Re: [PATCH v2 1/3] btrfs: change how we calculate the nrptrs for btrfs_buffered_write()

Nikolay Borisov
2020-08-24T10:33:08Z

On 24.08.20 г. 10:59 ч., Qu Wenruo wrote:
> @nrptrs of btrfs_bufferd_write() determines the up limit of pages we can
> process in one batch.
>
> Normally we want it to be as large as possible as btrfs metadata/data
> reserve and release can cost quite a lot of time.
>
> Commit 142349f541d0 ("btrfs: lower the dirty balance poll interval")
> introduced two extra limitations which are suspicious now:
> - limit the page number to nr_dirtied_pause - nr_dirtied
> However I can't find any mainline fs nor iomap utilize these two
> members.
> Although page write back still uses those two members, as no other fs
> utilizeing them at all, I doubt about the usefulness.
>
> - ensure we always have 8 pages allocates
> The 8 lower limit looks pretty strange, this means even we're just
> writing 4K, we will allocate page* for 8 pages no matter what.
> To me, this 8 pages look more like a upper limit.
>
> This patch will change it by:
> - Extract the calculation into another function
> This allows us to add more comment explaining every calculation.
>
> - Do proper page alignment calculation
> The old calculation, DIV_ROUND_UP(iov_iter_count(i), PAGE_SIZE)
> doesn't take @pos into consideration.
> In fact we can easily have iov_iter_count(i) == 2, but still cross two
> pages. (pos == page_offset() + PAGE_SIZE - 1).
>
> - Remove the useless max(8)
>
> - Use PAGE_SIZE independent up limit
> Now we use 64K as nr_pages limit, so we should get similar performance
> between different arches.
>
> Signed-off-by: Qu Wenruo <wqu@suse.com>
> ---
> fs/btrfs/file.c | 28 ++++++++++++++++++++++++----
> 1 file changed, 24 insertions(+), 4 deletions(-)
>
> diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
> index 5a818ebcb01f..c592350a5a82 100644
> --- a/fs/btrfs/file.c
> +++ b/fs/btrfs/file.c
> @@ -1620,6 +1620,29 @@ void btrfs_check_nocow_unlock(struct btrfs_inode *inode)
> btrfs_drew_write_unlock(&inode->root->snapshot_lock);
> }
>
> +/* Helper to get how many pages we should alloc for the batch */
> +static int get_nr_pages(struct btrfs_fs_info *fs_info, loff_t pos,
> + struct iov_iter *iov)

Drop the fs_info parameter as it's unsued. Also rename the function to
calc_nr_pages since it's closer to what the function is actually doing.

> +{
> + int nr_pages;
> +
> + /*
> + * Try to cover the full iov range, as btrfs metadata/data reserve
> + * and release can be pretty slow, thus the more pages we process in
> + * one batch the better.
> + */
> + nr_pages = (round_up(pos + iov_iter_count(iov), PAGE_SIZE) -
> + round_down(pos, PAGE_SIZE)) / PAGE_SIZE;
> +
> + /*
> + * But still limit it to 64KiB, so we can still get a similar
> + * buffered write performance between different page sizes
> + */
> + nr_pages = min_t(int, nr_pages, SZ_64K / PAGE_SIZE);
> +
> + return nr_pages;
> +}
> +
> static noinline ssize_t btrfs_buffered_write(struct kiocb *iocb,
> struct iov_iter *i)
> {
> @@ -1638,10 +1661,7 @@ static noinline ssize_t btrfs_buffered_write(struct kiocb *iocb,
> bool only_release_metadata = false;
> bool force_page_uptodate = false;
>
> - nrptrs = min(DIV_ROUND_UP(iov_iter_count(i), PAGE_SIZE),
> - PAGE_SIZE / (sizeof(struct page *)));
> - nrptrs = min(nrptrs, current->nr_dirtied_pause - current->nr_dirtied);
> - nrptrs = max(nrptrs, 8);
> + nrptrs = get_nr_pages(fs_info, pos, i);
> pages = kmalloc_array(nrptrs, sizeof(struct page *), GFP_KERNEL);
> if (!pages)
> return -ENOMEM;
>