From: Andrew Morton <akpm@linux-foundation.org>
To: Shaohua Li <shaohua.li@intel.com>
Cc: Andi Kleen <andi@firstfloor.org>, linux-mm <linux-mm@kvack.org>,
lkml <linux-kernel@vger.kernel.org>,
Rik van Riel <riel@redhat.com>, Hugh Dickins <hughd@google.com>
Subject: Re: [PATCH]mmap: add alignment for some variables
Date: Tue, 29 Mar 2011 18:25:44 -0700 [thread overview]
Message-ID: <20110329182544.6ad4eccb.akpm@linux-foundation.org> (raw)
In-Reply-To: <1301447843.3981.48.camel@sli10-conroe>
On Wed, 30 Mar 2011 09:17:23 +0800 Shaohua Li <shaohua.li@intel.com> wrote:
> On Wed, 2011-03-30 at 09:06 +0800, Andrew Morton wrote:
> > On Wed, 30 Mar 2011 09:01:22 +0800 Shaohua Li <shaohua.li@intel.com> wrote:
> >
> > > +/*
> > > + * 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;
> >
> > The mystery deepens. The only cross-cpu writeable fields in there are
> > percpu_counter.lock and its companion percpu_counter.count. If CPUs
> > are contending for the lock then that itself is a problem - how does
> > adding some padding to the struct help anything?
> I had another patch trying to address the lock contention (for case
> OVERCOMMIT_GUESS), will send out soon. But thought better to have the
> correct alignment for OVERCOMMIT_NEVER case.
I still don't understand why adding
____cacheline_internodealigned_in_smp to vm_committed_as improves
anything.
Here it is:
struct percpu_counter {
spinlock_t lock;
s64 count;
#ifdef CONFIG_HOTPLUG_CPU
struct list_head list; /* All percpu_counters are on a list */
#endif
s32 __percpu *counters;
};
and your patch effectively converts this to
struct percpu_counter {
spinlock_t lock;
s64 count;
#ifdef CONFIG_HOTPLUG_CPU
struct list_head list; /* All percpu_counters are on a list */
#endif
s32 __percpu *counters;
+ char large_waste_of_space[lots];
};
how is it that this improves things?
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next prev parent reply other threads:[~2011-03-30 1:29 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-03-28 1:58 [PATCH]mmap: add alignment for some variables Shaohua Li
2011-03-28 16:55 ` Andi Kleen
2011-03-29 0:54 ` Shaohua Li
2011-03-29 22:24 ` Andrew Morton
2011-03-29 22:38 ` Christoph Lameter
2011-03-30 1:01 ` Shaohua Li
2011-03-30 1:06 ` Andrew Morton
2011-03-30 1:17 ` Shaohua Li
2011-03-30 1:25 ` Andrew Morton [this message]
2011-03-30 1:36 ` Shaohua Li
2011-03-30 1:41 ` Andrew Morton
2011-03-30 1:54 ` Shaohua Li
2011-03-30 2:10 ` Andrew Morton
2011-03-30 2:35 ` Shaohua Li
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