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SubjectRe: [RFC PATCH 0/2] Add predictive memory reclamation and compaction
On Tue 13-08-19 09:20:51, Khalid Aziz wrote:
> On 8/13/19 8:05 AM, Michal Hocko wrote:
> > On Mon 12-08-19 19:40:10, Khalid Aziz wrote:
> > [...]
> >> Patch 1 adds code to maintain a sliding lookback window of (time, number
> >> of free pages) points which can be updated continuously and adds code to
> >> compute best fit line across these points. It also adds code to use the
> >> best fit lines to determine if kernel must start reclamation or
> >> compaction.
> >>
> >> Patch 2 adds code to collect data points on free pages of various orders
> >> at different points in time, uses code in patch 1 to update sliding
> >> lookback window with these points and kicks off reclamation or
> >> compaction based upon the results it gets.
> >
> > An important piece of information missing in your description is why
> > do we need to keep that logic in the kernel. In other words, we have
> > the background reclaim that acts on a wmark range and those are tunable
> > from the userspace. The primary point of this background reclaim is to
> > keep balance and prevent from direct reclaim. Why cannot you implement
> > this or any other dynamic trend watching watchdog and tune watermarks
> > accordingly? Something similar applies to kcompactd although we might be
> > lacking a good interface.
> >
> Hi Michal,
> That is a very good question. As a matter of fact the initial prototype
> to assess the feasibility of this approach was written in userspace for
> a very limited application. We wrote the initial prototype to monitor
> fragmentation and used /sys/devices/system/node/node*/compact to trigger
> compaction. The prototype demonstrated this approach has merits.
> The primary reason to implement this logic in the kernel is to make the
> kernel self-tuning.

What makes this particular self-tuning an universal win? In other words
there are many ways to analyze the memory pressure and feedback it back
that I can think of. It is quite likely that very specific workloads
would have very specific demands there. I have seen cases where are
trivial increase of min_free_kbytes to normally insane value worked
really great for a DB workload because the wasted memory didn't matter
for example.

> The more knobs we have externally, the more complex
> it becomes to tune the kernel externally.

I agree on this point. Is the current set of tunning sufficient? What
would be missing if not?
Michal Hocko

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