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Subject[PATCH 11/18] block: add bdi flag to indicate risk of io queue underrun
Hurry it up when there are less than 3 async requests in the block io queue:

1) don't dirty throttle the current dirtier

2) wakeup the flusher for background writeout (XXX: the flusher may then
abort not being aware of the underrun)

When doing 1-dd write test with dirty_bytes=1MB, it increased the XFS
writeout throughput from 5MB/s to 55MB/s and increased disk utilization
from ~3% to ~85%. ext4 achieves almost the same. However btrfs is not
good: it only does 1MB/s normally, with sudden rushes to 10-60MB/s.

CC: Tejun Heo <tj@kernel.org>
CC: Jens Axboe <axboe@kernel.dk>
CC: Li Shaohua <shaohua.li@intel.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
block/blk-core.c | 7 +++++++
include/linux/backing-dev.h | 18 ++++++++++++++++++
include/linux/blkdev.h | 12 ++++++++++++
mm/page-writeback.c | 3 +++
4 files changed, 40 insertions(+)
--- linux-next.orig/block/blk-core.c 2011-08-31 10:27:11.000000000 +0800
+++ linux-next/block/blk-core.c 2011-08-31 14:41:38.000000000 +0800
@@ -637,6 +637,10 @@ static void __freed_request(struct reque
{
struct request_list *rl = &q->rq;

+ if (rl->count[sync] <= q->in_flight[sync] &&
+ rl->count[!sync] == 0)
+ blk_set_queue_underrun(q, sync);
+
if (rl->count[sync] < queue_congestion_off_threshold(q))
blk_clear_queue_congested(q, sync);

@@ -738,6 +742,9 @@ static struct request *get_request(struc
if (rl->count[is_sync] >= (3 * q->nr_requests / 2))
goto out;

+ if (rl->count[is_sync] >= q->in_flight[is_sync] + BLK_UNDERRUN_REQUESTS)
+ blk_clear_queue_underrun(q, is_sync);
+
rl->count[is_sync]++;
rl->starved[is_sync] = 0;

--- linux-next.orig/include/linux/blkdev.h 2011-08-31 10:27:11.000000000 +0800
+++ linux-next/include/linux/blkdev.h 2011-08-31 10:49:43.000000000 +0800
@@ -699,6 +699,18 @@ static inline void blk_set_queue_congest
set_bdi_congested(&q->backing_dev_info, sync);
}

+#define BLK_UNDERRUN_REQUESTS 3
+
+static inline void blk_clear_queue_underrun(struct request_queue *q, int sync)
+{
+ clear_bdi_underrun(&q->backing_dev_info, sync);
+}
+
+static inline void blk_set_queue_underrun(struct request_queue *q, int sync)
+{
+ set_bdi_underrun(&q->backing_dev_info, sync);
+}
+
extern void blk_start_queue(struct request_queue *q);
extern void blk_stop_queue(struct request_queue *q);
extern void blk_sync_queue(struct request_queue *q);
--- linux-next.orig/include/linux/backing-dev.h 2011-08-31 10:27:11.000000000 +0800
+++ linux-next/include/linux/backing-dev.h 2011-08-31 10:49:43.000000000 +0800
@@ -32,6 +32,7 @@ enum bdi_state {
BDI_sync_congested, /* The sync queue is getting full */
BDI_registered, /* bdi_register() was done */
BDI_writeback_running, /* Writeback is in progress */
+ BDI_async_underrun, /* The async queue is getting underrun */
BDI_unused, /* Available bits start here */
};

@@ -301,6 +302,23 @@ void set_bdi_congested(struct backing_de
long congestion_wait(int sync, long timeout);
long wait_iff_congested(struct zone *zone, int sync, long timeout);

+static inline void clear_bdi_underrun(struct backing_dev_info *bdi, int sync)
+{
+ if (sync == BLK_RW_ASYNC)
+ clear_bit(BDI_async_underrun, &bdi->state);
+}
+
+static inline void set_bdi_underrun(struct backing_dev_info *bdi, int sync)
+{
+ if (sync == BLK_RW_ASYNC)
+ set_bit(BDI_async_underrun, &bdi->state);
+}
+
+static inline int bdi_async_underrun(struct backing_dev_info *bdi)
+{
+ return bdi->state & (1 << BDI_async_underrun);
+}
+
static inline bool bdi_cap_writeback_dirty(struct backing_dev_info *bdi)
{
return !(bdi->capabilities & BDI_CAP_NO_WRITEBACK);
--- linux-next.orig/mm/page-writeback.c 2011-08-31 10:49:43.000000000 +0800
+++ linux-next/mm/page-writeback.c 2011-08-31 14:40:58.000000000 +0800
@@ -1067,6 +1067,9 @@ static void balance_dirty_pages(struct a
nr_dirty, bdi_thresh, bdi_dirty,
start_time);

+ if (unlikely(!dirty_exceeded && bdi_async_underrun(bdi)))
+ break;
+
dirty_ratelimit = bdi->dirty_ratelimit;
pos_ratio = bdi_position_ratio(bdi, dirty_thresh,
background_thresh, nr_dirty,



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