mm: Improve swap path scalability with batched operations

The page swap out path is not scalable due to the numerous locks acquired and released along the way, which are all executed on a page by page basis, e.g.:

1. The acquisition of the mapping tree lock in swap cache when adding a page to swap cache, and then again when deleting a page from swap cache after it has been swapped out.
2. The acquisition of the lock on swap device to allocate a swap slot for a page to be swapped out.

With the advent of high speed block devices that's several orders of magnitude faster than the old spinning disks, these bottlenecks become fairly significant, especially on server class machines with many theads running.  To reduce these locking costs, this patch series attempt to batch the pages on the following oprations needed on for swap:
1. Allocate swap slots in large batches, so locks on the swap device don't need to be acquired as often.
2. Add anonymous pages to the swap cache for the same swap device in batches, so the mapping tree lock can be acquired less.
3. Delete pages from swap cache also in batches.

We experimented the effect of this patches. We set up N threads to access memory in excess of memory capcity, causing swap.  In experiments using a single pmem based fast block device on a 2 socket machine, we saw that for 1 thread, there is a ~25% increase in swap throughput and for 16 threads, the swap throughput increase by ~85%, when compared with the vanilla kernel. Batching helps even for 1 thread because of contention with kswapd when doing direct memory reclaim.

Feedbacks and reviews to this patch series are much appreciated.

Thanks.

Tim

Tim Chen (7):
  mm: Cleanup - Reorganize the shrink_page_list code into smaller functions
  mm: Group the processing of anonymous pages to be swapped in shrink_page_list
  mm: Add new functions to allocate swap slots in batches
  mm: Shrink page list batch allocates swap slots for page swapping
  mm: Batch addtion of pages to swap cache
  mm: Cleanup - Reorganize code to group handling of page
  mm: Batch unmapping of pages that are in swap cache

 include/linux/swap.h |  29 ++-
 mm/swap_state.c      | 253 +++++++++++++-----
 mm/swapfile.c        | 215 +++++++++++++--
 mm/vmscan.c          | 725 ++++++++++++++++++++++++++++++++++++++-------------

 4 files changed, 945 insertions(+), 277 deletions(-)

--
2.5.5