From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754618AbbBJWKg (ORCPT ); Tue, 10 Feb 2015 17:10:36 -0500 Received: from smtp.infotech.no ([82.134.31.41]:43698 "EHLO smtp.infotech.no" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751707AbbBJWKe (ORCPT ); Tue, 10 Feb 2015 17:10:34 -0500 Message-ID: <54DA81CA.2030303@interlog.com> Date: Tue, 10 Feb 2015 17:10:18 -0500 From: Douglas Gilbert Reply-To: dgilbert@interlog.com User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.4.0 MIME-Version: 1.0 To: Andrej Gelenberg , JBottomley@parallels.com, linux-scsi@vger.kernel.org, linux-kernel@vger.kernel.org, hch@lst.de, martin.petersen@oracle.com, elliott@hp.com Subject: Re: CONFIG_SCSI_MQ_DEFAULT cause slow direct write speed to usb stick References: <20150210195036.3dce0e9e@bigpc> In-Reply-To: <20150210195036.3dce0e9e@bigpc> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 15-02-10 01:50 PM, Andrej Gelenberg wrote: > Hi, > > i had found a problem with CONFIG_SCSI_MQ_DEFAULT config option. If it > activated, then the write speeds to the /dev/sd* of an usb stick drops > dramatically: it's only about 250 kb/CONFIG_SCSI_MQ_DEFAULTs, but > should be about 7 Mb/s. Git bisect also points to commit > 24c20f10583647e30afe87b6f6d5e14bc7b1cbc6 'scsi: add a > CONFIG_SCSI_MQ_DEFAULT option' (i always set CONFIG_SCSI_MQ_DEFAULT to > y, because it sounded interesting). Same problem is also in 3.19 > present. Write speeds to a filesystem on that USB-Stick were not so > bad, but as i tried to dd in Live-DVD Ubuntu image it was painfully > slow. After i disabled CONFIG_SCSI_MQ_DEFAULT write speeds are back to > normal. > > I created bug report in bugzilla: > https://bugzilla.kernel.org/show_bug.cgi?id=92571 Perhaps a related datapoint: when reading from a uSD card via a USB dongle in lk 3.19 (SCSI_MQ set), blk_get_request() sometimes returns EAGAIN to the sg driver. I'm not sure that I have seen the SG_IO ioctl return EAGAIN via this route before (i.e. before it can even set up the SCSI command). Arguably doing that breaks the long standing interface of ioctl(sg, SG_IO). Faster storage devices do not seem to have this problem. The laptop involved has plenty of ram and was lightly loaded. This was a synchronous copy of a slow device onto local storage (a SSD) so it is hard to see why there might be a resource problem. By adjusting my user space code (ddpt and sg_dd) to repeat the ioctl, the copy continues normally. Here is an example: # ddpt if=/dev/sg1 bs=512 of=x.bin 1953792+0 records in 1953792+0 records out 1281 retries after EAGAIN error(s) during IO time to transfer data: 59.678788 secs at 16.76 MB/sec That is not a bad transfer time but other utilities or drivers might dwell longer on those (nuisance) EAGAINs. Doug Gilbert