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btrfs: do unsigned integer division in the extent buffer binary search loop
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>
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@ -853,8 +853,8 @@ int btrfs_realloc_node(struct btrfs_trans_handle *trans,
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/*
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* Search for a key in the given extent_buffer.
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*
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* The lower boundary for the search is specified by the slot number @low. Use a
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* value of 0 to search over the whole extent buffer.
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* The lower boundary for the search is specified by the slot number @first_slot.
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* Use a value of 0 to search over the whole extent buffer.
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*
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* The slot in the extent buffer is returned via @slot. If the key exists in the
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* extent buffer, then @slot will point to the slot where the key is, otherwise
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@ -863,18 +863,23 @@ int btrfs_realloc_node(struct btrfs_trans_handle *trans,
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* Slot may point to the total number of items (i.e. one position beyond the last
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* key) if the key is bigger than the last key in the extent buffer.
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*/
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int btrfs_generic_bin_search(struct extent_buffer *eb, int low,
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int btrfs_generic_bin_search(struct extent_buffer *eb, int first_slot,
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const struct btrfs_key *key, int *slot)
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{
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unsigned long p;
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int item_size;
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int high = btrfs_header_nritems(eb);
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/*
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* Use unsigned types for the low and high slots, so that we get a more
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* efficient division in the search loop below.
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*/
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u32 low = first_slot;
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u32 high = btrfs_header_nritems(eb);
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int ret;
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const int key_size = sizeof(struct btrfs_disk_key);
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if (low > high) {
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if (unlikely(low > high)) {
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btrfs_err(eb->fs_info,
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"%s: low (%d) > high (%d) eb %llu owner %llu level %d",
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"%s: low (%u) > high (%u) eb %llu owner %llu level %d",
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__func__, low, high, eb->start,
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btrfs_header_owner(eb), btrfs_header_level(eb));
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return -EINVAL;
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@ -508,7 +508,7 @@ int btrfs_trim_fs(struct btrfs_fs_info *fs_info, struct fstrim_range *range);
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int __init btrfs_ctree_init(void);
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void __cold btrfs_ctree_exit(void);
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int btrfs_generic_bin_search(struct extent_buffer *eb, int low,
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int btrfs_generic_bin_search(struct extent_buffer *eb, int first_slot,
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const struct btrfs_key *key, int *slot);
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/*
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