Message-ID: <4B27A417.3040206@redhat.com>
Date: Tue, 15 Dec 2009 09:58:31 -0500
From: Rik van Riel <riel@redhat.com>
Organization: Red Hat, Inc
User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.4pre) Gecko/20090922 Fedora/3.0-3.9.b4.fc12 Lightning/1.0pre Thunderbird/3.0b4
MIME-Version: 1.0
To: Mike Galbraith <efault@gmx.de>
CC: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>, lwoodman@redhat.com,
       akpm@linux-foundation.org, linux-mm@kvack.org,
       linux-kernel@vger.kernel.org, minchan.kim@gmail.com
Subject: Re: [PATCH 4/8] Use prepare_to_wait_exclusive() instead prepare_to_wait()
References: <20091214212936.BBBA.A69D9226@jp.fujitsu.com>	 <4B264CCA.5010609@redhat.com> <20091215085631.CDAD.A69D9226@jp.fujitsu.com> <1260855146.6126.30.camel@marge.simson.net>
In-Reply-To: <1260855146.6126.30.camel@marge.simson.net>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1723
Lines: 44

On 12/15/2009 12:32 AM, Mike Galbraith wrote:
> On Tue, 2009-12-15 at 09:45 +0900, KOSAKI Motohiro wrote:
>>> On 12/14/2009 07:30 AM, KOSAKI Motohiro wrote:
>>>> if we don't use exclusive queue, wake_up() function wake _all_ waited
>>>> task. This is simply cpu wasting.
>>>>
>>>> Signed-off-by: KOSAKI Motohiro<kosaki.motohiro@jp.fujitsu.com>
>>>
>>>>    		if (zone_watermark_ok(zone, sc->order, low_wmark_pages(zone),
>>>>    					0, 0)) {
>>>> -			wake_up(wq);
>>>> +			wake_up_all(wq);
>>>>    			finish_wait(wq,&wait);
>>>>    			sc->nr_reclaimed += sc->nr_to_reclaim;
>>>>    			return -ERESTARTSYS;
>>>
>>> I believe we want to wake the processes up one at a time
>>> here.

>> Actually, wake_up() and wake_up_all() aren't different so much.
>> Although we use wake_up(), the task wake up next task before
>> try to alloate memory. then, it's similar to wake_up_all().

That is a good point.  Maybe processes need to wait a little
in this if() condition, before the wake_up().  That would give
the previous process a chance to allocate memory and we can
avoid waking up too many processes.

> What happens to waiters should running tasks not allocate for a while?

When a waiter is woken up, it will either:
1) see that there is enough free memory and wake up the next guy, or
2) run shrink_zone and wake up the next guy

Either way, the processes that just got woken up will ensure that
the sleepers behind them in the queue will get woken up.

-- 
All rights reversed.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/