Re: [PATCH v3 5/5] Use 2GB memory block size on large x86-64 systems

From: Thomas Gleixner <tglx@linutronix.de>
To: Daniel J Blueman <daniel@numascale.com>
Cc: Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
	Bjorn Helgaas <bhelgaas@google.com>,
	x86@kernel.org, linux-kernel@vger.kernel.org,
	linux-pci@vger.kernel.org, Steffen Persvold <sp@numascale.com>
Subject: Re: [PATCH v3 5/5] Use 2GB memory block size on large x86-64 systems
Date: Tue, 4 Nov 2014 00:36:57 +0100 (CET)	[thread overview]
Message-ID: <alpine.DEB.2.11.1411040033530.5308@nanos> (raw)
In-Reply-To: <54580C9E.7070706@numascale.com>

On Tue, 4 Nov 2014, Daniel J Blueman wrote:

> On 11/04/2014 03:38 AM, Thomas Gleixner wrote:
> > On Sun, 2 Nov 2014, Daniel J Blueman wrote:
> > 
> > > On larger x64-64 systems, use a 2GB memory block size to reduce sysfs
> > > entry creation time by 16x. Large is defined as 64GB or more memory.
> > 
> > This changelog sucks.
> > 
> > It neither tells which sysfs entries are meant nor does it explain
> > what the actual effect of this change is aside of speeding up some
> > random sysfs thingy.
> 
> How about this?
> 
> On large-memory systems of 64GB or more with memory hot-plug enabled, use a
> 2GB memory block size. Eg with 64GB memory, this reduces the number of
> directories in /sys/devices/system/memory from 512 to 32, making it more
> manageable, and reducing the creation time accordingly.

It still does not tell what the downside is of this and why you think
it does not matter.

> > > @@ -1247,9 +1246,9 @@ static unsigned long probe_memory_block_size(void)
> > >   	/* start from 2g */
> > >   	unsigned long bz = 1UL<<31;
> > > 
> > > -#ifdef CONFIG_X86_UV
> > > -	if (is_uv_system()) {
> > > -		printk(KERN_INFO "UV: memory block size 2GB\n");
> > > +#ifdef CONFIG_X86_64
> > 
> > And this brainless 's/CONFIG_X86_UV/CONFIG_X86_64/' sucks even
> > more. I'm sure you can figure out the WHY yourself.
> 
> The benefit of this is applicable to other architectures. I'm unable to test
> the change, but if you agree it's conservative enough, I'll drop the ifdef?

Which other architectures? Care to turn on your brain before replying?

Thanks,

	tglx