md: introduce get_priority_stripe() to improve raid456 write performance

Improve write performance by preventing the delayed_list from dumping all its
stripes onto the handle_list in one shot.  Delayed stripes are now further
delayed by being held on the 'hold_list'.  The 'hold_list' is bypassed when:

  * a STRIPE_IO_STARTED stripe is found at the head of 'handle_list'
  * 'handle_list' is empty and i/o is being done to satisfy full stripe-width
    write requests
  * 'bypass_count' is less than 'bypass_threshold'.  By default the threshold
    is 1, i.e. every other stripe handled is a preread stripe provided the
    top two conditions are false.

Benchmark data:
System: 2x Xeon 5150, 4x SATA, mem=1GB
Baseline: 2.6.24-rc7
Configuration: mdadm --create /dev/md0 /dev/sd[b-e] -n 4 -l 5 --assume-clean
Test1: dd if=/dev/zero of=/dev/md0 bs=1024k count=2048
  * patched:  +33% (stripe_cache_size = 256), +25% (stripe_cache_size = 512)

Test2: tiobench --size 2048 --numruns 5 --block 4096 --block 131072 (XFS)
  * patched: +13%
  * patched + preread_bypass_threshold = 0: +37%

Changes since v1:
* reduce bypass_threshold from (chunk_size / sectors_per_chunk) to (1) and
  make it configurable.  This defaults to fairness and modest performance
  gains out of the box.
Changes since v2:
* [neilb@suse.de]: kill STRIPE_PRIO_HI and preread_needed as they are not
  necessary, the important change was clearing STRIPE_DELAYED in
  add_stripe_bio and this has been moved out to make_request for the hang
  fix.
* [neilb@suse.de]: simplify get_priority_stripe
* [dan.j.williams@intel.com]: reset the bypass_count when ->hold_list is
  sampled empty (+11%)
* [dan.j.williams@intel.com]: decrement the bypass_count at the detection
  of stripes being naturally promoted off of hold_list +2%.  Note, resetting
  bypass_count instead of decrementing on these events yields +4% but that is
  probably too aggressive.
Changes since v3:
* cosmetic fixups

Tested-by: James W. Laferriere <babydr@baby-dragons.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 7 insertions, 0 deletions

diff --git a/include/linux/raid/raid5.h b/include/linux/raid/raid5.h
index 93678f57ccbe..f0827d31ae6f 100644
--- a/include/linux/raid/raid5.h
+++ b/include/linux/raid/raid5.h
@@ -252,6 +252,8 @@ struct r6_state {
 #define STRIPE_EXPANDING        9
 #define STRIPE_EXPAND_SOURCE    10
 #define STRIPE_EXPAND_READY     11
+#define STRIPE_IO_STARTED       12 /* do not count towards 'bypass_count' */
+#define STRIPE_FULL_WRITE       13 /* all blocks are set to be overwritten */
 /*
  * Operations flags (in issue order)
  */
@@ -316,12 +318,17 @@ struct raid5_private_data {
         int                     previous_raid_disks;
 
         struct list_head        handle_list; /* stripes needing handling */
+        struct list_head        hold_list; /* preread ready stripes */
         struct list_head        delayed_list; /* stripes that have plugged requests */
         struct list_head        bitmap_list; /* stripes delaying awaiting bitmap update */
         struct bio              *retry_read_aligned; /* currently retrying aligned bios   */
         struct bio              *retry_read_aligned_list; /* aligned bios retry list  */
         atomic_t                preread_active_stripes; /* stripes with scheduled io */
         atomic_t                active_aligned_reads;
+        atomic_t                pending_full_writes; /* full write backlog */
+        int                     bypass_count; /* bypassed prereads */
+        int                     bypass_threshold; /* preread nice */
+        struct list_head        *last_hold; /* detect hold_list promotions */
 
         atomic_t                reshape_stripes; /* stripes with pending writes for reshape */
         /* unfortunately we need two cache names as we temporarily have