* [PATCH] btrfs: reduce kmap_atomic time for checksumming
@ 2019-03-07 16:14 Johannes Thumshirn
2019-03-07 17:42 ` David Sterba
0 siblings, 1 reply; 2+ messages in thread
From: Johannes Thumshirn @ 2019-03-07 16:14 UTC (permalink / raw)
To: David Sterba; +Cc: Linux BTRFS Mailinglist, Johannes Thumshirn
Since commit c40a3d38aff4 ("Btrfs: Compute and look up csums based on
sectorsized blocks") we do a kmap_atomic() on the contents of a bvec.
kmap_atomic() in turn does a preempt_disable() and pagefault_disable(),
so we shouldn't map the data for too long. Reduce the time the bvec's
page is mapped to when we actually need it.
Performance wise it doesn't seem to make a huge difference with a 2 vcpu VM
on a /dev/zram device:
vanilla patched delta
write 17.4MiB/s 17.8MiB/s +0.4MiB/s (+2%)
read 40.6MiB/s 41.5MiB/s +0.9MiB/s (+2%)
The following fio job profile was used in the comparision:
[global]
ioengine=libaio
direct=1
sync=1
norandommap
time_based
runtime=10m
size=100m
group_reporting
numjobs=2
[test]
filename=/mnt/test/fio
rw=randrw
rwmixread=70
Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
---
fs/btrfs/file-item.c | 8 ++------
1 file changed, 2 insertions(+), 6 deletions(-)
diff --git a/fs/btrfs/file-item.c b/fs/btrfs/file-item.c
index 920bf3b4b0ef..aa40e66df1e1 100644
--- a/fs/btrfs/file-item.c
+++ b/fs/btrfs/file-item.c
@@ -453,8 +453,6 @@ blk_status_t btrfs_csum_one_bio(struct inode *inode, struct bio *bio,
BUG_ON(!ordered); /* Logic error */
}
- data = kmap_atomic(bvec.bv_page);
-
nr_sectors = BTRFS_BYTES_TO_BLKS(fs_info,
bvec.bv_len + fs_info->sectorsize
- 1);
@@ -464,7 +462,6 @@ blk_status_t btrfs_csum_one_bio(struct inode *inode, struct bio *bio,
offset < ordered->file_offset) {
unsigned long bytes_left;
- kunmap_atomic(data);
sums->len = this_sum_bytes;
this_sum_bytes = 0;
btrfs_add_ordered_sum(inode, ordered, sums);
@@ -482,16 +479,16 @@ blk_status_t btrfs_csum_one_bio(struct inode *inode, struct bio *bio,
sums->bytenr = ((u64)bio->bi_iter.bi_sector << 9)
+ total_bytes;
index = 0;
-
- data = kmap_atomic(bvec.bv_page);
}
sums->sums[index] = ~(u32)0;
+ data = kmap_atomic(bvec.bv_page);
sums->sums[index]
= btrfs_csum_data(data + bvec.bv_offset
+ (i * fs_info->sectorsize),
sums->sums[index],
fs_info->sectorsize);
+ kunmap_atomic(data);
btrfs_csum_final(sums->sums[index],
(char *)(sums->sums + index));
index++;
@@ -500,7 +497,6 @@ blk_status_t btrfs_csum_one_bio(struct inode *inode, struct bio *bio,
total_bytes += fs_info->sectorsize;
}
- kunmap_atomic(data);
}
this_sum_bytes = 0;
btrfs_add_ordered_sum(inode, ordered, sums);
--
2.16.4
^ permalink raw reply related [flat|nested] 2+ messages in thread
* Re: [PATCH] btrfs: reduce kmap_atomic time for checksumming
2019-03-07 16:14 [PATCH] btrfs: reduce kmap_atomic time for checksumming Johannes Thumshirn
@ 2019-03-07 17:42 ` David Sterba
0 siblings, 0 replies; 2+ messages in thread
From: David Sterba @ 2019-03-07 17:42 UTC (permalink / raw)
To: Johannes Thumshirn; +Cc: David Sterba, Linux BTRFS Mailinglist
On Thu, Mar 07, 2019 at 05:14:00PM +0100, Johannes Thumshirn wrote:
> Since commit c40a3d38aff4 ("Btrfs: Compute and look up csums based on
> sectorsized blocks") we do a kmap_atomic() on the contents of a bvec.
>
> kmap_atomic() in turn does a preempt_disable() and pagefault_disable(),
> so we shouldn't map the data for too long. Reduce the time the bvec's
> page is mapped to when we actually need it.
>
> Performance wise it doesn't seem to make a huge difference with a 2 vcpu VM
> on a /dev/zram device:
>
> vanilla patched delta
> write 17.4MiB/s 17.8MiB/s +0.4MiB/s (+2%)
> read 40.6MiB/s 41.5MiB/s +0.9MiB/s (+2%)
>
> The following fio job profile was used in the comparision:
>
> [global]
> ioengine=libaio
> direct=1
> sync=1
> norandommap
> time_based
> runtime=10m
> size=100m
> group_reporting
> numjobs=2
>
> [test]
> filename=/mnt/test/fio
> rw=randrw
> rwmixread=70
>
> Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
> ---
> fs/btrfs/file-item.c | 8 ++------
> 1 file changed, 2 insertions(+), 6 deletions(-)
>
> diff --git a/fs/btrfs/file-item.c b/fs/btrfs/file-item.c
> index 920bf3b4b0ef..aa40e66df1e1 100644
> --- a/fs/btrfs/file-item.c
> +++ b/fs/btrfs/file-item.c
> @@ -453,8 +453,6 @@ blk_status_t btrfs_csum_one_bio(struct inode *inode, struct bio *bio,
> BUG_ON(!ordered); /* Logic error */
> }
>
> - data = kmap_atomic(bvec.bv_page);
> -
> nr_sectors = BTRFS_BYTES_TO_BLKS(fs_info,
> bvec.bv_len + fs_info->sectorsize
> - 1);
> @@ -464,7 +462,6 @@ blk_status_t btrfs_csum_one_bio(struct inode *inode, struct bio *bio,
> offset < ordered->file_offset) {
> unsigned long bytes_left;
>
> - kunmap_atomic(data);
> sums->len = this_sum_bytes;
> this_sum_bytes = 0;
> btrfs_add_ordered_sum(inode, ordered, sums);
> @@ -482,16 +479,16 @@ blk_status_t btrfs_csum_one_bio(struct inode *inode, struct bio *bio,
> sums->bytenr = ((u64)bio->bi_iter.bi_sector << 9)
> + total_bytes;
> index = 0;
> -
> - data = kmap_atomic(bvec.bv_page);
> }
>
> sums->sums[index] = ~(u32)0;
> + data = kmap_atomic(bvec.bv_page);
> sums->sums[index]
> = btrfs_csum_data(data + bvec.bv_offset
> + (i * fs_info->sectorsize),
> sums->sums[index],
> fs_info->sectorsize);
> + kunmap_atomic(data);
The code before c40a3d38aff4 did exactly this, ie. kmap only around the
checksumming. I don't actually see anywhere from the context why it
would need to be around the whole bvec as none of the operations uses
that.
So I think your patch is correct.
Reviewed-by: David Sterba <dsterba@suse.com>
^ permalink raw reply [flat|nested] 2+ messages in thread
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2019-03-07 17:42 ` David Sterba
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