linux-kernel - Re: [PATCH 0/4] hugetlb: add hugepagesnid= command-line option

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Message-ID: <20140211103624.7edf1423@redhat.com>
Date:	Tue, 11 Feb 2014 10:36:24 -0500
From:	Luiz Capitulino <lcapitulino@...hat.com>
To:	David Rientjes <rientjes@...gle.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>, mtosatti@...hat.com,
	Mel Gorman <mgorman@...e.de>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andi Kleen <andi@...stfloor.org>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 0/4] hugetlb: add hugepagesnid= command-line option

On Mon, 10 Feb 2014 18:54:20 -0800 (PST)
David Rientjes <rientjes@...gle.com> wrote:

> On Mon, 10 Feb 2014, Luiz Capitulino wrote:
> 
> > HugeTLB command-line option hugepages= allows the user to specify how many
> > huge pages should be allocated at boot. On NUMA systems, this argument
> > automatically distributes huge pages allocation among nodes, which can
> > be undesirable.
> > 
> 
> And when hugepages can no longer be allocated on a node because it is too 
> small, the remaining hugepages are distributed over nodes with memory 
> available, correct?

No. hugepagesnid= tries to obey what was specified by the uses as much as
possible. So, if you specify that 10 1G huge pages should be allocated from
node0 but only 7 1G pages can actually be allocated, then hugepagesnid= will
do just that.

> > The hugepagesnid= option introduced by this commit allows the user
> > to specify which NUMA nodes should be used to allocate boot-time HugeTLB
> > pages. For example, hugepagesnid=0,2,2G will allocate two 2G huge pages
> > from node 0 only. More details on patch 3/4 and patch 4/4.
> > 
> 
> Strange, it would seem better to just reserve as many hugepages as you 
> want so that you get the desired number on each node and then free the 
> ones you don't need at runtime.

You mean, for example, if I have a 2 node system and want 2 1G huge pages
from node 1, then I have to allocate 4 1G huge pages and then free 2 pages
on node 0 after boot? That seems very cumbersome to me. Besides, what if
node0 needs this memory during boot?

> That probably doesn't work because we can't free very large hugepages that 
> are reserved at boot, would fixing that issue reduce the need for this 
> patchset?

I don't think so.
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