linux-kernel - Re: [PATCH v2] x86/mm: use max memory block size on bare metal

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Date:   Thu, 11 Jun 2020 12:59:10 -0400
From:   Daniel Jordan <daniel.m.jordan@...cle.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Daniel Jordan <daniel.m.jordan@...cle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        David Hildenbrand <david@...hat.com>,
        Michal Hocko <mhocko@...nel.org>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Sistare <steven.sistare@...cle.com>
Subject: Re: [PATCH v2] x86/mm: use max memory block size on bare metal

On Thu, Jun 11, 2020 at 07:16:02AM -0700, Dave Hansen wrote:
> On 6/9/20 3:54 PM, Daniel Jordan wrote:
> > +	/*
> > +	 * Use max block size to minimize overhead on bare metal, where
> > +	 * alignment for memory hotplug isn't a concern.
> > +	 */
> > +	if (hypervisor_is_type(X86_HYPER_NATIVE)) {
> > +		bz = MAX_BLOCK_SIZE;
> > +		goto done;
> > +	}
> 
> What ends up being the worst case scenario?  Booting a really small
> bare-metal x86 system, say with 64MB or 128MB of RAM?  What's the
> overhead there?

Might not be following you, so bear with me, but we only get to this check on a
system with a physical address end of at least MEM_SIZE_FOR_LARGE_BLOCK (64G),
and this would still (ever so slightly...) reduce overhead of memory block init
at boot in that case.