Re: [RFC v3 0/3] vmpressure_fd: Linux VM pressure notifications

[Anton Vorontsov <anton.vorontsov@linaro.org>]

On Wed, Nov 07, 2012 at 02:11:10PM +0200, Kirill A. Shutemov wrote:
[...]
> >    We can have plenty of "free" memory, of which say 90% will be caches,
> >    and say 10% idle. But we do want to differentiate these types of memory
> >    (although not going into details about it), i.e. we want to get
> >    notified when kernel is reclaiming. And we also want to know when the
> >    memory comes from swapping others' pages out (well, actually we don't
> >    call it swap, it's "new allocations cost becomes high" -- it might be a
> >    result of many factors (swapping, fragmentation, etc.) -- and userland
> >    might analyze the situation when this happens).
> > 
> >    Exposing all the VM details to userland is not an option
> 
> IIUC, you want MemFree + Buffers + Cached + SwapCached, right?
> It's already exposed to userspace.

How? If you mean vmstat, then no, that interface is not efficient at all:
we have to poll it from userland, which is no go for embedded (although,
as a workaround it can be done via deferrable timers in userland, which I
posted a few months ago).

But even with polling vmstat via deferrable timers, it leaves us with the
ugly timers-based approach (and no way to catch the pre-OOM conditions).
With vmpressure_fd() we have the synchronous notifications right from the
core (upon which, you can, if you want to, analyze the vmstat).

>> 2. The last time I checked, cgroups memory controller did not (and I guess
>>    still does not) not account kernel-owned slabs. I asked several times
>>    why so, but nobody answered.
>
> Almost there. Glauber works on it.

It's good to hear, but still, the number of "used KBs" is a bad (or
irrelevant) metric for the pressure. We'd still need to analyze the memory
in more details, and "'limit - used' KBs" doesn't tell us anything about
the cost of the available memory.

Thanks,
Anton.

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