Re: [PATCH 7/8] writeback: sync old inodes first in background writeback

From: Minchan Kim
Date: Mon Jul 26 2010 - 00:12:34 EST

On Mon, Jul 26, 2010 at 12:27 PM, Wu Fengguang <fengguang.wu@xxxxxxxxx> wrote:
> On Sun, Jul 25, 2010 at 08:03:45PM +0800, Minchan Kim wrote:
>> On Sun, Jul 25, 2010 at 07:43:20PM +0900, KOSAKI Motohiro wrote:
>> > Hi
>> >
>> > sorry for the delay.
>> >
>> > > Will you be picking it up or should I? The changelog should be more or less
>> > > the same as yours and consider it
>> > >
>> > > Signed-off-by: Mel Gorman <mel@xxxxxxxxx>
>> > >
>> > > It'd be nice if the original tester is still knocking around and willing
>> > > to confirm the patch resolves his/her problem. I am running this patch on
>> > > my desktop at the moment and it does feel a little smoother but it might be
>> > > my imagination. I had trouble with odd stalls that I never pinned down and
>> > > was attributing to the machine being commonly heavily loaded but I haven't
>> > > noticed them today.
>> > >
>> > > It also needs an Acked-by or Reviewed-by from Kosaki Motohiro as it alters
>> > > logic he introduced in commit [78dc583: vmscan: low order lumpy reclaim also
>> > > should use PAGEOUT_IO_SYNC]
>> >
>> > My reviewing doesn't found any bug. however I think original thread have too many guess
>> > and we need to know reproduce way and confirm it.
>> >
>> > At least, we need three confirms.
>> >  o original issue is still there?
>> >  o DEF_PRIORITY/3 is best value?
>> I agree. Wu, how do you determine DEF_PRIORITY/3 of LRU?
>> I guess system has 512M and 22M writeback pages.
>> So you may determine it for skipping max 32M writeback pages.
>> Is right?
> For 512M mem, DEF_PRIORITY/3 means 32M dirty _or_ writeback pages.
> Because shrink_inactive_list() first calls
> shrink_page_list(PAGEOUT_IO_ASYNC) then optionally
> shrink_page_list(PAGEOUT_IO_SYNC), so dirty pages will first be
> converted to writeback pages and then optionally be waited on.
> The dirty/writeback pages may go up to 512M*20% = 100M. So 32M looks
> a reasonable value.

Why do you think it's a reasonable value?
I mean why isn't it good 12.5% or 3.125%? Why do you select 6.25%?
I am not against you. Just out of curiosity and requires more explanation.
It might be thing _only I_ don't know. :(

>> And I have a question of your below comment.
>> "As the default dirty throttle ratio is 20%, sync write&wait
>> will hardly be triggered by pure dirty pages"
>> I am not sure exactly what you mean but at least DEF_PRIOIRTY/3 seems to be
>> related to dirty_ratio. It always can be changed by admin.
>> Then do we have to determine magic value(DEF_PRIORITY/3)  proportional to dirty_ratio?
> Yes DEF_PRIORITY/3 is already proportional to the _default_
> dirty_ratio. We could do explicit comparison with dirty_ratio
> just in case dirty_ratio get changed by user. It's mainly a question
> of whether deserving to add such overheads and complexity. I'd prefer
> to keep the current simple form :)

What I suggest is that couldn't we use recent_writeback/recent_scanned ratio?
I think scan_control's new filed and counting wouldn't be a big
overhead and complexity.
I am not sure which ratio is best. but at least, it would make the
logic scalable and sense to me. :)

> Thanks,
> Fengguang

Kind regards,
Minchan Kim
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