linux-kernel - Re: [PATCH RFC v2 4/4] memcg: implement memory thresholds

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Date:	Tue, 15 Dec 2009 20:09:27 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	containers@...ts.linux-foundation.org, linux-mm@...ck.org,
	Paul Menage <menage@...gle.com>,
	Li Zefan <lizf@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Pavel Emelyanov <xemul@...nvz.org>,
	Dan Malek <dan@...eddedalley.com>,
	Vladislav Buzov <vbuzov@...eddedalley.com>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC v2 4/4] memcg: implement memory thresholds

On Tue, 15 Dec 2009 12:46:32 +0200
"Kirill A. Shutemov" <kirill@...temov.name> wrote:

> On Tue, Dec 15, 2009 at 3:58 AM, KAMEZAWA Hiroyuki
> <kamezawa.hiroyu@...fujitsu.com> wrote:
> > On Sat, 12 Dec 2009 00:59:19 +0200
> > "Kirill A. Shutemov" <kirill@...temov.name> wrote:

> > If you use have to use spinlock here, this is a system-wide spinlock,
> > threshold as "100" is too small, I think.
> 
> What is reasonable value for THRESHOLDS_EVENTS_THRESH for you?
> 
> In most cases spinlock taken only for two checks. Is it significant time?
> 
I tend to think about "bad case" when I see spinlock. 

And...I'm not sure but, recently, there are many VM users.
spinlock can be a big pitfall in some enviroment if not para-virtualized.
(I'm sorry I misunderstand somehing and VM handle this well...)

> Unfortunately, I can't test it on a big box. I have only dual-core system.
> It's not enough to test scalability.
> 

please leave it as 100 for now. But there is a chance to do simple optimization
for reducing the number of checks.

example)
static void mem_cgroup_threshold(struct mem_cgroup *memcg, bool swap)
{
	/* For handle memory allocation in rush, check jiffies */
	*/
	smp_rmb();
	if (memcg->last_checkpoint_jiffies == jiffies)
		return;   /* reset event to half value ..*/
	memcg->last_checkpoint_jiffies = jiffies;
	smp_wmb();
	.....

I think this kind of check is necessary for handle "Rushing" memory allocation
in scalable way. Above one is just an example, 1 tick may be too long.

Other simple plan is

	/* Allow only one thread to do scan the list at the same time. */
	if (atomic_inc_not_zero(&memcg->threahold_scan_count) {
		atomic_dec(&memcg->threshold_scan_count);
		return;
	}
	...
	atomic_dec(&memcg->threahold_scan_count)

Some easy logic (as above) for taking care of scalability and commenary for that
is enough at 1st stage. Then, if there seems to be a trouble/concern, someone
(me?) will do some work later.




Thanks,
-Kame

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