From: Wu Fengguang <fengguang.wu@intel.com> To: Neil Brown <neilb@suse.de> Cc: Con Kolivas <kernel@kolivas.org>, KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>, Andrew Morton <akpm@linux-foundation.org>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>, "linux-mm@kvack.org" <linux-mm@kvack.org>, "riel@redhat.com" <riel@redhat.com>, "david@fromorbit.com" <david@fromorbit.com>, "hch@lst.de" <hch@lst.de>, "axboe@kernel.dk" <axboe@kernel.dk> Subject: Re: [PATCH] writeback: remove the internal 5% low bound on dirty_ratio Date: Mon, 23 Aug 2010 14:23:59 +0800 [thread overview] Message-ID: <20100823062359.GA19586@localhost> (raw) In-Reply-To: <20100823144248.15fbb700@notabene> On Mon, Aug 23, 2010 at 12:42:48PM +0800, Neil Brown wrote: > On Fri, 20 Aug 2010 15:50:54 +1000 > Con Kolivas <kernel@kolivas.org> 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 <jack@suse.cz> > > > > CC: Neil Brown <neilb@suse.de> > > > > Signed-off-by: Wu Fengguang <fengguang.wu@intel.com> > > > > > > Thank you. > > > Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> > > > > 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. You are right, vm.dirty_ratio=0 will block applications for ever.. > > 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. How about imposing an explicit bound of 1%? That's more natural and its risk of breaking user space should be lower than 5%. Thanks, Fengguang --- writeback: remove the internal 5% low bound on dirty_ratio The dirty_ratio was silently limited in global_dirty_limits() to >= 5%. This is not a user expected behavior. And it's inconsistent with calc_period_shift(), which uses the plain vm_dirty_ratio value. So let's rip the internal bound. At the same time, force a user visible low bound of 1% for the vm.dirty_ratio interface. Applications trying to write 0 will be rejected with -EINVAL. This will break user space applications if they 1) try to write 0 to vm.dirty_ratio 2) and check the return value That is very weird combination, so the risk of breaking user space is low. CC: Jan Kara <jack@suse.cz> CC: Neil Brown <neilb@suse.de> CC: Rik van Riel <riel@redhat.com> CC: Con Kolivas <kernel@kolivas.org> CC: Peter Zijlstra <a.p.zijlstra@chello.nl> CC: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com> --- kernel/sysctl.c | 2 +- mm/page-writeback.c | 10 ++-------- 2 files changed, 3 insertions(+), 9 deletions(-) --- linux-next.orig/mm/page-writeback.c 2010-08-20 20:14:11.000000000 +0800 +++ linux-next/mm/page-writeback.c 2010-08-23 10:31:01.000000000 +0800 @@ -415,14 +415,8 @@ void global_dirty_limits(unsigned long * if (vm_dirty_bytes) dirty = DIV_ROUND_UP(vm_dirty_bytes, PAGE_SIZE); - else { - int dirty_ratio; - - dirty_ratio = vm_dirty_ratio; - if (dirty_ratio < 5) - dirty_ratio = 5; - dirty = (dirty_ratio * available_memory) / 100; - } + else + dirty = (vm_dirty_ratio * available_memory) / 100; if (dirty_background_bytes) background = DIV_ROUND_UP(dirty_background_bytes, PAGE_SIZE); --- linux-next.orig/kernel/sysctl.c 2010-08-23 14:06:11.000000000 +0800 +++ linux-next/kernel/sysctl.c 2010-08-23 14:07:30.000000000 +0800 @@ -1029,7 +1029,7 @@ static struct ctl_table vm_table[] = { .maxlen = sizeof(vm_dirty_ratio), .mode = 0644, .proc_handler = dirty_ratio_handler, - .extra1 = &zero, + .extra1 = &one, .extra2 = &one_hundred, }, {
WARNING: multiple messages have this Message-ID (diff)
From: Wu Fengguang <fengguang.wu@intel.com> To: Neil Brown <neilb@suse.de> Cc: Con Kolivas <kernel@kolivas.org>, KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>, Andrew Morton <akpm@linux-foundation.org>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>, "linux-mm@kvack.org" <linux-mm@kvack.org>, "riel@redhat.com" <riel@redhat.com>, "david@fromorbit.com" <david@fromorbit.com>, "hch@lst.de" <hch@lst.de>, "axboe@kernel.dk" <axboe@kernel.dk> Subject: Re: [PATCH] writeback: remove the internal 5% low bound on dirty_ratio Date: Mon, 23 Aug 2010 14:23:59 +0800 [thread overview] Message-ID: <20100823062359.GA19586@localhost> (raw) In-Reply-To: <20100823144248.15fbb700@notabene> On Mon, Aug 23, 2010 at 12:42:48PM +0800, Neil Brown wrote: > On Fri, 20 Aug 2010 15:50:54 +1000 > Con Kolivas <kernel@kolivas.org> 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 <jack@suse.cz> > > > > CC: Neil Brown <neilb@suse.de> > > > > Signed-off-by: Wu Fengguang <fengguang.wu@intel.com> > > > > > > Thank you. > > > Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> > > > > 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. You are right, vm.dirty_ratio=0 will block applications for ever.. > > 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. How about imposing an explicit bound of 1%? That's more natural and its risk of breaking user space should be lower than 5%. Thanks, Fengguang --- writeback: remove the internal 5% low bound on dirty_ratio The dirty_ratio was silently limited in global_dirty_limits() to >= 5%. This is not a user expected behavior. And it's inconsistent with calc_period_shift(), which uses the plain vm_dirty_ratio value. So let's rip the internal bound. At the same time, force a user visible low bound of 1% for the vm.dirty_ratio interface. Applications trying to write 0 will be rejected with -EINVAL. This will break user space applications if they 1) try to write 0 to vm.dirty_ratio 2) and check the return value That is very weird combination, so the risk of breaking user space is low. CC: Jan Kara <jack@suse.cz> CC: Neil Brown <neilb@suse.de> CC: Rik van Riel <riel@redhat.com> CC: Con Kolivas <kernel@kolivas.org> CC: Peter Zijlstra <a.p.zijlstra@chello.nl> CC: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com> --- kernel/sysctl.c | 2 +- mm/page-writeback.c | 10 ++-------- 2 files changed, 3 insertions(+), 9 deletions(-) --- linux-next.orig/mm/page-writeback.c 2010-08-20 20:14:11.000000000 +0800 +++ linux-next/mm/page-writeback.c 2010-08-23 10:31:01.000000000 +0800 @@ -415,14 +415,8 @@ void global_dirty_limits(unsigned long * if (vm_dirty_bytes) dirty = DIV_ROUND_UP(vm_dirty_bytes, PAGE_SIZE); - else { - int dirty_ratio; - - dirty_ratio = vm_dirty_ratio; - if (dirty_ratio < 5) - dirty_ratio = 5; - dirty = (dirty_ratio * available_memory) / 100; - } + else + dirty = (vm_dirty_ratio * available_memory) / 100; if (dirty_background_bytes) background = DIV_ROUND_UP(dirty_background_bytes, PAGE_SIZE); --- linux-next.orig/kernel/sysctl.c 2010-08-23 14:06:11.000000000 +0800 +++ linux-next/kernel/sysctl.c 2010-08-23 14:07:30.000000000 +0800 @@ -1029,7 +1029,7 @@ static struct ctl_table vm_table[] = { .maxlen = sizeof(vm_dirty_ratio), .mode = 0644, .proc_handler = dirty_ratio_handler, - .extra1 = &zero, + .extra1 = &one, .extra2 = &one_hundred, }, { -- 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: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
next prev parent reply other threads:[~2010-08-23 6:24 UTC|newest] Thread overview: 33+ messages / expand[flat|nested] mbox.gz Atom feed top 2010-08-20 3:25 [PATCH] writeback: remove the internal 5% low bound on dirty_ratio Wu Fengguang 2010-08-20 3:25 ` Wu Fengguang 2010-08-20 3:46 ` Rik van Riel 2010-08-20 3:46 ` Rik van Riel 2010-08-20 4:13 ` KOSAKI Motohiro 2010-08-20 4:13 ` KOSAKI Motohiro 2010-08-20 5:50 ` Con Kolivas 2010-08-20 5:50 ` Con Kolivas 2010-08-20 5:56 ` Wu Fengguang 2010-08-20 5:56 ` Wu Fengguang 2010-08-23 4:42 ` Neil Brown 2010-08-23 4:42 ` Neil Brown 2010-08-23 6:23 ` Wu Fengguang [this message] 2010-08-23 6:23 ` Wu Fengguang 2010-08-23 6:30 ` Con Kolivas 2010-08-23 6:30 ` Con Kolivas 2010-08-23 7:15 ` Wu Fengguang 2010-08-23 7:15 ` Wu Fengguang 2010-08-24 0:00 ` KOSAKI Motohiro 2010-08-24 0:00 ` KOSAKI Motohiro [not found] ` <201008241620.54048.kernel@kolivas.org> [not found] ` <20100824071440.GA14598@localhost> [not found] ` <201008251840.00532.kernel@kolivas.org> 2010-08-26 1:29 ` Wu Fengguang 2010-08-26 1:29 ` Wu Fengguang 2010-08-26 1:36 ` Neil Brown 2010-08-26 1:36 ` Neil Brown 2010-08-26 4:22 ` KOSAKI Motohiro 2010-08-26 4:22 ` KOSAKI Motohiro 2010-08-27 10:36 Wu Fengguang 2010-08-27 10:36 ` Wu Fengguang 2010-08-27 10:39 ` Peter Zijlstra 2010-08-27 10:39 ` Peter Zijlstra 2010-08-27 10:39 ` Peter Zijlstra 2010-08-27 13:47 ` Rik van Riel 2010-08-27 13:47 ` Rik van Riel
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