From owner-freebsd-threads@FreeBSD.ORG  Thu Dec 24 01:20:30 2009
Return-Path: <owner-freebsd-threads@FreeBSD.ORG>
Delivered-To: threads@freebsd.org
Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34])
	by hub.freebsd.org (Postfix) with ESMTP id CEF7A106566B;
	Thu, 24 Dec 2009 01:20:30 +0000 (UTC)
	(envelope-from davidxu@freebsd.org)
Received: from freefall.freebsd.org (freefall.freebsd.org
	[IPv6:2001:4f8:fff6::28])
	by mx1.freebsd.org (Postfix) with ESMTP id BD53A8FC16;
	Thu, 24 Dec 2009 01:20:30 +0000 (UTC)
Received: from apple.my.domain (localhost [127.0.0.1])
	by freefall.freebsd.org (8.14.3/8.14.3) with ESMTP id nBO1KSg6001520;
	Thu, 24 Dec 2009 01:20:29 GMT (envelope-from davidxu@freebsd.org)
Message-ID: <4B32C1DC.9080308@freebsd.org>
Date: Thu, 24 Dec 2009 09:20:28 +0800
From: David Xu <davidxu@freebsd.org>
User-Agent: Thunderbird 2.0.0.9 (X11/20080612)
MIME-Version: 1.0
To: John Baldwin <jhb@freebsd.org>
References: <4B317741.8080004@freebsd.org> <200912230936.35998.jhb@freebsd.org>
In-Reply-To: <200912230936.35998.jhb@freebsd.org>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
Cc: threads@freebsd.org, freebsd-threads@freebsd.org
Subject: Re: first patch for process-shared semaphore
X-BeenThere: freebsd-threads@freebsd.org
X-Mailman-Version: 2.1.5
Precedence: list
List-Id: Threading on FreeBSD <freebsd-threads.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-threads>, 
	<mailto:freebsd-threads-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-threads>
List-Post: <mailto:freebsd-threads@freebsd.org>
List-Help: <mailto:freebsd-threads-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-threads>,
	<mailto:freebsd-threads-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Thu, 24 Dec 2009 01:20:30 -0000

John Baldwin wrote:
> On Tuesday 22 December 2009 8:49:53 pm David Xu wrote:
>> This is my first attempt to make process-shared mutex work, this means
>> you can mmap(MAP_SHARED) a memory area, and put semaphore there,
>> or you can sem_open a named semaphore, and just use it between
>> processes, the named semaphore uses file system and mmap(), directory 
>> /tmp/.semaphore is used as IPC directory, any named semaphore
>> locates in the directory. old semaphore implementation still exists
>> to make it binary compatible, it uses symbol version.
>>
>> http://people.freebsd.org/~davidxu/patch/shared_semaphore_1.patch
> 
> I would suggest that you leave named semaphores as they currently exist and 
> follow this approach instead:
>  
> 1) Named semaphores use ksem_*() still.
> 2) sem_init/sem_destroy operate on UTMX-backed semaphores identical to the 
> ones used in the current libthr code.  The semid_t structure now becomes the 
> full structure that libthr currently allocates with a flag to indicate if it 
> is a "system" semaphore or otherwise.  The pshared flag passed to sem_init() 
> can be used to set the sharing properties of the UMTX.
> 3) All of sem_init/sem_destroy is just in libc.  Just move the libthr 
> implementation bits into libc.
> 

ksem base shared semaphore is slow because whenever you call
sem_wait(), it always enters kernel even if count is non-zero,
sem_post() also always enters kernel even if there is no waiter.
but the new implementation is as simple as just an atomic operation
in these cases, I know another competitor OS is doing things in
this way.