For folks who are interested in getting started with ext4, here's a relatively simple todo item. Wu Fenguang has reported a scalability issue with ext4 when running a VM stress test where there are multiple processes all hammering the file with random mmap requests. (Although this would have shown up as well if we had multiple threads all trying to read from the same file descriptor at the same time.) The point of contention is ext4_inode_info.i_block_reservation_lock(), and the problem is that we're using this spinlock to protect a simple singleton extent cache, i_cached_extent. See the functions ext4_ext_put_gap_in_cache() and ext4_ext_in_cache(). Your mission should you choose to accept it :-), is to replace i_cached_extent with an RCU protected data structure. For bonus points, try making it support caching more than a single extent. Also, while you're at it, fix a race condition with move_inode.c where we should make should the cached extents have been removed and an RCU grace period has elapsed before proceeding with the inode defragmentation. - Ted -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
