Re: [PATCH 4/4] hugetlb: add hugepages_node= command-line option

[Marcelo Tosatti]

On Mon, Feb 17, 2014 at 03:23:16PM -0800, David Rientjes wrote:
> On Mon, 17 Feb 2014, Luiz Capitulino wrote:
> 
> > hugepages= and hugepages_node= are similar, but have different semantics.
> > 
> > hugepagesz= and hugepages= create a pool of huge pages of the specified size.
> > This means that the number of times you specify those options are limited by
> > the number of different huge pages sizes an arch supports. For x86_64 for
> > example, this limit is two so one would not specify those options more than
> > two times. And this doesn't count default_hugepagesz=, which allows you to
> > drop one hugepagesz= option.
> > 
> > hugepages_node= allows you to allocate huge pages per node, so the number of
> > times you can specify this option is limited by the number of nodes. Also,
> > hugepages_node= create the pools, if necessary (at least one will be). For
> > this reason I think it makes a lot of sense to have different options.
> > 
> 
> I understand you may want to add as much code as you can to the boot code 
> so that you can parse all this information in short-form, and it's 
> understood that it's possible to specify a different number of varying 
> hugepage sizes on individual nodes, but let's come back down to reality 
> here.
> 
> Lacking from your entire patchset is a specific example of what you want 
> to do.  So I think we're all guessing what exactly your usecase is and we 
> aren't getting any help.  Are you really suggesting that a customer wants 
> to allocate 4 1GB hugepages on node 0, 12 2MB hugepages on node 0, 6 1GB 
> hugepages on node 1, 24 2MB hugepages on node 1, 2 1GB hugepages on node 
> 2, 100 2MB hugepages on node 3, etc?  Please.

Customer has 32GB machine. He wants 8 1GB pages for his performance
critical application on node0 (KVM guest), and other guests and
pagecache etc. using the remaining 26GB of memory.

> If that's actually the usecase then I'll renew my objection to the entire 
> patchset and say you want to add the ability to dynamically allocate 1GB 
> pages and free them at runtime early in initscripts.  If something is 
> going to be added to init code in the kernel then it better be trivial 
> since all this can be duplicated in userspace if you really want to be 
> fussy about it.

Not sure what is the point here. The command line interface addition
being proposed is simple, is it not?

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