Read side was already fully supported, and with the write side
appropriately punted to the worker queue, all that's needed now is
setting FOP_DONTCACHE in the file_operations structure to enable full
support for read and write uncached IO.
This provides similar benefits to using RWF_DONTCACHE with reads. Testing
buffered writes on 32 files:
writing bs 65536, uncached 0
1s: 196035MB/sec
2s: 132308MB/sec
3s: 132438MB/sec
4s: 116528MB/sec
5s: 103898MB/sec
6s: 108893MB/sec
7s: 99678MB/sec
8s: 106545MB/sec
9s: 106826MB/sec
10s: 101544MB/sec
11s: 111044MB/sec
12s: 124257MB/sec
13s: 116031MB/sec
14s: 114540MB/sec
15s: 115011MB/sec
16s: 115260MB/sec
17s: 116068MB/sec
18s: 116096MB/sec
where it's quite obvious where the page cache filled, and performance
dropped from to about half of where it started, settling in at around
115GB/sec. Meanwhile, 32 kswapds were running full steam trying to
reclaim pages.
Running the same test with uncached buffered writes:
writing bs 65536, uncached 1
1s: 198974MB/sec
2s: 189618MB/sec
3s: 193601MB/sec
4s: 188582MB/sec
5s: 193487MB/sec
6s: 188341MB/sec
7s: 194325MB/sec
8s: 188114MB/sec
9s: 192740MB/sec
10s: 189206MB/sec
11s: 193442MB/sec
12s: 189659MB/sec
13s: 191732MB/sec
14s: 190701MB/sec
15s: 191789MB/sec
16s: 191259MB/sec
17s: 190613MB/sec
18s: 191951MB/sec
and the behavior is fully predictable, performing the same throughout
even after the page cache would otherwise have fully filled with dirty
data. It's also about 65% faster, and using half the CPU of the system
compared to the normal buffered write.
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
---
fs/xfs/xfs_file.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
index f7a7d89c345e..358987b6e2f8 100644
--- a/fs/xfs/xfs_file.c
+++ b/fs/xfs/xfs_file.c
@@ -1626,7 +1626,8 @@ const struct file_operations xfs_file_operations = {
.fadvise = xfs_file_fadvise,
.remap_file_range = xfs_file_remap_range,
.fop_flags = FOP_MMAP_SYNC | FOP_BUFFER_RASYNC |
- FOP_BUFFER_WASYNC | FOP_DIO_PARALLEL_WRITE,
+ FOP_BUFFER_WASYNC | FOP_DIO_PARALLEL_WRITE |
+ FOP_DONTCACHE,
};
const struct file_operations xfs_dir_file_operations = {
--
2.47.2
