Btrfs: Fix streaming read performance with checksumming on

Large streaming reads make for large bios, which means each entry on the
list async work queues represents a large amount of data.  IO
congestion throttling on the device was kicking in before the async
worker threads decided a single thread was busy and needed some help.

The end result was that a streaming read would result in a single CPU
running at 100% instead of balancing the work off to other CPUs.

This patch also changes the pre-IO checksum lookup done by reads to
work on a per-bio basis instead of a per-page.  This results in many
extra btree lookups on large streaming reads.  Doing the checksum lookup
right before bio submit allows us to reuse searches while processing
adjacent offsets.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

1 files changed, 15 insertions, 0 deletions

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index e826730d750f..d2d1cc87e8ad 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -1357,10 +1357,25 @@ struct btrfs_root *open_ctree(struct super_block *sb,
          */
         btrfs_init_workers(&fs_info->workers, fs_info->thread_pool_size);
         btrfs_init_workers(&fs_info->submit_workers, fs_info->thread_pool_size);
+
+        /* a higher idle thresh on the submit workers makes it much more
+         * likely that bios will be send down in a sane order to the
+         * devices
+         */
+        fs_info->submit_workers.idle_thresh = 64;
+
         btrfs_init_workers(&fs_info->fixup_workers, 1);
         btrfs_init_workers(&fs_info->endio_workers, fs_info->thread_pool_size);
         btrfs_init_workers(&fs_info->endio_write_workers,
                            fs_info->thread_pool_size);
+
+        /*
+         * endios are largely parallel and should have a very
+         * low idle thresh
+         */
+        fs_info->endio_workers.idle_thresh = 4;
+        fs_info->endio_write_workers.idle_thresh = 4;
+
         btrfs_start_workers(&fs_info->workers, 1);
         btrfs_start_workers(&fs_info->submit_workers, 1);
         btrfs_start_workers(&fs_info->fixup_workers, 1);