In the search loop of the binary search function, we are doing a division
by 2 of the sum of the high and low slots. Because the slots are integers,
the generated assembly code for it is the following on x86_64:
0x00000000000141f1 <+145>: mov %eax,%ebx
0x00000000000141f3 <+147>: shr $0x1f,%ebx
0x00000000000141f6 <+150>: add %eax,%ebx
0x00000000000141f8 <+152>: sar %ebx
It's a few more instructions than a simple right shift, because signed
integer division needs to round towards zero. However we know that slots
can never be negative (btrfs_header_nritems() returns an u32), so we
can instead use unsigned types for the low and high slots and therefore
use unsigned integer division, which results in a single instruction on
x86_64:
0x00000000000141f0 <+144>: shr %ebx
So use unsigned types for the slots and therefore unsigned division.
This is part of a small patchset comprised of the following two patches:
btrfs: eliminate extra call when doing binary search on extent buffer
btrfs: do unsigned integer division in the extent buffer binary search loop
The following fs_mark test was run on a non-debug kernel (Debian's default
kernel config) before and after applying the patchset:
$ cat test.sh
#!/bin/bash
DEV=/dev/sdi
MNT=/mnt/sdi
MOUNT_OPTIONS="-o ssd"
MKFS_OPTIONS="-O no-holes -R free-space-tree"
FILES=100000
THREADS=$(nproc --all)
FILE_SIZE=0
umount $DEV &> /dev/null
mkfs.btrfs -f $MKFS_OPTIONS $DEV
mount $MOUNT_OPTIONS $DEV $MNT
OPTS="-S 0 -L 6 -n $FILES -s $FILE_SIZE -t $THREADS -k"
for ((i = 1; i <= $THREADS; i++)); do
OPTS="$OPTS -d $MNT/d$i"
done
fs_mark $OPTS
umount $MNT
Results before applying patchset:
FSUse% Count Size Files/sec App Overhead
2
1200000 0 174472.0
11549868
4
2400000 0 253503.0
11694618
4
3600000 0 257833.1
11611508
6
4800000 0 247089.5
11665983
6
6000000 0 211296.1
12121244
10
7200000 0 187330.6
12548565
Results after applying patchset:
FSUse% Count Size Files/sec App Overhead
2
1200000 0 207556.0
11393252
4
2400000 0 266751.1
11347909
4
3600000 0 274397.5
11270058
6
4800000 0 259608.4
11442250
6
6000000 0 238895.8
11635921
8
7200000 0 211942.2
11873825
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
/*
* Search for a key in the given extent_buffer.
*
- * The lower boundary for the search is specified by the slot number @low. Use a
- * value of 0 to search over the whole extent buffer.
+ * The lower boundary for the search is specified by the slot number @first_slot.
+ * Use a value of 0 to search over the whole extent buffer.
*
* The slot in the extent buffer is returned via @slot. If the key exists in the
* extent buffer, then @slot will point to the slot where the key is, otherwise
* Slot may point to the total number of items (i.e. one position beyond the last
* key) if the key is bigger than the last key in the extent buffer.
*/
-int btrfs_generic_bin_search(struct extent_buffer *eb, int low,
+int btrfs_generic_bin_search(struct extent_buffer *eb, int first_slot,
const struct btrfs_key *key, int *slot)
{
unsigned long p;
int item_size;
- int high = btrfs_header_nritems(eb);
+ /*
+ * Use unsigned types for the low and high slots, so that we get a more
+ * efficient division in the search loop below.
+ */
+ u32 low = first_slot;
+ u32 high = btrfs_header_nritems(eb);
int ret;
const int key_size = sizeof(struct btrfs_disk_key);
- if (low > high) {
+ if (unlikely(low > high)) {
btrfs_err(eb->fs_info,
- "%s: low (%d) > high (%d) eb %llu owner %llu level %d",
+ "%s: low (%u) > high (%u) eb %llu owner %llu level %d",
__func__, low, high, eb->start,
btrfs_header_owner(eb), btrfs_header_level(eb));
return -EINVAL;
int __init btrfs_ctree_init(void);
void __cold btrfs_ctree_exit(void);
-int btrfs_generic_bin_search(struct extent_buffer *eb, int low,
+int btrfs_generic_bin_search(struct extent_buffer *eb, int first_slot,
const struct btrfs_key *key, int *slot);
/*