From: Christoph Hellwig Date: Tue, 21 Jun 2022 06:26:27 +0000 (+0200) Subject: btrfs: increase direct io read size limit to 256 sectors X-Git-Url: http://git.maquefel.me/?a=commitdiff_plain;h=ee5b46a353afe23d04fac464e83a8c788342d453;p=linux.git btrfs: increase direct io read size limit to 256 sectors Btrfs currently limits direct I/O reads to a single sector, which goes back to commit c329861da406 ("Btrfs: don't allocate a separate csums array for direct reads") from Josef. That commit changes the direct I/O code to ".. use the private part of the io_tree for our csums.", but ten years later that isn't how checksums for direct reads work, instead they use a csums allocation on a per-btrfs_dio_private basis (which have their own performance problem for small I/O, but that will be addressed later). There is no fundamental limit in btrfs itself to limit the I/O size except for the size of the checksum array that scales linearly with the number of sectors in an I/O. Pick a somewhat arbitrary limit of 256 limits, which matches what the buffered reads typically see as the upper limit as the limit for direct I/O as well. This significantly improves direct read performance. For example a fio run doing 1 MiB aio reads with a queue depth of 1 roughly triples the throughput: Baseline: READ: bw=65.3MiB/s (68.5MB/s), 65.3MiB/s-65.3MiB/s (68.5MB/s-68.5MB/s), io=19.1GiB (20.6GB), run=300013-300013msec With this patch: READ: bw=196MiB/s (206MB/s), 196MiB/s-196MiB/s (206MB/s-206MB/s), io=57.5GiB (61.7GB), run=300006-300006msc Reviewed-by: Qu Wenruo Reviewed-by: Nikolay Borisov Signed-off-by: Christoph Hellwig Signed-off-by: David Sterba --- diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c index d535ed39c3918..03771f09a9a21 100644 --- a/fs/btrfs/inode.c +++ b/fs/btrfs/inode.c @@ -7578,8 +7578,12 @@ static int btrfs_dio_iomap_begin(struct inode *inode, loff_t start, const u64 data_alloc_len = length; bool unlock_extents = false; + /* + * Cap the size of reads to that usually seen in buffered I/O as we need + * to allocate a contiguous array for the checksums. + */ if (!write) - len = min_t(u64, len, fs_info->sectorsize); + len = min_t(u64, len, fs_info->sectorsize * BTRFS_MAX_BIO_SECTORS); lockstart = start; lockend = start + len - 1; diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h index b61508723d5d2..9537d82bb7a20 100644 --- a/fs/btrfs/volumes.h +++ b/fs/btrfs/volumes.h @@ -354,6 +354,13 @@ struct btrfs_fs_devices { - 2 * sizeof(struct btrfs_chunk)) \ / sizeof(struct btrfs_stripe) + 1) +/* + * Maximum number of sectors for a single bio to limit the size of the + * checksum array. This matches the number of bio_vecs per bio and thus the + * I/O size for buffered I/O. + */ +#define BTRFS_MAX_BIO_SECTORS (256) + /* * Additional info to pass along bio. *