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

[Brian Geffon]

To expand on the ChromeOS use case we're in a very similar situation
to Android. For example, the Chrome browser uses a separate process
for each individual tab (with some exceptions) and over time many tabs
remain open in a back-grounded or idle state. Given that we have a lot
of information about the weight of a tab, when it was last active,
etc, we can benefit tremendously from per-process reclaim. We're
working on getting real world numbers but all of our initial testing
shows very promising results.


On Tue, May 21, 2019 at 5:57 AM Shakeel Butt <shakeelb@xxxxxxxxxx> wrote:
>
> On Mon, May 20, 2019 at 7:55 PM Anshuman Khandual
> <anshuman.khandual@xxxxxxx> 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.
> >
> > >
> > >> 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.
> >
> > > 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.
> >
> > > 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.
>
> Chrome OS is another real world use-case for this user hinted LRU
> management approach by proactively reclaiming reclaim from tabs not
> accessed by the user for some time.