Acked-by: Mikulas Patocka <mpatocka@xxxxxxxxxx> On Sun, 17 Mar 2019, Nikos Tsironis wrote: > dm-snapshot uses a single mutex to serialize every access to the > snapshot state, including accesses to the exception hash tables. This > mutex is a bottleneck preventing dm-snapshot to scale as the number of > threads doing IO increases. > > The major contention points are __origin_write()/snapshot_map() and > pending_complete(), i.e., the submission and completion of pending > exceptions. > > This patchset substitutes the single mutex with: > > * A read-write semaphore, which protects the mostly read fields of the > snapshot structure. > > * Per-bucket bit spinlocks, that protect accesses to the exception > hash tables. > > fio benchmarks using the null_blk device show significant performance > improvements as the number of worker processes increases. Write latency > is almost halved and write IOPS are nearly doubled. > > The relevant patch provides detailed benchmark results. > > A summary of the patchset follows: > > 1. The first patch removes an unnecessary use of WRITE_ONCE() in > hlist_add_behind(). > > 2. The second patch adds two helper functions to linux/list_bl.h, > which is used to implement the per-bucket bit spinlocks in > dm-snapshot. > > 3. The third patch removes the need to sleep holding the snapshot lock > in pending_complete(), thus allowing us to replace the mutex with > the per-bucket bit spinlocks. > > 4. Patches 4, 5 and 6 change the locking scheme, as described > previously. > > Changes in v3: > - Don't use WRITE_ONCE() in hlist_bl_add_behind(), as it's not needed. > - Fix hlist_add_behind() to also not use WRITE_ONCE(). > - Use uintptr_t instead of unsigned long in hlist_bl_add_before(). > > v2: https://www.redhat.com/archives/dm-devel/2019-March/msg00007.html > > Changes in v2: > - Split third patch of v1 into three patches: 3/5, 4/5, 5/5. > > v1: https://www.redhat.com/archives/dm-devel/2018-December/msg00161.html > > Nikos Tsironis (6): > list: Don't use WRITE_ONCE() in hlist_add_behind() > list_bl: Add hlist_bl_add_before/behind helpers > dm snapshot: Don't sleep holding the snapshot lock > dm snapshot: Replace mutex with rw semaphore > dm snapshot: Make exception tables scalable > dm snapshot: Use fine-grained locking scheme > > drivers/md/dm-exception-store.h | 3 +- > drivers/md/dm-snap.c | 359 +++++++++++++++++++++++++++------------- > include/linux/list.h | 2 +- > include/linux/list_bl.h | 26 +++ > 4 files changed, 269 insertions(+), 121 deletions(-) > > -- > 2.11.0 > -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
