Problem
-------
romfs sequential read performance has regressed very badly since
v3.10. Currently, reading a large file inside a romfs image is
up to 12x slower compared to reading the romfs image directly.
Benchmarks:
- use a romfs image which contains a single 250M file
- calculate the md5sum of the romfs image directly (test 1)
$ time md5sum image.romfs
- loop-mount the romfs image, and calc the md5sum of the file
inside it (test 2)
$ mount -o loop,ro image.romfs /mnt/romfs
$ time md5sum /mnt/romfs/file
- drop caches in between
$ echo 3 > /proc/sys/vm/drop_caches
imx6 (arm cortex a9) on emmc, running v5.7.2:
(test 1) 5 seconds
(test 2) 60 seconds (12x slower)
Intel i7-3630QM on Samsung SSD 850 EVO (EMT02B6Q),
running Ubuntu with v4.15.0-106-generic:
(test 1) 1.3 seconds
(test 2) 3.3 seconds (2.5x slower)
To show that a regression has occurred since v3.10:
imx6 on emmc, running v3.10.17:
(test 1) 16 seconds
(test 2) 18 seconds
Proposed Solution
-----------------
Increase the blocksize from 1K to PAGE_SIZE. This brings the
sequential read performance close to where it was on v3.10:
imx6 on emmc, running v5.7.2:
(test 2 1K blocksize) 60 seconds
(test 2 4K blocksize) 22 seconds
Intel on Ubuntu running v4.15:
(test 2 1K blocksize) 3.3 seconds
(test 2 4K blocksize) 1.9 seconds
There is a risk that this may increase latency on random-
access workloads. But the test below suggests that this
is not a concern:
Benchmark:
- use a 630M romfs image consisting of 9600 files
- loop-mount the romfs image
$ mount -o loop,ro image.romfs /mnt/romfs
- drop all caches
- list all files in the filesystem (test 3)
$ time find /mnt/romfs > /dev/null
imx6 on emmc, running v5.7.2:
(test 3 1K blocksize) 9.5 seconds
(test 3 4K blocksize) 9 seconds
Intel on Ubuntu, running v4.15:
(test 3 1K blocksize) 1.4 seconds
(test 3 4K blocksize) 1.2 seconds
Practical Solution
------------------
Introduce a mount-option called 'largeblocks'. If present,
increase the blocksize for much better sequential performance.
Note that the Linux block layer can only support n-K blocks if
the underlying block device length is also aligned to n-K. This
may not always be the case. Therefore, the driver will pick the
largest blocksize which the underlying block device can support.
Cc: Al Viro <v...@zeniv.linux.org.uk>
Cc: Deepa Dinamani <deepa.ker...@gmail.com>
Cc: David Howells <dhowe...@redhat.com>
Cc: "Darrick J. Wong" <darrick.w...@oracle.com>
Cc: Janos Farkas <chexum+...@gmail.com>
Cc: Jeff Layton <jlay...@kernel.org>
To: linux-kernel@vger.kernel.org
Signed-off-by: Sven Van Asbroeck <thesve...@gmail.com>
---
fs/romfs/super.c | 62 ++++++++++++++++++++++++++++++++++++++++++++----
1 file changed, 57 insertions(+), 5 deletions(-)
diff --git a/fs/romfs/super.c b/fs/romfs/super.c
index 6fecdea791f1..93565aeaa43c 100644
--- a/fs/romfs/super.c
+++ b/fs/romfs/super.c
@@ -65,7 +65,7 @@
#include <linux/slab.h>
#include <linux/init.h>
#include <linux/blkdev.h>
-#include <linux/fs_context.h>
+#include <linux/fs_parser.h>
#include <linux/mount.h>
#include <linux/namei.h>
#include <linux/statfs.h>
@@ -460,6 +460,54 @@ static __u32 romfs_checksum(const void *data, int size)
return sum;
}
+enum romfs_param {
+ Opt_largeblocks,
+};
+
+static const struct fs_parameter_spec romfs_fs_parameters[] = {
+ fsparam_flag("largeblocks", Opt_largeblocks),
+ {}
+};
+
+/*
+ * Parse a single mount parameter.
+ */
+static int romfs_parse_param(struct fs_context *fc, struct fs_parameter *param)
+{
+ struct fs_parse_result result;
+ int opt;
+
+ opt = fs_parse(fc, romfs_fs_parameters, param, &result);
+ if (opt < 0)
+ return opt;
+
+ switch (opt) {
+ case Opt_largeblocks:
+ fc->fs_private = (void *) 1;
+ break;
+ default:
+ return -EINVAL;
+ }
+
+ return 0;
+}
+
+/*
+ * pick the largest blocksize which the underlying block device
+ * is a multiple of. Or fall back to legacy (ROMBSIZE).
+ */
+static int romfs_largest_blocksize(struct super_block *sb)
+{
+ loff_t device_sz = i_size_read(sb->s_bdev->bd_inode);
+ int blksz;
+
+ for (blksz = PAGE_SIZE; blksz > ROMBSIZE; blksz >>= 1)
+ if ((device_sz % blksz) == 0)
+ break;
+
+ return blksz;
+}
+
/*
* fill in the superblock
*/
@@ -467,17 +515,19 @@ static int romfs_fill_super(struct super_block *sb,
struct fs_context *fc)
{
struct romfs_super_block *rsb;
struct inode *root;
- unsigned long pos, img_size;
+ unsigned long pos, img_size, dev_blocksize;
const char *storage;
size_t len;
int ret;
#ifdef CONFIG_BLOCK
+ dev_blocksize = fc->fs_private ? romfs_largest_blocksize(sb) :
+ ROMBSIZE;
if (!sb->s_mtd) {
- sb_set_blocksize(sb, ROMBSIZE);
+ sb_set_blocksize(sb, dev_blocksize);
} else {
- sb->s_blocksize = ROMBSIZE;
- sb->s_blocksize_bits = blksize_bits(ROMBSIZE);
+ sb->s_blocksize = dev_blocksize;
+ sb->s_blocksize_bits = blksize_bits(dev_blocksize);
}
#endif
@@ -573,6 +623,7 @@ static int romfs_get_tree(struct fs_context *fc)
static const struct fs_context_operations romfs_context_ops = {
.get_tree = romfs_get_tree,
.reconfigure = romfs_reconfigure,
+ .parse_param = romfs_parse_param,
};
/*
@@ -607,6 +658,7 @@ static struct file_system_type romfs_fs_type = {
.owner = THIS_MODULE,
.name = "romfs",
.init_fs_context = romfs_init_fs_context,
+ .parameters = romfs_fs_parameters,
.kill_sb = romfs_kill_sb,
.fs_flags = FS_REQUIRES_DEV,
};
--
2.17.1
