid mentioned > > earlier on how his desktop took more than 30secs to boot up because of > > the caches using up a lot of memory. > > Rather than allowing any unused memory to be the page cache, would it be > > a good idea to fix a size for the caches and elastically change the size > > based on the workload? > > I do not think so. Limiting the pagecache is unlikely to help as it is > really cheap to reclaim most of the time. In those cases when this is > not the case (e.g. the underlying FS needs to flush and/or metadata) > then the same would be possible in a restricted page cache situation > and you could easily end up stalled waiting for pagecache (e.g. any > executable/library) while there is a lot of memory. That makes sense to me. > I cannot comment on the Khalid's example because there were no details > there but I would be really surprised if the primary source of stall was > the pagecache. Should have done more research before talking :) Sorry about that. > -- > Michal Hocko > SUSE Labs
