Re: [RFC 0/4] ZRAM: make it just store the high compression rate page

[Minchan Kim]

On Mon, Sep 05, 2016 at 01:12:05PM +0800, Hui Zhu wrote:
> On Mon, Sep 5, 2016 at 10:18 AM, Minchan Kim <minchan@xxxxxxxxxx> wrote:
> > On Thu, Aug 25, 2016 at 04:25:30PM +0800, Hui Zhu wrote:
> >> On Thu, Aug 25, 2016 at 2:09 PM, Sergey Senozhatsky
> >> <sergey.senozhatsky.work@xxxxxxxxx> wrote:
> >> > Hello,
> >> >
> >> > On (08/22/16 16:25), Hui Zhu wrote:
> >> >>
> >> >> Current ZRAM just can store all pages even if the compression rate
> >> >> of a page is really low.  So the compression rate of ZRAM is out of
> >> >> control when it is running.
> >> >> In my part, I did some test and record with ZRAM.  The compression rate
> >> >> is about 40%.
> >> >>
> >> >> This series of patches make ZRAM can just store the page that the
> >> >> compressed size is smaller than a value.
> >> >> With these patches, I set the value to 2048 and did the same test with
> >> >> before.  The compression rate is about 20%.  The times of lowmemorykiller
> >> >> also decreased.
> >> >
> >> > I haven't looked at the patches in details yet. can you educate me a bit?
> >> > is your test stable? why the number of lowmemorykill-s has decreased?
> >> > ... or am reading "The times of lowmemorykiller also decreased" wrong?
> >> >
> >> > suppose you have X pages that result in bad compression size (from zram
> >> > point of view). zram stores such pages uncompressed, IOW we have no memory
> >> > savings - swapped out page lands in zsmalloc PAGE_SIZE class. now you
> >> > don't try to store those pages in zsmalloc, but keep them as unevictable.
> >> > so the page still occupies PAGE_SIZE; no memory saving again. why did it
> >> > improve LMK?
> >>
> >> No, zram will not save this page uncompressed with these patches.  It
> >> will set it as non-swap and kick back to shrink_page_list.
> >> Shrink_page_list will remove this page from swapcache and kick it to
> >> unevictable list.
> >> Then this page will not be swaped before it get write.
> >> That is why most of code are around vmscan.c.
> >
> > If I understand Sergey's point right, he means there is no gain
> > to save memory between before and after.
> >
> > With your approach, you can prevent unnecessary pageout(i.e.,
> > uncompressible page swap out) but it doesn't mean you save the
> > memory compared to old so why does your patch decrease the number of
> > lowmemory killing?
> >
> > A thing I can imagine is without this feature, zram could be full of
> > uncompressible pages so good-compressible page cannot be swapped out.
> > Hui, is this scenario right for your case?
> >
> 
> That is one reason.  But it is not the principal one.
> 
> Another reason is when swap is running to put page to zram, what the
> system wants is to get memory.
> Then the deal is system spends cpu time and memory to get memory. If
> the zram just access the high compression rate pages, system can get
> more memory with the same amount of memory. It will pull system from
> low memory status earlier. (Maybe more cpu time, because the
> compression rate checks. But maybe less, because fewer pages need to
> digress. That is the interesting part. :)
> I think that is why lmk times decrease.
> 
> And yes, all of this depends on the number of high compression rate
> pages. So you cannot just set a non_swap limit to the system and get
> everything. You need to do a lot of test around it to make sure the
> non_swap limit is good for your system.
> 
> And I think use AOP_WRITEPAGE_ACTIVATE without kicking page to a
> special list will make cpu too busy sometimes.

Yes, and it would same with your patch if new arraival write on CoWed
page is uncompressible data.

> I did some tests before I kick page to a special list. The shrink task

What kinds of test? Could you elaborate a bit more?
shrink task. What does it mean?

> will be moved around, around and around because low compression rate
> pages just moved from one list to another a lot of times, again, again
> and again.
> And all this low compression rate pages always stay together.

I cannot understand with detail description. :(
Could you explain more?