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    • Dave Chinner's avatar
      xfs: track AGs with reclaimable inodes in per-ag radix tree · 16fd5367
      Dave Chinner authored
      https://bugzilla.kernel.org/show_bug.cgi?id=16348
      
      
      
      When the filesystem grows to a large number of allocation groups,
      the summing of recalimable inodes gets expensive. In many cases,
      most AGs won't have any reclaimable inodes and so we are wasting CPU
      time aggregating over these AGs. This is particularly important for
      the inode shrinker that gets called frequently under memory
      pressure.
      
      To avoid the overhead, track AGs with reclaimable inodes in the
      per-ag radix tree so that we can find all the AGs with reclaimable
      inodes via a simple gang tag lookup. This involves setting the tag
      when the first reclaimable inode is tracked in the AG, and removing
      the tag when the last reclaimable inode is removed from the tree.
      Then the summation process becomes a loop walking the radix tree
      summing AGs with the reclaim tag set.
      
      This significantly reduces the overhead of scanning - a 6400 AG
      filesystea now only uses about 25% of a cpu in kswapd while slab
      reclaim progresses instead of being permanently stuck at 100% CPU
      and making little progress. Clean filesystems filesystems will see
      no overhead and the overhead only increases linearly with the number
      of dirty AGs.
      
      Signed-off-by: default avatarDave Chinner <dchinner@redhat.com>
      Reviewed-by: default avatarChristoph Hellwig <hch@lst.de>
      16fd5367