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Subject: Re: [PATCH 4/4] hugetlb: add hugepages_node= command-line option
From: Davidlohr Bueso <davidlohr@hp.com>
To: David Rientjes <rientjes@google.com>
Cc: Luiz Capitulino <lcapitulino@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>, mtosatti@redhat.com,
        Mel Gorman <mgorman@suse.de>, Andrea Arcangeli <aarcange@redhat.com>,
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Date: Mon, 17 Feb 2014 21:47:36 -0800
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On Sat, 2014-02-15 at 02:06 -0800, David Rientjes wrote:
> On Fri, 14 Feb 2014, Luiz Capitulino wrote:
> 
> > > Again, I think this syntax is horrendous and doesn't couple well with the 
> > > other hugepage-related kernel command line options.  We already have 
> > > hugepages= and hugepagesz= which you can interleave on the command line to 
> > > get 100 2M hugepages and 10 1GB hugepages, for example.
> > > 
> > > This patchset is simply introducing another variable to the matter: the 
> > > node that the hugepages should be allocated on.  So just introduce a 
> > > hugepagesnode= parameter to couple with the others so you can do
> > > 
> > > 	hugepagesz=<size> hugepagesnode=<nid> hugepages=<#>
> > 
> > That was my first try but it turned out really bad. First, for every node
> > you specify you need three options.
> 
> Just like you need two options today to specify a number of hugepages of a 
> particular non-default size.  You only need to use hugepagesz= or 
> hugepagenode= if you want a non-default size or a specify a particular 
> node.
> 
> > So, if you want to setup memory for
> > three nodes you'll need to specify nine options.
> 
> And you currently need six if you want to specify three different hugepage 
> sizes (?).  But who really specifies three different hugepage sizes on the 
> command line that are needed to be reserved at boot?
> 
> If that's really the usecase, it seems like you want the old 
> CONFIG_PAGE_SHIFT patch.
> 
> > And it gets worse, because
> > hugepagesz= and hugepages= have strict ordering (which is a mistake, IMHO) so
> > you have to specify them in the right order otherwise things don't work as
> > expected and you have no idea why (have been there myself).
> > 
> 
> How is that difficult?  hugepages= is the "noun", hugepagesz= is the 
> "adjective".  hugepages=100 hugepagesz=1G hugepages=4 makes perfect sense 
> to me, and I actually don't allocate hugepages on the command line, nor 
> have I looked at Documentation/kernel-parameters.txt to check if I'm 
> constructing it correctly.  It just makes sense and once you learn it it's 
> just natural.

This can get annoying _really_ fast for larger systems.

> > IMO, hugepages_node=<nid>:<nr_pages>:<size>,... is good enough. It's concise,
> > and don't depend on any other option to function. Also, there are lots of other
> > kernel command-line options that require you to specify multiple fields, so
> > it's not like hugepages_node= is totally different in that regard.
> > 

Agreed.

> 
> I doubt Andrew is going to want a completely different format for hugepage 
> allocations that want to specify a node and have to deal with people who 
> say hugepages_node=2:1:1G and constantly have to lookup if it's 2 
> hugepages on node 1 or 1 hugepage on node 2.

I guess most users won't even be aware of this new parameter and those
who really care will have the choice.


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