From dc516164869300efd0bdbf6f894defc306588b75 Mon Sep 17 00:00:00 2001 From: David Sterba Date: Thu, 29 Oct 2020 15:33:45 +0100 Subject: [PATCH] btrfs: reorder extent buffer members for better packing After the rwsem replaced the tree lock implementation, the extent buffer got smaller but leaving some holes behind. By changing log_index type and reordering, we can squeeze the size further to 240 bytes, measured on release config on x86_64. Log_index spans only 3 values and needs to be signed. Before: struct extent_buffer { u64 start; /* 0 8 */ long unsigned int len; /* 8 8 */ long unsigned int bflags; /* 16 8 */ struct btrfs_fs_info * fs_info; /* 24 8 */ spinlock_t refs_lock; /* 32 4 */ atomic_t refs; /* 36 4 */ atomic_t io_pages; /* 40 4 */ int read_mirror; /* 44 4 */ struct callback_head callback_head __attribute__((__aligned__(8))); /* 48 16 */ /* --- cacheline 1 boundary (64 bytes) --- */ pid_t lock_owner; /* 64 4 */ bool lock_recursed; /* 68 1 */ /* XXX 3 bytes hole, try to pack */ struct rw_semaphore lock; /* 72 40 */ short int log_index; /* 112 2 */ /* XXX 6 bytes hole, try to pack */ struct page * pages[16]; /* 120 128 */ /* size: 248, cachelines: 4, members: 14 */ /* sum members: 239, holes: 2, sum holes: 9 */ /* forced alignments: 1 */ /* last cacheline: 56 bytes */ } __attribute__((__aligned__(8))); After: struct extent_buffer { u64 start; /* 0 8 */ long unsigned int len; /* 8 8 */ long unsigned int bflags; /* 16 8 */ struct btrfs_fs_info * fs_info; /* 24 8 */ spinlock_t refs_lock; /* 32 4 */ atomic_t refs; /* 36 4 */ atomic_t io_pages; /* 40 4 */ int read_mirror; /* 44 4 */ struct callback_head callback_head __attribute__((__aligned__(8))); /* 48 16 */ /* --- cacheline 1 boundary (64 bytes) --- */ pid_t lock_owner; /* 64 4 */ bool lock_recursed; /* 68 1 */ s8 log_index; /* 69 1 */ /* XXX 2 bytes hole, try to pack */ struct rw_semaphore lock; /* 72 40 */ struct page * pages[16]; /* 112 128 */ /* size: 240, cachelines: 4, members: 14 */ /* sum members: 238, holes: 1, sum holes: 2 */ /* forced alignments: 1 */ /* last cacheline: 48 bytes */ } __attribute__((__aligned__(8))); Reviewed-by: Josef Bacik Signed-off-by: David Sterba --- fs/btrfs/extent_io.h | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) diff --git a/fs/btrfs/extent_io.h b/fs/btrfs/extent_io.h index 5403354de0e1..3c2bf21c54eb 100644 --- a/fs/btrfs/extent_io.h +++ b/fs/btrfs/extent_io.h @@ -88,10 +88,10 @@ struct extent_buffer { struct rcu_head rcu_head; pid_t lock_owner; bool lock_recursed; - struct rw_semaphore lock; - /* >= 0 if eb belongs to a log tree, -1 otherwise */ - short log_index; + s8 log_index; + + struct rw_semaphore lock; struct page *pages[INLINE_EXTENT_BUFFER_PAGES]; #ifdef CONFIG_BTRFS_DEBUG