Re: [PATCH -mm v9 0/8] idle memory tracking

[Vladimir Davydov]

On Thu, Jul 30, 2015 at 11:07:09AM +0200, Michal Hocko wrote:
> On Wed 29-07-15 19:29:08, Vladimir Davydov wrote:
> > On Wed, Jul 29, 2015 at 05:47:18PM +0200, Michal Hocko wrote:
> [...]
> > > If you use the low limit for isolating an important load then you do not
> > > have to care about the others that much. All you care about is to set
> > > the reasonable protection level and let others to compete for the rest.
> > 
> > That's a use case, you're right. Well, it's a natural limitation of this
> > API - you just have to perform a full PFN scan then. You can avoid
> > costly rmap walks for the cgroups you are not interested in by filtering
> > them out using /proc/kpagecgroup though.
> 
> You still have to read through the whole memory and that is inherent to
> the API and there no way for a better implementation later on other than
> a new exported file.

I don't deny that. Nevertheless, PFN-walk is something that will always
be useful, simply because PFN-range is an invariant - it will always
exist. If one day a better page iterator appear (e.g. LRU walk) and the
need for it is justified well enough, we can add one more file. Note, it
won't deprecate the original PFN map - they both can be used for
different use cases then. If we move kpageidle to /sys/kernel/mm attr
group, which I'm doing now, it will be trivial to do and won't pollute
/proc.

> 
> [...]
> 
> > > > Because there is too much to be taken care of in the kernel with such an
> > > > approach and chances are high that it won't satisfy everyone. What
> > > > should the scan period be equal too?
> > > 
> > > No, just gather the data on the read request and let the userspace
> > > to decide when/how often etc. If we are clever enough we can cache
> > > the numbers and prevent from the walk. Write to the file and do the
> > > mark_idle stuff.
> > 
> > Still, scan rate limiting would be an issue IMO.
> 
> Not sure what you mean here. Scan rate would be defined by the userspace
> by reading/writing to the knob. No background kernel thread is really
> necessary.

Nevertheless, it means more logic in the kernel (rate limiter) and a
wider interface (+ rate limit value).

> 
> > > > Knob. How many kthreads do we want?
> > > > Knob. I want to keep history for last N intervals (this was a part of
> > > > Michel's implementation), what should N be equal to? Knob.
> > > 
> > > This all relates to the kernel thread implementation which I wasn't
> > > suggesting. I was referring to Michel's work which might induce that.
> > > I was merely referring to a single number output. Sorry about the
> > > confusion.
> > 
> > Still, what about idle stats history? I mean having info about how many
> > pages were idle for N scans. It might be useful for more robust/accurate
> > wss estimation.
> 
> Why cannot userspace remember those numbers?

Because they must be per-page - you have to remember for how many
periods *each particular* page has been idle. To achieve this, Michel
had to introduce a byte array referenced by PFN in his work. With
kpageidle file one can store this array in the userspace.

> 
> > > > I want to be
> > > > able to choose between an instant scan and a scan distributed in time.
> > > > Knob. I want to see stats for anon/locked/file/dirty memory separately,
> > > 
> > > Why is this useful for the memcg limits setting or the wss estimation? I
> > > can imagine that a further drop down numbers might be interesting
> > > from the debugging POV but I fail to see what kind of decisions from
> > > userspace you would do based on them.
> > 
> > A couple examples that pop up in my mind:
> > 
> > It's difficult to make wss estimation perfect. By mlocking pages, a
> > workload might give a hint to the system that it will be really unhappy
> > if they are evicted.
> > 
> > One might want to consider anon pages and/or dirty pages as not idle in
> > order to protect them and hence avoid expensive pageout/swapout.
> 
> I still seem to miss the point. How do you do that via the proposed
> interface which doesn't influence the reclaim AFAIU and you do not have
> means to achieve the above (except for swappiness). What am I missing?

You can consider idle only those pages that are clean, and then set the
low limit appropriately for your workload. You can find out which pages
are clean by reading /proc/kpageflags. Of course, this won't stop the
reclaimer from evicting them, but it will make the reclaimer less
aggressive with respect to your workload.

Thanks,
Vladimir
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