From a6f23657d3072bde6844055bbc2290e497f33fbc Mon Sep 17 00:00:00 2001 From: Jens Axboe Date: Fri, 24 Oct 2008 12:52:42 +0200 Subject: [PATCH] block: add one-hit cache for disk partition lookup disk_map_sector_rcu() returns a partition from a sector offset, which we use for IO statistics on a per-partition basis. The lookup itself is an O(N) list lookup, where N is the number of partitions. This actually hurts performance quite a bit, even on the lower end partitions. On higher numbered partitions, it can get pretty bad. Solve this by adding a one-hit cache for partition lookup. This makes the lookup O(1) for the case where we do most IO to one partition. Even for mixed partition workloads, amortized cost is pretty close to O(1) since the natural IO batching makes the one-hit cache last for lots of IOs. Signed-off-by: Jens Axboe --- block/genhd.c | 23 +++++++++++++++++++---- include/linux/genhd.h | 1 + 2 files changed, 20 insertions(+), 4 deletions(-) diff --git a/block/genhd.c b/block/genhd.c index 2f7feda61e35..d84a7df1e2a0 100644 --- a/block/genhd.c +++ b/block/genhd.c @@ -181,6 +181,12 @@ void disk_part_iter_exit(struct disk_part_iter *piter) } EXPORT_SYMBOL_GPL(disk_part_iter_exit); +static inline int sector_in_part(struct hd_struct *part, sector_t sector) +{ + return part->start_sect <= sector && + sector < part->start_sect + part->nr_sects; +} + /** * disk_map_sector_rcu - map sector to partition * @disk: gendisk of interest @@ -199,16 +205,22 @@ EXPORT_SYMBOL_GPL(disk_part_iter_exit); struct hd_struct *disk_map_sector_rcu(struct gendisk *disk, sector_t sector) { struct disk_part_tbl *ptbl; + struct hd_struct *part; int i; ptbl = rcu_dereference(disk->part_tbl); + part = rcu_dereference(ptbl->last_lookup); + if (part && sector_in_part(part, sector)) + return part; + for (i = 1; i < ptbl->len; i++) { - struct hd_struct *part = rcu_dereference(ptbl->part[i]); + part = rcu_dereference(ptbl->part[i]); - if (part && part->start_sect <= sector && - sector < part->start_sect + part->nr_sects) + if (part && sector_in_part(part, sector)) { + rcu_assign_pointer(ptbl->last_lookup, part); return part; + } } return &disk->part0; } @@ -888,8 +900,11 @@ static void disk_replace_part_tbl(struct gendisk *disk, struct disk_part_tbl *old_ptbl = disk->part_tbl; rcu_assign_pointer(disk->part_tbl, new_ptbl); - if (old_ptbl) + + if (old_ptbl) { + rcu_assign_pointer(old_ptbl->last_lookup, NULL); call_rcu(&old_ptbl->rcu_head, disk_free_ptbl_rcu_cb); + } } /** diff --git a/include/linux/genhd.h b/include/linux/genhd.h index 3df7742ce246..16948eaecae3 100644 --- a/include/linux/genhd.h +++ b/include/linux/genhd.h @@ -126,6 +126,7 @@ struct blk_scsi_cmd_filter { struct disk_part_tbl { struct rcu_head rcu_head; int len; + struct hd_struct *last_lookup; struct hd_struct *part[]; }; -- 2.20.1