From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jeff Moyer Subject: Re: trying to understand READ_META, READ_SYNC, WRITE_SYNC & co Date: Mon, 21 Jun 2010 14:52:41 -0400 Message-ID: References: <20100621094828.GA30748@lst.de> <4C1F3916.4070608@kernel.dk> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Christoph Hellwig , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org To: Jens Axboe Return-path: Received: from mx1.redhat.com ([209.132.183.28]:36783 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752828Ab0FUSws (ORCPT ); Mon, 21 Jun 2010 14:52:48 -0400 In-Reply-To: <4C1F3916.4070608@kernel.dk> (Jens Axboe's message of "Mon, 21 Jun 2010 12:04:06 +0200") Sender: linux-fsdevel-owner@vger.kernel.org List-ID: Jens Axboe writes: > On 2010-06-21 11:48, Christoph Hellwig wrote: >> Now how do we use these flags in the block layer? >> >> - REQ_META >> >> The only place where we ever use this flag is inside the >> cfq scheduler. In cfq_choose_req we use it to give a meta >> request priority over one that doesn't have it. But before >> that we already do the same preference check with rw_is_sync, >> which evaluates to true for requests with that are either >> reads or have REQ_SYNC set. So for reads the REQ_META flag >> here effectively is a no-op, and for writes it gives less >> priority than REQ_SYNC. >> In addition to that we use it to account for pending metadata >> requests in cfq_rq_enqueued/cfq_remove_request which gets >> checked in cfq_should_preempt to give priority to a meta >> request if the other queue doesn't have any pending meta >> requests. But again this priority comes after a similar >> check for sync requests that checks if the other queue has >> been marked to have sync requests pending. > > It's also annotation for blktrace, so you can tell which parts of the IO > is meta data etc. The scheduler impact is questionable, I doubt it makes > a whole lot of difference. Really? Even after I showed the performance impact of setting that bit for journal I/O? http://lkml.org/lkml/2010/4/1/344 Cheers, Jeff