From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail172.messagelabs.com (mail172.messagelabs.com [216.82.254.3]) by kanga.kvack.org (Postfix) with ESMTP id DB0C36B00C8 for ; Sun, 4 Jan 2009 19:21:12 -0500 (EST) Date: Sun, 4 Jan 2009 19:21:09 -0500 From: Theodore Tso Subject: Re: [PATCH, RFC] Use WRITE_SYNC in __block_write_full_page() if WBC_SYNC_ALL Message-ID: <20090105002109.GI22958@mit.edu> References: <20090104142303.98762f81.akpm@linux-foundation.org> <20090104224351.GF22958@mit.edu> <20090104151927.1f1603c6.akpm@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20090104151927.1f1603c6.akpm@linux-foundation.org> Sender: owner-linux-mm@kvack.org To: Andrew Morton Cc: linux-mm@kvack.org, linux-ext4@vger.kernel.org, Arjan van de Ven , Jens Axboe List-ID: On Sun, Jan 04, 2009 at 03:19:27PM -0800, Andrew Morton wrote: > > Still, if we are > > submitting I/O which we are going to end up waiting on, we really > > should submit it with WRITE_SYNC, and this patch should optimize > > writes in other situations; for example, if we fsync() a file, we will > > also end up calling block_write_full_page(), and so supplying the > > WRITE_SYNC hint to the block layer would be a Good Thing. > > Is it? WRITE_SYNC means "unplug the queue after this bh/BIO". By setting > it against every bh, don't we risk the generation of more BIOs and > the loss of merging opportunities? Good point, yeah, that's a problem. Some of IO schedulers also use REQ_RW_SYNC to prioritize the I/O's above non-sync I/O's. That's an orthognal issue to unplugging the queue; it would be useful to be able to mark an I/O as "this is bio is one that we will eventually end up waiting to complete", separately from "please unplug the the queue after this bio submitted". BTW, I notice that the CFQ io scheduler prioritizes REQ_RW_META bio's behind REQ_RW_SYNC bio's, but ahead of normal bio requeuss. But as far as I can tell nothing is actually marking requests REQ_RW_META. What is the intended use for this, and are there plans to make other I/O schedulers honor REQ_RW_META? - Ted -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org