linux-kernel - Re: [PATCH]mmap: add alignment for some variables

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Date:	Wed, 30 Mar 2011 09:01:22 +0800
From:	Shaohua Li <shaohua.li@...el.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Andi Kleen <andi@...stfloor.org>, linux-mm <linux-mm@...ck.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Rik van Riel <riel@...hat.com>,
	Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH]mmap: add alignment for some variables

On Wed, 2011-03-30 at 06:24 +0800, Andrew Morton wrote:
> On Tue, 29 Mar 2011 08:54:14 +0800
> Shaohua Li <shaohua.li@...el.com> wrote:
> 
> > -struct percpu_counter vm_committed_as;
> > +struct percpu_counter vm_committed_as ____cacheline_internodealigned_in_smp;
> 
> Why ____cacheline_internodealigned_in_smp?  That's pretty aggressive.
> 
> afacit the main benefit from this will occur if the read-only
> vm_committed_as.counters lands in the same cacheline as some
> write-frequently storage.
vm_committed_as can be frequently updated in some workloads too.

> But that's a complete mad guess and I'd prefer not to have to guess.
is below updated patch better to you?

Make some variables have correct alignment/section to avoid cache issue.
In a workload which heavily does mmap/munmap, the variables will be used
frequently.

Signed-off-by: Shaohua Li <shaohua.li@...el.com>
---
 mm/mmap.c |   10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

Index: linux/mm/mmap.c
===================================================================
--- linux.orig/mm/mmap.c	2011-03-30 08:45:05.000000000 +0800
+++ linux/mm/mmap.c	2011-03-30 08:59:23.000000000 +0800
@@ -84,10 +84,14 @@ pgprot_t vm_get_page_prot(unsigned long
 }
 EXPORT_SYMBOL(vm_get_page_prot);
 
-int sysctl_overcommit_memory = OVERCOMMIT_GUESS;  /* heuristic overcommit */
-int sysctl_overcommit_ratio = 50;	/* default is 50% */
+int sysctl_overcommit_memory __read_mostly = OVERCOMMIT_GUESS;  /* heuristic overcommit */
+int sysctl_overcommit_ratio __read_mostly = 50;	/* default is 50% */
 int sysctl_max_map_count __read_mostly = DEFAULT_MAX_MAP_COUNT;
-struct percpu_counter vm_committed_as;
+/*
+ * Make sure vm_committed_as in one cacheline and not cacheline shared with
+ * other variables. It can be updated by several CPUs frequently.
+ */
+struct percpu_counter vm_committed_as ____cacheline_internodealigned_in_smp;
 
 /*
  * Check that a process has enough memory to allocate a new virtual



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