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

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Date:   Tue, 9 Jun 2020 19:03:45 -0400
From:   Daniel Jordan <daniel.m.jordan@...cle.com>
To:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc:     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>,
        Daniel Jordan <daniel.m.jordan@...cle.com>
Subject: Re: [PATCH v2] x86/mm: use max memory block size on bare metal

On Tue, Jun 09, 2020 at 06:54:51PM -0400, Daniel Jordan wrote:
> Some of our servers spend significant time at kernel boot initializing
> memory block sysfs directories and then creating symlinks between them
> and the corresponding nodes.  The slowness happens because the machines
> get stuck with the smallest supported memory block size on x86 (128M),
> which results in 16,288 directories to cover the 2T of installed RAM.
> The search for each memory block is noticeable even with
> commit 4fb6eabf1037 ("drivers/base/memory.c: cache memory blocks in
> xarray to accelerate lookup").
> 
> Commit 078eb6aa50dc ("x86/mm/memory_hotplug: determine block size based
> on the end of boot memory") chooses the block size based on alignment
> with memory end.  That addresses hotplug failures in qemu guests, but
> for bare metal systems whose memory end isn't aligned to even the
> smallest size, it leaves them at 128M.
> 
> Make kernels that aren't running on a hypervisor use the largest
> supported size (2G) to minimize overhead on big machines.  Kernel boot
> goes 7% faster on the aforementioned servers, shaving off half a second.
> 
> Signed-off-by: Daniel Jordan <daniel.m.jordan@...cle.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Andy Lutomirski <luto@...nel.org>
> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: David Hildenbrand <david@...hat.com>
> Cc: Michal Hocko <mhocko@...nel.org>
> Cc: Pavel Tatashin <pasha.tatashin@...een.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Steven Sistare <steven.sistare@...cle.com>
> Cc: linux-mm@...ck.org
> Cc: linux-kernel@...r.kernel.org
> ---

Forgot the v1 changes:

 - Thanks to David for the idea to make this conditional based on
   virtualization.
 - Update performance numbers to account for 4fb6eabf1037 (David)