linux-kernel - Re: [PATCH v2 5/5] Numachip: use 2GB memory block size

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Date:	Mon, 20 Oct 2014 14:02:43 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Daniel J Blueman <daniel@...ascale.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
	Steffen Persvold <sp@...ascale.com>
Subject: Re: [PATCH v2 5/5] Numachip: use 2GB memory block size


* Daniel J Blueman <daniel@...ascale.com> wrote:

> On 19/10/2014 17:23, Ingo Molnar wrote:
> >
> >* Daniel J Blueman <daniel@...ascale.com> wrote:
> >
> >>Use appropriate memory block size to reduce sysfs entry creation time
> >>by 16x.
> >>
> >>Boot-tested with the four permutations of X86_UV and X86_NUMACHIP.
> >>
> >>Signed-off-by: Daniel J Blueman <daniel@...ascale.com>
> >>---
> >>  arch/x86/mm/init_64.c | 7 ++++---
> >>  1 file changed, 4 insertions(+), 3 deletions(-)
> >>
> >>diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
> >>index 5621c47..22ea6de 100644
> >>--- a/arch/x86/mm/init_64.c
> >>+++ b/arch/x86/mm/init_64.c
> >>@@ -53,6 +53,7 @@
> >>  #include <asm/cacheflush.h>
> >>  #include <asm/init.h>
> >>  #include <asm/uv/uv.h>
> >>+#include <asm/numachip/numachip.h>
> >>  #include <asm/setup.h>
> >>
> >>  #include "mm_internal.h"
> >>@@ -1235,9 +1236,9 @@ 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
> >>+	if (is_uv_system() || is_numachip_system()) {
> >>+		pr_info("Memory block size 2GB for large-SMP system\n");
> >>  		return 2UL * 1024 * 1024 * 1024;
> >
> >It would be a lot cleaner and more robust to have a more
> >intelligent decision here.
> >
> >Is there a reliable indicator for large 'sysfs entry creation
> >time', such as a lot of RAM present?
> 
> Yes, agreed exactly.
> 
> > Also, it would be nice to list the pros/cons of this change, 
> > an advantage is reduced overhead - what are the 
> > disadvantages?
> 
> The single disadvantage is that small-memory systems won't be 
> able to have finer control of memory offlining, though the 
> impact of that depend on why the user is offlining memory of 
> course.
> 
> If it seems reasonable for x86-64 systems with >64GB memory to 
> have 2GB memory block sizes, I could prepare that change 
> instead and document the above if preferred?

I'd make it >= 64GB, but yes, that sounds like a good limit.

Thanks,

	Ingo
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