From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH 8/8] vm: Add an tuning knob for vm.max_writeback_mb
Date: Thu, 24 Sep 2009 17:38:16 +0200
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Cc: Jan Kara <jack@suse.cz>, Chris Mason <chris.mason@oracle.com>,
	Artem Bityutskiy <dedekind1@gmail.com>,
	Jens Axboe <jens.axboe@oracle.com>,
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	Theodore Ts'o <tytso@mit.edu>
To: Wu Fengguang <fengguang.wu@intel.com>
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On Thu, 2009-09-24 at 16:33 +0800, Wu Fengguang wrote:

> 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
> 
> 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;

Right, something like that ought to work well, or at least sounds like
worth a try ;-)