From mboxrd@z Thu Jan 1 00:00:00 1970 From: Neil Brown Subject: Re: [PATCH] writeback: remove the internal 5% low bound on dirty_ratio Date: Mon, 23 Aug 2010 14:42:48 +1000 Message-ID: <20100823144248.15fbb700@notabene> References: <20100820032506.GA6662@localhost> <20100820131249.5FF4.A69D9226@jp.fujitsu.com> <201008201550.54164.kernel@kolivas.org> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Cc: KOSAKI Motohiro , Wu Fengguang , Andrew Morton , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, riel@redhat.com, david@fromorbit.com, hch@lst.de, axboe@kernel.dk To: Con Kolivas Return-path: In-Reply-To: <201008201550.54164.kernel@kolivas.org> Sender: owner-linux-mm@kvack.org List-Id: linux-fsdevel.vger.kernel.org On Fri, 20 Aug 2010 15:50:54 +1000 Con Kolivas wrote: > On Fri, 20 Aug 2010 02:13:25 pm KOSAKI Motohiro wrote: > > > The dirty_ratio was silently limited to >= 5%. This is not a user > > > expected behavior. Let's rip it. > > > > > > It's not likely the user space will depend on the old behavior. > > > So the risk of breaking user space is very low. > > > > > > CC: Jan Kara > > > CC: Neil Brown > > > Signed-off-by: Wu Fengguang > > > > Thank you. > > Reviewed-by: KOSAKI Motohiro > > I have tried to do this in the past, and setting this value to 0 on some > machines caused the machine to come to a complete standstill with small > writes to disk. It seemed there was some kind of "minimum" amount of data > required by the VM before anything would make it to the disk and I never > quite found out where that blockade occurred. This was some time ago (3 years > ago) so I'm not sure if the problem has since been fixed in the VM since > then. I suggest you do some testing with this value set to zero before > approving this change. > If it is appropriate to have a lower limit, that should be imposed where the sysctl is defined in kernel/sysctl.c, not imposed after the fact where the value is used. As we now have dirty_bytes which over-rides dirty_ratio, there is little cost in having a lower_limit for dirty_ratio - it could even stay at 5% - but it really shouldn't be silent. Writing a number below the limit to the sysctl file should fail. NeilBrown -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org