From: Andrew Morton Subject: Re: [PATCH 1/3] block_write_full_page: Use synchronous writes for WBC_SYNC_ALL writebacks Date: Mon, 6 Apr 2009 23:50:52 -0700 Message-ID: <20090406235052.1ea47513.akpm@linux-foundation.org> References: <1238185471-31152-1-git-send-email-tytso@mit.edu> <1238185471-31152-2-git-send-email-tytso@mit.edu> <20090406232141.ebdb426a.akpm@linux-foundation.org> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit To: "Theodore Ts'o" , Linux Kernel Developers List , Ext4 Developers List , jack@suse.cz, Jens Axboe Received: from smtp1.linux-foundation.org ([140.211.169.13]:49493 "EHLO smtp1.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751908AbZDGGys (ORCPT ); Tue, 7 Apr 2009 02:54:48 -0400 In-Reply-To: <20090406232141.ebdb426a.akpm@linux-foundation.org> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Mon, 6 Apr 2009 23:21:41 -0700 Andrew Morton wrote: > I mean, let's graph it: > > WRITE_SYNC -> WRITE_SYNC_PLUG -> BIO_RW_SYNCIO -> bio_sync() -> REQ_RW_SYNC -> rw_is_sync() -> does something mysterious in get_request() > -> rq_is_sync() -> does something mysterious in IO schedulers > -> BIO_RW_NOIDLE -> bio_noidle() -> REQ_NOIDLE -> rq_noidle() -> does something mysterious in cfq-iosched only > -> BIO_RW_UNPLUG -> bio_unplug() -> REQ_UNPLUG -> OK, the cognoscenti know what this is supposed to do, but it is unused! whoop, I found a use of bio_unplug() in __make_request(). So it appears that the intent of your patch is to cause an unplug after submission of each WB_SYNC_ALL block? But what about all the other stuff which WRITE_SYNC might or might not do? What does WRITE_SYNC _actually_ do, and what are the actual effects of this change?? And what effect will this large stream of unplugs have upon merging?