Re: [RFC 0/7] introduce memory hinting API for external process

[Minchan Kim]

On Tue, May 21, 2019 at 08:25:55AM +0530, Anshuman Khandual wrote:
> 
> 
> On 05/20/2019 10:29 PM, Tim Murray wrote:
> > On Sun, May 19, 2019 at 11:37 PM Anshuman Khandual
> > <anshuman.khandual@xxxxxxx> wrote:
> >>
> >> Or Is the objective here is reduce the number of processes which get killed by
> >> lmkd by triggering swapping for the unused memory (user hinted) sooner so that
> >> they dont get picked by lmkd. Under utilization for zram hardware is a concern
> >> here as well ?
> > 
> > The objective is to avoid some instances of memory pressure by
> > proactively swapping pages that userspace knows to be cold before
> > those pages reach the end of the LRUs, which in turn can prevent some
> > apps from being killed by lmk/lmkd. As soon as Android userspace knows
> > that an application is not being used and is only resident to improve
> > performance if the user returns to that app, we can kick off
> > process_madvise on that process's pages (or some portion of those
> > pages) in a power-efficient way to reduce memory pressure long before
> > the system hits the free page watermark. This allows the system more
> > time to put pages into zram versus waiting for the watermark to
> > trigger kswapd, which decreases the likelihood that later memory
> > allocations will cause enough pressure to trigger a kill of one of
> > these apps.
> 
> So this opens up bit of LRU management to user space hints. Also because the app
> in itself wont know about the memory situation of the entire system, new system
> call needs to be called from an external process.

That's why process_madvise is introduced here.

> 
> > 
> >> Swapping out memory into zram wont increase the latency for a hot start ? Or
> >> is it because as it will prevent a fresh cold start which anyway will be slower
> >> than a slow hot start. Just being curious.
> > 
> > First, not all swapped pages will be reloaded immediately once an app
> > is resumed. We've found that an app's working set post-process_madvise
> > is significantly smaller than what an app allocates when it first
> > launches (see the delta between pswpin and pswpout in Minchan's
> > results). Presumably because of this, faulting to fetch from zram does
> 
> pswpin      417613    1392647     975034     233.00
> pswpout    1274224    2661731    1387507     108.00
> 
> IIUC the swap-in ratio is way higher in comparison to that of swap out. Is that
> always the case ? Or it tend to swap out from an active area of the working set
> which faulted back again.

I think it's because apps are alive longer via reducing being killed
so turn into from pgpgin to swapin.

> 
> > not seem to introduce a noticeable hot start penalty, not does it
> > cause an increase in performance problems later in the app's
> > lifecycle. I've measured with and without process_madvise, and the
> > differences are within our noise bounds. Second, because we're not
> 
> That is assuming that post process_madvise() working set for the application is
> always smaller. There is another challenge. The external process should ideally
> have the knowledge of active areas of the working set for an application in
> question for it to invoke process_madvise() correctly to prevent such scenarios.

There are several ways to detect workingset more accurately at the cost
of runtime. For example, with idle page tracking or clear_refs. Accuracy
is always trade-off of overhead for LRU aging.

> 
> > preemptively evicting file pages and only making them more likely to
> > be evicted when there's already memory pressure, we avoid the case
> > where we process_madvise an app then immediately return to the app and
> > reload all file pages in the working set even though there was no
> > intervening memory pressure. Our initial version of this work evicted
> 
> That would be the worst case scenario which should be avoided. Memory pressure
> must be a parameter before actually doing the swap out. But pages if know to be
> inactive/cold can be marked high priority to be swapped out.
> 
> > file pages preemptively and did cause a noticeable slowdown (~15%) for
> > that case; this patch set avoids that slowdown. Finally, the benefit
> > from avoiding cold starts is huge. The performance improvement from
> > having a hot start instead of a cold start ranges from 3x for very
> > small apps to 50x+ for larger apps like high-fidelity games.
> 
> Is there any other real world scenario apart from this app based ecosystem where
> user hinted LRU management might be helpful ? Just being curious. Thanks for the
> detailed explanation. I will continue looking into this series.