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

Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
include/linux/writeback.h | 2 +
mm/page-writeback.c | 41 ++++++++++++++++++++++++++++++++++++
2 files changed, 43 insertions(+)
--- linux-next.orig/include/linux/writeback.h 2011-06-19 22:56:18.000000000 +0800
+++ linux-next/include/linux/writeback.h 2011-06-19 22:59:29.000000000 +0800
@@ -88,6 +88,8 @@ static inline void laptop_sync_completio
#endif
void throttle_vm_writeout(gfp_t gfp_mask);

+extern unsigned long global_dirty_limit;
+
/* These are exported to sysctl. */
extern int dirty_background_ratio;
extern unsigned long dirty_background_bytes;
--- linux-next.orig/mm/page-writeback.c 2011-06-19 22:56:18.000000000 +0800
+++ linux-next/mm/page-writeback.c 2011-06-19 22:59:29.000000000 +0800
@@ -116,6 +116,7 @@ EXPORT_SYMBOL(laptop_mode);

/* End of sysctl-exported parameters */

+unsigned long global_dirty_limit;

/*
* Scale the writeback cache size proportional to the relative writeout speeds.
@@ -510,6 +511,43 @@ static void bdi_update_write_bandwidth(s
bdi->avg_write_bandwidth = avg;
}

+static void update_dirty_limit(unsigned long thresh,
+ unsigned long dirty)
+{
+ unsigned long limit = global_dirty_limit;
+
+ if (limit < thresh) {
+ limit = thresh;
+ goto update;
+ }
+
+ if (limit > thresh &&
+ limit > dirty) {
+ limit -= (limit - max(thresh, dirty)) >> 5;
+ goto update;
+ }
+ return;
+update:
+ global_dirty_limit = limit;
+}
+
+static void global_update_bandwidth(unsigned long thresh,
+ unsigned long dirty,
+ unsigned long now)
+{
+ static DEFINE_SPINLOCK(dirty_lock);
+
+ if (now - default_backing_dev_info.bw_time_stamp < MAX_PAUSE)
+ return;
+
+ spin_lock(&dirty_lock);
+ if (now - default_backing_dev_info.bw_time_stamp >= MAX_PAUSE) {
+ update_dirty_limit(thresh, dirty);
+ default_backing_dev_info.bw_time_stamp = now;
+ }
+ spin_unlock(&dirty_lock);
+}
+
void __bdi_update_bandwidth(struct backing_dev_info *bdi,
unsigned long thresh,
unsigned long dirty,
@@ -535,6 +573,9 @@ void __bdi_update_bandwidth(struct backi
if (elapsed > HZ && time_before(bdi->bw_time_stamp, start_time))
goto snapshot;

+ if (thresh)
+ global_update_bandwidth(thresh, dirty, now);
+
bdi_update_write_bandwidth(bdi, elapsed, written);

snapshot:



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