From: Zou Mingzhe <mingzhe.zou@xxxxxxxxxxxx>
When attaching a cached device (a.k.a backing device) to a cache
device, bch_sectors_dirty_init() is called to count dirty sectors
and stripes (see what bcache_dev_sectors_dirty_add() does) on the
cache device.
When bcache_dev_sectors_dirty_add() is called, set_bit(stripe,
d->full_dirty_stripes) or clear_bit(stripe, d->full_dirty_stripes)
operation will always be performed. In full_dirty_stripes, each 1bit
represents stripe_size (8192) sectors (512B), so 1bit=4MB (8192*512),
and each CPU cache line=64B=512bit=2048MB. When 20 threads process
a cached disk with 100G dirty data, a single thread processes about
23M at a time, and 20 threads total 460M. These full_dirty_stripes
bits corresponding to the 460M data is likely to fall in the same CPU
cache line. When one of these threads performs a set_bit or clear_bit
operation, the same CPU cache line of other threads will become invalid
and must read the full_dirty_stripes from the main memory again. Compared
with single thread, the time of a bcache_dev_sectors_dirty_add()
call is increased by about 50 times in our test (100G dirty data,
20 threads, bcache_dev_sectors_dirty_add() is called more than
20 million times).
This patch tries to test_bit before set_bit or clear_bit operation.
Therefore, a lot of force set and clear operations will be avoided,
and most of bcache_dev_sectors_dirty_add() calls will only read CPU
cache line.
Signed-off-by: Mingzhe Zou <mingzhe.zou@xxxxxxxxxxxx>
---
drivers/md/bcache/writeback.c | 11 +++++++----
1 file changed, 7 insertions(+), 4 deletions(-)
diff --git a/drivers/md/bcache/writeback.c b/drivers/md/bcache/writeback.c
index 68e75c692dd4..4afe22875d4f 100644
--- a/drivers/md/bcache/writeback.c
+++ b/drivers/md/bcache/writeback.c
@@ -596,10 +596,13 @@ void bcache_dev_sectors_dirty_add(struct cache_set *c, unsigned int inode,
sectors_dirty = atomic_add_return(s,
d->stripe_sectors_dirty + stripe);
- if (sectors_dirty == d->stripe_size)
- set_bit(stripe, d->full_dirty_stripes);
- else
- clear_bit(stripe, d->full_dirty_stripes);
+ if (sectors_dirty == d->stripe_size) {
+ if (!test_bit(stripe, d->full_dirty_stripes))
+ set_bit(stripe, d->full_dirty_stripes);
+ } else {
+ if (test_bit(stripe, d->full_dirty_stripes))
+ clear_bit(stripe, d->full_dirty_stripes);
+ }
nr_sectors -= s;
stripe_offset = 0;
--
2.17.1
