Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751755AbZI0Qme (ORCPT ); Sun, 27 Sep 2009 12:42:34 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1750841AbZI0Qmd (ORCPT ); Sun, 27 Sep 2009 12:42:33 -0400 Received: from brick.kernel.dk ([93.163.65.50]:57737 "EHLO kernel.dk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750699AbZI0Qmd (ORCPT ); Sun, 27 Sep 2009 12:42:33 -0400 Date: Sun, 27 Sep 2009 18:42:36 +0200 From: Jens Axboe To: Mike Galbraith Cc: Vivek Goyal , Ulrich Lukas , linux-kernel@vger.kernel.org, containers@lists.linux-foundation.org, dm-devel@redhat.com, nauman@google.com, dpshah@google.com, lizf@cn.fujitsu.com, mikew@google.com, fchecconi@gmail.com, paolo.valente@unimore.it, ryov@valinux.co.jp, fernando@oss.ntt.co.jp, jmoyer@redhat.com, dhaval@linux.vnet.ibm.com, balbir@linux.vnet.ibm.com, righi.andrea@gmail.com, m-ikeda@ds.jp.nec.com, agk@redhat.com, akpm@linux-foundation.org, peterz@infradead.org, jmarchan@redhat.com, torvalds@linux-foundation.org, mingo@elte.hu, riel@redhat.com Subject: Re: IO scheduler based IO controller V10 Message-ID: <20090927164235.GA23126@kernel.dk> References: <1253820332-10246-1-git-send-email-vgoyal@redhat.com> <4ABC28DE.7050809@datenparkplatz.de> <20090925202636.GC15007@redhat.com> <1253976676.7005.40.camel@marge.simson.net> <1254034500.7933.6.camel@marge.simson.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1254034500.7933.6.camel@marge.simson.net> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2086 Lines: 61 On Sun, Sep 27 2009, Mike Galbraith wrote: > My dd vs load non-cached binary woes seem to be coming from backmerge. > > #if 0 /*MIKEDIDIT sand in gearbox?*/ > /* > * See if our hash lookup can find a potential backmerge. > */ > __rq = elv_rqhash_find(q, bio->bi_sector); > if (__rq && elv_rq_merge_ok(__rq, bio)) { > *req = __rq; > return ELEVATOR_BACK_MERGE; > } > #endif It's a given that not merging will provide better latency. We can't disable that or performance will suffer A LOT on some systems. There are ways to make it better, though. One would be to make the max request size smaller, but that would also hurt for streamed workloads. Can you try whether the below patch makes a difference? It will basically disallow merges to a request that isn't the last one. We should probably make the merging logic a bit more clever, since the below wont work well for two (or more) streamed cases. I'll think a bit about that. Note this is totally untested! diff --git a/block/elevator.c b/block/elevator.c index 1975b61..d00a72b 100644 --- a/block/elevator.c +++ b/block/elevator.c @@ -497,9 +497,17 @@ int elv_merge(struct request_queue *q, struct request **req, struct bio *bio) * See if our hash lookup can find a potential backmerge. */ __rq = elv_rqhash_find(q, bio->bi_sector); - if (__rq && elv_rq_merge_ok(__rq, bio)) { - *req = __rq; - return ELEVATOR_BACK_MERGE; + if (__rq) { + /* + * If requests are queued behind this one, disallow merge. This + * prevents streaming IO from continually passing new IO. + */ + if (elv_latter_request(q, __rq)) + return ELEVATOR_NO_MERGE; + if (elv_rq_merge_ok(__rq, bio)) { + *req = __rq; + return ELEVATOR_BACK_MERGE; + } } if (e->ops->elevator_merge_fn) -- Jens Axboe -- 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/