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author | Aaron Lu <ziqian.lzq@antfin.com> | 2019-07-12 06:55:41 +0300 |
---|---|---|
committer | Linus Torvalds <torvalds@linux-foundation.org> | 2019-07-12 21:05:43 +0300 |
commit | 4efaceb1c5f8136d5fec3f26549d294b8e898bd7 (patch) | |
tree | 9bb4b1ee853f4b25dfc021ac2c9248a118b73fc5 /include | |
parent | 054f1d1faaed6a7930b77286d607ae45c01d0443 (diff) | |
download | linux-4efaceb1c5f8136d5fec3f26549d294b8e898bd7.tar.xz |
mm, swap: use rbtree for swap_extent
swap_extent is used to map swap page offset to backing device's block
offset. For a continuous block range, one swap_extent is used and all
these swap_extents are managed in a linked list.
These swap_extents are used by map_swap_entry() during swap's read and
write path. To find out the backing device's block offset for a page
offset, the swap_extent list will be traversed linearly, with
curr_swap_extent being used as a cache to speed up the search.
This works well as long as swap_extents are not huge or when the number
of processes that access swap device are few, but when the swap device
has many extents and there are a number of processes accessing the swap
device concurrently, it can be a problem. On one of our servers, the
disk's remaining size is tight:
$df -h
Filesystem Size Used Avail Use% Mounted on
... ...
/dev/nvme0n1p1 1.8T 1.3T 504G 72% /home/t4
When creating a 80G swapfile there, there are as many as 84656 swap
extents. The end result is, kernel spends abou 30% time in
map_swap_entry() and swap throughput is only 70MB/s.
As a comparison, when I used smaller sized swapfile, like 4G whose
swap_extent dropped to 2000, swap throughput is back to 400-500MB/s and
map_swap_entry() is about 3%.
One downside of using rbtree for swap_extent is, 'struct rbtree' takes
24 bytes while 'struct list_head' takes 16 bytes, that's 8 bytes more
for each swap_extent. For a swapfile that has 80k swap_extents, that
means 625KiB more memory consumed.
Test:
Since it's not possible to reboot that server, I can not test this patch
diretly there. Instead, I tested it on another server with NVMe disk.
I created a 20G swapfile on an NVMe backed XFS fs. By default, the
filesystem is quite clean and the created swapfile has only 2 extents.
Testing vanilla and this patch shows no obvious performance difference
when swapfile is not fragmented.
To see the patch's effects, I used some tweaks to manually fragment the
swapfile by breaking the extent at 1M boundary. This made the swapfile
have 20K extents.
nr_task=4
kernel swapout(KB/s) map_swap_entry(perf) swapin(KB/s) map_swap_entry(perf)
vanilla 165191 90.77% 171798 90.21%
patched 858993 +420% 2.16% 715827 +317% 0.77%
nr_task=8
kernel swapout(KB/s) map_swap_entry(perf) swapin(KB/s) map_swap_entry(perf)
vanilla 306783 92.19% 318145 87.76%
patched 954437 +211% 2.35% 1073741 +237% 1.57%
swapout: the throughput of swap out, in KB/s, higher is better 1st
map_swap_entry: cpu cycles percent sampled by perf swapin: the
throughput of swap in, in KB/s, higher is better. 2nd map_swap_entry:
cpu cycles percent sampled by perf
nr_task=1 doesn't show any difference, this is due to the curr_swap_extent
can be effectively used to cache the correct swap extent for single task
workload.
[akpm@linux-foundation.org: s/BUG_ON(1)/BUG()/]
Link: http://lkml.kernel.org/r/20190523142404.GA181@aaronlu
Signed-off-by: Aaron Lu <ziqian.lzq@antfin.com>
Cc: Huang Ying <ying.huang@intel.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'include')
-rw-r--r-- | include/linux/swap.h | 5 |
1 files changed, 2 insertions, 3 deletions
diff --git a/include/linux/swap.h b/include/linux/swap.h index 6358a6185634..de2c67a33b7e 100644 --- a/include/linux/swap.h +++ b/include/linux/swap.h @@ -148,7 +148,7 @@ struct zone; * We always assume that blocks are of size PAGE_SIZE. */ struct swap_extent { - struct list_head list; + struct rb_node rb_node; pgoff_t start_page; pgoff_t nr_pages; sector_t start_block; @@ -248,8 +248,7 @@ struct swap_info_struct { unsigned int cluster_next; /* likely index for next allocation */ unsigned int cluster_nr; /* countdown to next cluster search */ struct percpu_cluster __percpu *percpu_cluster; /* per cpu's swap location */ - struct swap_extent *curr_swap_extent; - struct swap_extent first_swap_extent; + struct rb_root swap_extent_root;/* root of the swap extent rbtree */ struct block_device *bdev; /* swap device or bdev of swap file */ struct file *swap_file; /* seldom referenced */ unsigned int old_block_size; /* seldom referenced */ |