Re: [PATCH 3/7] writeback: introduce smoothed global dirty limit

[Wu Fengguang <fengguang.wu@intel.com>]

On Sun, Jun 19, 2011 at 11:36:37PM +0800, Christoph Hellwig wrote:
> On Sun, Jun 19, 2011 at 11:01:11PM +0800, Wu Fengguang wrote:
> > The start of a heavy weight application (ie. KVM) may instantly knock
> > down determine_dirtyable_memory() and hence the global/bdi dirty
> > thresholds.
> > 
> > So introduce global_dirty_limit for tracking the global dirty threshold
> > with policies
> > 
> > - follow downwards slowly
> > - follow up in one shot
> > 
> > global_dirty_limit can effectively mask out the impact of sudden drop of
> > dirtyable memory. It will be used in the next patch for two new type of
> > dirty limits.
> 
> This needs to be explained in more detail in comments near the actual
> code.

Good point! This is the added comment for the update_dirty_limit() function.

/*
 * The global dirtyable memory and dirty threshold could be suddenly knocked
 * down by a large amount (eg. on the startup of KVM). This may throw the
 * system into deep dirty exceeded state and throttled to "death" for a couple
 * of seconds. The solution is to maintain global_dirty_limit for tracking
 * slowly down to the knocked down dirty threshold.
 */
static void update_dirty_limit(unsigned long thresh,
                               unsigned long dirty)

Thanks,
Fengguang