Re: [PATCH] writeback: remove the internal 5% low bound on dirty_ratio
From: Neil Brown <hidden>
Date: 2010-08-26 01:37:07
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On Thu, 26 Aug 2010 09:29:45 +0800 Wu Fengguang [off-list ref] wrote:
On Wed, Aug 25, 2010 at 04:40:00PM +0800, Con Kolivas wrote:quoted
On Tue, 24 Aug 2010 05:14:40 pm you wrote:quoted
On Tue, Aug 24, 2010 at 02:20:54PM +0800, Con Kolivas wrote:quoted
On Mon, 23 Aug 2010 05:15:35 pm you wrote:quoted
On Mon, Aug 23, 2010 at 02:30:40PM +0800, Con Kolivas wrote:quoted
On Mon, 23 Aug 2010 04:23:59 pm Wu Fengguang wrote:quoted
On Mon, Aug 23, 2010 at 12:42:48PM +0800, Neil Brown wrote:quoted
On Fri, 20 Aug 2010 15:50:54 +1000 Con Kolivas [off-list ref] wrote:quoted
On Fri, 20 Aug 2010 02:13:25 pm KOSAKI Motohiro wrote:quoted
quoted
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 <redacted> Signed-off-by: Wu Fengguang <redacted>Thank you. Reviewed-by: KOSAKI Motohiro [off-list ref]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..Indeed. And while you shouldn't set the lower limit to zero to avoid this problem, it doesn't answer _why_ this happens. What is this "minimum write" that blocks everything, will 1% be enough, and is it hiding another real bug somewhere in the VM?Good question. This simple change will unblock the application even with vm_dirty_ratio=0. # echo 0 > /proc/sys/vm/dirty_ratio # echo 0 > /proc/sys/vm/dirty_background_ratio # vmmon nr_dirty nr_writeback nr_unstable nr_dirty nr_writeback nr_unstable 0 444 1369 37 37 326 0 0 37 74 772 694 0 0 19 0 0 1406 0 0 23 0 0 0 0 370 186 74 1073 1221 0 12 26 0 703 1147 37 0 999 37 37 1517 0 888 63 0 0 0 0 0 20 37 0 0 37 74 1776 0 0 8 37 629 333 0 12 19 Even with it, the 1% explicit bound still looks reasonable for me. Who will want to set it to 0%? That would destroy IO inefficient.Thanks for your work in this area. I'll experiment with these later. There are low latency applications that would benefit with it set to zero.It might be useful to some users. Shall we give the rope to users, heh? Note that for these applications, they may well use /proc/sys/vm/dirty_bytes for more fine grained control. That interface only imposes a low limit of 2 pages.I don't see why there needs to be a limit. Users fiddling with sysctls should know what they're messing with, and there may well be a valid use out there somewhere for it.OK, the following patch gives users the full freedom. I tested 1 single dirtier and 9 parallel dirtiers, the system remains alive, but with much slower IO throughput. Maybe not all users care IO performance in all situations? 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. Let's rip the internal bound. At the same time, fix balance_dirty_pages() to work with the dirty_thresh=0 case. This allows applications to proceed when dirty+writeback pages are all cleaned.
And ">" fits with the name "exceeded" better than ">=" does. I think it is an aesthetic improvement as well as a functional one. Reviewed-by: NeilBrown <redacted> Thanks, NeilBrown
quoted hunk ↗ jump to hunk
CC: Jan Kara <jack@suse.cz> CC: Neil Brown <redacted> CC: Con Kolivas <redacted> CC: Rik van Riel <redacted> CC: Peter Zijlstra <redacted> CC: KOSAKI Motohiro <redacted> Signed-off-by: Wu Fengguang <redacted> --- mm/page-writeback.c | 14 ++++---------- 1 file changed, 4 insertions(+), 10 deletions(-)--- linux-next.orig/mm/page-writeback.c 2010-08-26 08:37:31.000000000 +0800 +++ linux-next/mm/page-writeback.c 2010-08-26 08:37:55.000000000 +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);@@ -542,8 +536,8 @@ static void balance_dirty_pages(struct a * the last resort safeguard. */ dirty_exceeded = - (bdi_nr_reclaimable + bdi_nr_writeback >= bdi_thresh) - || (nr_reclaimable + nr_writeback >= dirty_thresh); + (bdi_nr_reclaimable + bdi_nr_writeback > bdi_thresh) + || (nr_reclaimable + nr_writeback > dirty_thresh); if (!dirty_exceeded) break;
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