Nick Piggin [off-list ref] wrote:

3rd attempt for this patch ;)
 I have since addressed your concerns.

 So for example, with a 10/40 async/sync ratio, if the sync
 watermark is moved down to 20, the async mark will be moved
 to 5, preserving the ratio.

Disagree.

[vm-tune-writeout.patch  text/x-patch (1365 bytes)]

 Slightly change the writeout watermark calculations so we keep background
 and synchronous writeout watermarks in the same ratios after adjusting them.
 This ensures we should always attempt to start background writeout before
 synchronous writeout.

 Signed-off-by: Nick Piggin [off-list ref]


 ---

  linux-2.6-npiggin/mm/page-writeback.c |    8 +++++---
  1 files changed, 5 insertions(+), 3 deletions(-)

 diff -puN mm/page-writeback.c~vm-tune-writeout mm/page-writeback.c
 --- linux-2.6/mm/page-writeback.c~vm-tune-writeout	2004-08-06 14:48:45.000000000 +1000
 +++ linux-2.6-npiggin/mm/page-writeback.c	2004-08-06 14:48:45.000000000 +1000
 @@ -153,9 +153,11 @@ get_dirty_limits(struct writeback_state 
  	if (dirty_ratio < 5)
  		dirty_ratio = 5;
  
 -	background_ratio = dirty_background_ratio;
 -	if (background_ratio >= dirty_ratio)
 -		background_ratio = dirty_ratio / 2;
 +	/*
 +	 * Keep the ratio between dirty_ratio and background_ratio roughly
 +	 * what the sysctls are after dirty_ratio has been scaled (above).
 +	 */
 +	background_ratio = dirty_background_ratio * dirty_ratio/vm_dirty_ratio;
  
  	background = (background_ratio * total_pages) / 100;
  	dirty = (dirty_ratio * total_pages) / 100;

Look, these are sysadmin-settable sysctls.  The admin can set them to
whatever wild and whacky values he wants - it's his computer.

The only reason the check is there at all is because background_ratio >
dirty_ratio has never been even tested, and could explode, and I don't want
to have to test and support it.  Plus if the admin is in the process of
setting both tunables there might be a transient period of time when
they're in a bad state.

That's all!  Please, just pretend the code isn't there at all.  What the
admin sets, the admin gets, end of story.
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