linux-kernel - Re: [PATCH 8/8] vm: Add an tuning knob for vm.max_writeback

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Date:	Tue, 29 Sep 2009 19:35:06 +0200
From:	Jan Kara <jack@...e.cz>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Jan Kara <jack@...e.cz>, Peter Zijlstra <peterz@...radead.org>,
	Chris Mason <chris.mason@...cle.com>,
	Artem Bityutskiy <dedekind1@...il.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"david@...morbit.com" <david@...morbit.com>,
	"hch@...radead.org" <hch@...radead.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	Theodore Ts'o <tytso@....edu>
Subject: Re: [PATCH 8/8] vm: Add an tuning knob for vm.max_writeback_mb

On Thu 24-09-09 16:33:42, Wu Fengguang wrote:
> On Mon, Sep 14, 2009 at 07:17:21PM +0800, Jan Kara wrote:
> > On Thu 10-09-09 17:49:10, Peter Zijlstra wrote:
> > > On Wed, 2009-09-09 at 16:23 +0200, Jan Kara wrote:
> > > >   Well, what I imagined we could do is:
> > > > Have a per-bdi variable 'pages_written' - that would reflect the amount of
> > > > pages written to the bdi since boot (OK, we'd have to handle overflows but
> > > > that's doable).
> > > > 
> > > > There will be a per-bdi variable 'pages_waited'. When a thread should sleep
> > > > in balance_dirty_pages() because we are over limits, it kicks writeback thread
> > > > and does:
> > > >   to_wait =  max(pages_waited, pages_written) + sync_dirty_pages() (or
> > > > whatever number we decide)
> > > >   pages_waited = to_wait
> > > >   sleep until pages_written reaches to_wait or we drop below dirty limits.
> > > > 
> > > > That will make sure each thread will sleep until writeback threads have done
> > > > their duty for the writing thread.
> > > > 
> > > > If we make sure sleeping threads are properly ordered on the wait queue,
> > > > we could always wakeup just the first one and thus avoid the herding
> > > > effect. When we drop below dirty limits, we would just wakeup the whole
> > > > waitqueue.
> > > > 
> > > > Does this sound reasonable?
> > > 
> > > That seems to go wrong when there's multiple tasks waiting on the same
> > > bdi, you'd count each page for 1/n its weight.
> > > 
> > > Suppose pages_written = 1024, and 4 tasks block and compute their to
> > > wait as pages_written + 256 = 1280, then we'd release all 4 of them
> > > after 256 pages are written, instead of 4*256, which would be
> > > pages_written = 2048.
> >   Well, there's some locking needed of course. The intent is to stack
> > demands as they come. So in case pages_written = 1024, pages_waited = 1024
> > we would do:
> > THREAD 1:
> > 
> > spin_lock
> > to_wait = 1024 + 256
> > pages_waited = 1280
> > spin_unlock
> > 
> > THREAD 2:
> > 
> > spin_lock
> > to_wait = 1280 + 256
> > pages_waited = 1536
> > spin_unlock
> > 
> >   So weight of each page will be kept. The fact that second thread
> > effectively waits until the first thread has its demand satisfied looks
> > strange at the first sight but we don't do better currently and I think
> > it's fine - if they were two writer threads, then soon the thread released
> > first will queue behind the thread still waiting so long term the behavior
> > should be fair.
> 
> Yeah, FIFO queuing should be good enough.
> 
> I'd like to propose one more data structure for evaluation :)
> 
> - bdi->throttle_lock
> - bdi->throttle_list    pages to sync for each waiting task, taken from sync_writeback_pages()
> - bdi->throttle_pages   (counted down) pages to sync for the head task, shall be atomic_t
> 
> In balance_dirty_pages(), it would do
> 
>         nr_to_sync = sync_writeback_pages()
>         if (list_empty(bdi->throttle_list))  # I'm the only task
>                 bdi->throttle_pages = nr_to_sync
>         append nr_to_sync to bdi->throttle_list
>         kick off background writeback
>         wait
>         remove itself from bdi->throttle_list and wait list
>         set bdi->throttle_pages for new head task (or LONG_MAX)
> 
> In __bdi_writeout_inc(), it would do
> 
>         if (--bdi->throttle_pages <= 0)
>                 check and wake up head task
  Yeah, this would work as well. I don't see a big difference between my
approach and this so if you get to implementing this, I'm happy :).

> In wb_writeback(), it would do
> 
>         if (args->for_background && exiting)
>                 wake up all throttled tasks
> To prevent wake up too many tasks at the same time, it can relax the
> background threshold a bit, so that __bdi_writeout_inc() become the
> only wake up point in normal cases.
> 
>         if (args->for_background && !list_empty(bdi->throttle_list) &&
>                 over background_thresh - background_thresh / 32)
>                 keep write pages;
  We want to wakeup tasks when we get below dirty_limit (either global
or per-bdi). Not when we get below background threshold...

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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