Re: [PATCH v3] x86/mm: use max memory block size on bare metal

[Daniel Jordan]

On Tue, Jul 14, 2020 at 04:54:50PM -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@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: David Hildenbrand <david@xxxxxxxxxx>

Darn.  David, I forgot to add your ack from v2.  My assumption is that it still
stands after the minor change in this version, but please do correct me if I'm
wrong.