Re: [PATCH 6/9] writeback: introduce smoothed global dirty limit

Andrea Righi
2011-07-01T15:21:01Z

On Wed, Jun 29, 2011 at 10:52:51PM +0800, Wu Fengguang wrote:
> The start of a heavy weight application (ie. KVM) may instantly knock
> down determine_dirtyable_memory() if the swap is not enabled or full.
> global_dirty_limits() and bdi_dirty_limit() will in turn get global/bdi
> dirty thresholds that are _much_ lower than the global/bdi dirty pages.
>
> balance_dirty_pages() will then heavily throttle all dirtiers including
> the light ones, until the dirty pages drop below the new dirty thresholds.
> During this _deep_ dirty-exceeded state, the system may appear rather
> unresponsive to the users.
>
> About "deep" dirty-exceeded: task_dirty_limit() assigns 1/8 lower dirty
> threshold to heavy dirtiers than light ones, and the dirty pages will
> be throttled around the heavy dirtiers' dirty threshold and reasonably
> below the light dirtiers' dirty threshold. In this state, only the heavy
> dirtiers will be throttled and the dirty pages are carefully controlled
> to not exceed the light dirtiers' dirty threshold. However if the
> threshold itself suddenly drops below the number of dirty pages, the
> light dirtiers will get heavily throttled.
>
> 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. Note that the new dirty limits are not going to avoid
> throttling the light dirtiers, but could limit their sleep time to 200ms.
>
> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>

...

> +static void global_update_bandwidth(unsigned long thresh,
> + unsigned long dirty,
> + unsigned long now)
> +{
> + static DEFINE_SPINLOCK(dirty_lock);
> + static unsigned long update_time;
> +
> + /*
> + * Do a lockless check first to optimize away locking for most time.
> + */
> + if (now - update_time < MAX_PAUSE)

if (time_before(now, update_time + MAX_PAUSE))

> + return;
> +
> + spin_lock(&dirty_lock);
> + if (now - update_time >= MAX_PAUSE) {

if (time_after_eq(now, update_time + MAX_PAUSE))

Thanks,
-Andrea