• Zheng Liu's avatar
    ext4: improve extent cache shrink mechanism to avoid to burn CPU time · d3922a77
    Zheng Liu authored
    Now we maintain an proper in-order LRU list in ext4 to reclaim entries
    from extent status tree when we are under heavy memory pressure.  For
    keeping this order, a spin lock is used to protect this list.  But this
    lock burns a lot of CPU time.  We can use the following steps to trigger
    it.
    
      % cd /dev/shm
      % dd if=/dev/zero of=ext4-img bs=1M count=2k
      % mkfs.ext4 ext4-img
      % mount -t ext4 -o loop ext4-img /mnt
      % cd /mnt
      % for ((i=0;i<160;i++)); do truncate -s 64g $i; done
      % for ((i=0;i<160;i++)); do cp $i /dev/null &; done
      % perf record -a -g
      % perf report
    
    This commit tries to fix this problem.  Now a new member called
    i_touch_when is added into ext4_inode_info to record the last access
    time for an inode.  Meanwhile we never need to keep a proper in-order
    LRU list.  So this can avoid to burns some CPU time.  When we try to
    reclaim some entries from extent status tree, we use list_sort() to get
    a proper in-order list.  Then we traverse this list to discard some
    entries.  In ext4_sb_info, we use s_es_last_sorted to record the last
    time of sorting this list.  When we traverse the list, we skip the inode
    that is newer than this time, and move this inode to the tail of LRU
    list.  When the head of the list is newer than s_es_last_sorted, we will
    sort the LRU list again.
    
    In this commit, we break the loop if s_extent_cache_cnt == 0 because
    that means that all extents in extent status tree have been reclaimed.
    
    Meanwhile in this commit, ext4_es_{un}register_shrinker()'s prototype is
    changed to save a local variable in these functions.
    Reported-by: default avatarDave Hansen <dave.hansen@intel.com>
    Signed-off-by: default avatarZheng Liu <wenqing.lz@taobao.com>
    Signed-off-by: default avatar"Theodore Ts'o" <tytso@mit.edu>
    d3922a77
inode.c 147 KB