Re: ext4 extent status tree LRU locking

From: Theodore Ts'o
Date: Mon Jun 17 2013 - 22:47:37 EST

> Subject: [PATCH v2] ext4: improve extent cache shrink mechanism to avoid to burn CPU time
> From: Zheng Liu <wenqing.lz@xxxxxxxxxx>
> 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: Dave Hansen <dave.hansen@xxxxxxxxx>
> Cc: "Theodore Ts'o" <tytso@xxxxxxx>
> Signed-off-by: Zheng Liu <wenqing.lz@xxxxxxxxxx>

Applied, thanks.

- Ted
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