From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from merlin.infradead.org ([205.233.59.134]:43637 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932378Ab3E0NeB (ORCPT ); Mon, 27 May 2013 09:34:01 -0400 Date: Mon, 27 May 2013 15:33:32 +0200 From: Jens Axboe Subject: Re: Any Delay Introduced by FIO between submission of Two IOs? Message-ID: <20130527133332.GH29680@kernel.dk> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: fio-owner@vger.kernel.org List-Id: fio@vger.kernel.org To: sampath rapaka Cc: "fio@vger.kernel.org" On Mon, May 27 2013, sampath rapaka wrote: > hi > > i have run couple of fio runs. i am running this over raid1 device. i > have captured blktrace for each run. I was going through the > blktraces. what i have observed is there instances where there is > commit happening to disk at say time stamp X. I was expecting a new IO > to be submitted at X+1 time stamp. but i see next IO is submitted at > time stamp Y. > > Y-X is more than 1. Is there any delay introduced from FIO between two > fios. or what could be the probable reasons to see such delay? let me > know if my understanding is wrong. > > Below is the excerpt from blktrace: > > 21378 8,32 3 9162 0.116015463 0 C WS 574576152 + 8 [0] > 21379 8,16 3 10690 0.116020386 0 C WS 574576152 + 8 [0] > 21380 9,0 0 1 0.120074779 8797 Q WS 131300520 + 8 [fitness] > > from lines 21379(C) to 21380(Q) there is lots of delay for submission of new IO. > > So i was thinking in lines of any delay from FIO between submissions > of IO or they submit IO as soon as it is available. > > Question 2: > I have also observed larger delays in Q-D values. which is more than D-C value. > > What could be the possible reasons for this behavior ? Fio doesn't introduce any delays on purpose. What job file are you running? It could either be that fio is scheduled out or in other ways in a non-running state. Or perhaps fio is waiting for completions. Line 21380 above is something called 'fitness', what is that? -- Jens Axboe