From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S265119AbUEYVry (ORCPT ); Tue, 25 May 2004 17:47:54 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S265128AbUEYVrF (ORCPT ); Tue, 25 May 2004 17:47:05 -0400 Received: from fw.osdl.org ([65.172.181.6]:16622 "EHLO mail.osdl.org") by vger.kernel.org with ESMTP id S265119AbUEYVp4 (ORCPT ); Tue, 25 May 2004 17:45:56 -0400 Date: Tue, 25 May 2004 14:48:36 -0700 From: Andrew Morton To: Olaf Hering Cc: linux-kernel@vger.kernel.org Subject: Re: very low performance on SCSI disks if device node is in tmpfs Message-Id: <20040525144836.1af59a96.akpm@osdl.org> In-Reply-To: <20040525184732.GB26661@suse.de> References: <20040525184732.GB26661@suse.de> X-Mailer: Sylpheed version 0.9.7 (GTK+ 1.2.10; i586-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Olaf Hering wrote: > > Any ideas why the location of the device node makes such a big > difference? I always wondered why my firewire is so dog slow with 2.6. > Now I know the reason: /dev is in tmpfs. > I dont see that with IDE disks, only with SCSI. This is truly bizarre. Reading /dev/sda I get 24MB/sec at 700 context switches/sec. Reading /mnt/tmpfs/sda it's 14MB/sec, 7000 switches/sec. /mnt/ramfs/sda is slow too. /mnt/hda5/sda is fast. I'd assumed that the kernel got the backing_dev_info's screwed up and the tmpfs node isn't doing readahead but that appears to not be the case (/dev/sda is still fast with zero readahead). You really, really get the weird-bug-of-the-month award for this one. I'll poke at it some more later on.