Re: [PATCH] RFC: vmscan: add min_filelist_kbytes sysctl forprotecting the working set

[Minchan Kim]

On Fri, Oct 29, 2010 at 9:04 AM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:
> On Fri, 29 Oct 2010 08:28:23 +0900
> Minchan Kim <minchan.kim@xxxxxxxxx> wrote:
>
>> On Fri, Oct 29, 2010 at 7:03 AM, Mandeep Singh Baines <msb@xxxxxxxxxxxx> wrote:
>> > Andrew Morton (akpm@xxxxxxxxxxxxxxxxxxxx) wrote:
>> >> On Thu, 28 Oct 2010 12:15:23 -0700
>> >> Mandeep Singh Baines <msb@xxxxxxxxxxxx> wrote:
>> >>
>> >> > On ChromiumOS, we do not use swap.
>> >>
>> >> Well that's bad.  Why not?
>> >>
>> >
>> > We're using SSDs. We're still in the "make it work" phase so wanted
>> > avoid swap unless/until we learn how to use it effectively with
>> > an SSD.
>> >
>> > You'll want to tune swap differently if you're using an SSD. Not sure
>> > if swappiness is the answer. Maybe a new tunable to control how aggressive
>> > swap is unless such a thing already exits?
>> >
>> >> > When memory is low, the only way to
>> >> > free memory is to reclaim pages from the file list. This results in a
>> >> > lot of thrashing under low memory conditions. We see the system become
>> >> > unresponsive for minutes before it eventually OOMs. We also see very
>> >> > slow browser tab switching under low memory. Instead of an unresponsive
>> >> > system, we'd really like the kernel to OOM as soon as it starts to
>> >> > thrash. If it can't keep the working set in memory, then OOM.
>> >> > Losing one of many tabs is a better behaviour for the user than an
>> >> > unresponsive system.
>> >> >
>> >> > This patch create a new sysctl, min_filelist_kbytes, which disables reclaim
>> >> > of file-backed pages when when there are less than min_filelist_bytes worth
>> >> > of such pages in the cache. This tunable is handy for low memory systems
>> >> > using solid-state storage where interactive response is more important
>> >> > than not OOMing.
>> >> >
>> >> > With this patch and min_filelist_kbytes set to 50000, I see very little
>> >> > block layer activity during low memory. The system stays responsive under
>> >> > low memory and browser tab switching is fast. Eventually, a process a gets
>> >> > killed by OOM. Without this patch, the system gets wedged for minutes
>> >> > before it eventually OOMs. Below is the vmstat output from my test runs.
>> >> >
>> >> > BEFORE (notice the high bi and wa, also how long it takes to OOM):
>> >>
>> >> That's an interesting result.
>> >>
>> >> Having the machine "wedged for minutes" thrashing away paging
>> >> executable text is pretty bad behaviour.  I wonder how to fix it.
>> >> Perhaps simply declaring oom at an earlier stage.
>> >>
>> >> Your patch is certainly simple enough but a bit sad.  It says "the VM
>> >> gets this wrong, so lets just disable it all".  And thereby reduces the
>> >> motivation to fix it for real.
>> >>
>> >
>> > Yeah, I used the RFC label because we're thinking this is just a temporary
>> > bandaid until something better comes along.
>> >
>> > Couple of other nits I have with our patch:
>> > * Not really sure what to do for the cgroup case. We do something
>> >  reasonable for now.
>> > * One of my colleagues also brought up the point that we might want to do
>> >  something different if swap was enabled.
>> >
>> >> But the patch definitely improves the situation in real-world
>> >> situations and there's a case to be made that it should be available at
>> >> least as an interim thing until the VM gets fixed for real.  Which
>> >> means that the /proc tunable might disappear again (or become a no-op)
>> >> some time in the future.
>>
>> I think this feature that "System response time doesn't allow but OOM allow".
>> While we can control process to not killed by OOM using
>> /oom_score_adj, we can't control response time directly.
>> But in mobile system, we have to control response time. One of cause
>> to avoid swap is due to response time.
>>
>> How about using memcg?
>> Isolate processes related to system response(ex, rendering engine, IPC
>> engine and so no)  to another group.
>>
> Yes, this seems interesting topic on memcg.
>
> maybe configure cgroups as..
>
> /system       ....... limit to X % of the system.
> /application  ....... limit to 100-X % of the system.
>
> and put management software to /system. Then, the system software can check
> behavior of applicatoin and measure cpu time and I/O performance in /applicaiton.
> (And yes, it can watch memory usage.)
>
> Here, memory cgroup has oom-notifier, you may able to do something other than
> oom-killer by the system. If this patch is applied to global VM, I'll check
> memcg can support it or not.
> Hmm....checking anon/file rate in /application may be enough ?

I think anon/file/mapped_file is enough to do that.

>
> Or, as a google guy proosed, we may have to add "file-cache-only" memcg.
> For example, configure system as
>
> /system
> /application-anon
> /application-file-cache
>
> (But balancing file/anon must be done by user....this is difficult.)

Yes. I believe such fine-grained control can make system admin more annoying.

>
> BTW, can we know that "recently paged out file cache comes back immediately!"
> score ?

Not easy. If we can get it easily, we can enhance victim selection algorithm.
AFAIR, Rik tried it.
http://lwn.net/Articles/147879/


-- 
Kind regards,
Minchan Kim
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