Re: [PATCH] mm, meminit: Serially initialise deferred memory if trace_buf_size is specified

[Mel Gorman]

On Wed, Nov 29, 2017 at 10:41:59PM -0500, Pavel Tatashin wrote:
> Hi Mel,
> 
> Thank you very much for your feedback, my replies below:
> 
> > A lack of involvement from admins is indeed desirable. For example,
> > while I might concede on using a disable-everything-switch, I would not
> > be happy to introduce a switch that specified how much memory per node
> > to initialise.
> >
> > For the forth approach, I really would be only thinking of a blunt
> > "initialise everything instead of going OOM". I was wary of making things
> > too complicated and I worried about some side-effects I'll cover later.
> 
> I see, I misunderstood your suggestion. Switching to serial
> initialization when OOM works, however, boot time becomes
> unpredictable, with some configurations boot is fast with others it is
> slow. All of that depends on whether predictions in
> reset_deferred_meminit() were good or not which is not easy to debug
> for users. Also, overtime predictions in reset_deferred_meminit() can
> become very off, and I do not think that we want to continuously
> adjust this function.
> 

You could increase the probabilty of a report by doing a WARN_ON_ONCE if
the serialised meminit is used.

> >> With this approach we could always init a very small amount of struct
> >> pages, and allow the rest to be initialized on demand as boot requires
> >> until deferred struct pages are initialized. Since, having deferred
> >> pages feature assumes that the machine is large, there is no drawback
> >> of having some extra byte of dead code, especially that all the checks
> >> can be permanently switched of via static branches once deferred init
> >> is complete.
> >>
> >
> > This is where I fear there may be dragons. If we minimse the number of
> > struct pages and initialise serially as necessary, there is a danger that
> > we'll allocate remote memory in cases where local memory would have done
> > because a remote node had enough memory.
> 
> True, but is not what we have now has the same issue as well? If one
> node is gets out of memory we start using memory from another node,
> before deferred pages are initialized?
> 

It's possible but I'm not aware of it happening currently.

>  To offset that risk, it would be
> > necessary at boot-time to force allocations from local node where possible
> > and initialise more memory as necessary. That starts getting complicated
> > because we'd need to adjust gfp-flags in the fast path with init-and-retry
> > logic in the slow path and that could be a constant penalty. We could offset
> > that in the fast path by using static branches
> 
> I will try to implement this, and see how complicated the patch will
> be, if it gets too complicated for the problem I am trying to solve we
> can return to one of your suggestions.
> 
> I was thinking to do something like this:
> 
> Start with every small amount of initialized pages in every node.
> If allocation fails, initialize enough struct pages to cover this
> particular allocation with struct pages rounded up to section size but
> in every single node.
> 

Ok, just make sure it's all in the slow paths of the allocator when the
alternative is to fail the allocation.

> > but it's getting more and
> > more complex for what is a minor optimisation -- shorter boot times on
> > large machines where userspace itself could take a *long* time to get up
> > and running (think database reading in 1TB of data from disk as it warms up).
> 
> On M6-32 with 32T [1] of memory it saves over 4 minutes of boot time,
> and this is on SPARC with 8K pages, on x86 it would be around of 8
> minutes because of twice as many pages. This feature improves
> availability for larger machines quite a bit. Overtime, systems are
> growing, so I expect this feature to become a default configuration in
> the next several years on server configs.
> 

Ok, when developing the series originally, I had no machine even close
to 32T of memory.

-- 
Mel Gorman
SUSE Labs