Re: [RFC PATCH v2 -mm] provide estimated available memory in/proc/meminfo

[Minchan Kim]

Hi Hannes, Rik.

On Thu, Nov 07, 2013 at 04:21:32PM -0500, Johannes Weiner wrote:
> On Thu, Nov 07, 2013 at 10:13:45AM -0500, Rik van Riel wrote:
> > 
> > > >  fs/proc/meminfo.c | 36 ++++++++++++++++++++++++++++++++++++
> > > >  1 file changed, 36 insertions(+)
> > > 
> > > Documentation/filesystems/proc.txt told me it's feeling all offended.
> > 
> > You're right, of course.  Here is version 2 :)
> > 
> > ---8<---
> > 
> > Subject: provide estimated available memory in /proc/meminfo
> > 
> > Many load balancing and workload placing programs check /proc/meminfo
> > to estimate how much free memory is available. They generally do this
> > by adding up "free" and "cached", which was fine ten years ago, but
> > is pretty much guaranteed to be wrong today.
> > 
> > It is wrong because Cached includes memory that is not freeable as
> > page cache, for example shared memory segments, tmpfs, and ramfs,
> > and it does not include reclaimable slab memory, which can take up
> > a large fraction of system memory on mostly idle systems with lots
> > of files.
> > 
> > Currently, the amount of memory that is available for a new workload,
> > without pushing the system into swap, can be estimated from MemFree,
> > Active(file), Inactive(file), and SReclaimable, as well as the "low"
> > watermarks from /proc/zoneinfo.
> > 
> > However, this may change in the future, and user space really should
> > not be expected to know kernel internals to come up with an estimate
> > for the amount of free memory.
> > 
> > It is more convenient to provide such an estimate in /proc/meminfo.
> > If things change in the future, we only have to change it in one place.
> > 
> > Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>
> > Reported-by: Erik Mouw <erik.mouw_2@xxxxxxx>
> 
> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> 
> I have a suspicion that people will end up relying on this number to
> start new workloads in situations where lots of the page cache is
> actually heavily used.  We might not swap, but there will still be IO
> from thrashing cache.
> 
> Maybe we'll have to subtract mapped cache pages in the future to
> mitigate this risk somehow...

It might be huge false positive if there was mmaped used-once stream so
that userlevel could free some objects or kill someone to get a free memory.

And shouldn't we consider dirty + writeback, either?

Anyway, this feature is very handy. Swapping/LMK/OOM is very sensivie
subject for embedded people these days so we have been used some matrix
to get a ballpark estimate like

        "buffers + cached + Sreclaimable - (SHMEM + dirty + writeback +
         workingset)"

We included workingset to prevent thrashing page cache.
So, my point is we could include some tunable value in the expression
like workingset and default might be half of page cache like this patch
but admin can control it if the platform is aware of his workingset size.

> 
> Anyway, we can defer this to when it's proven to be an actual problem.
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-- 
Kind regards,
Minchan Kim
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