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    SubjectRe: [RFC] page-writeback: move indoes from one superblock together
    On Thu, Sep 24, 2009 at 02:54:20PM +0800, Li, Shaohua wrote:
    > __mark_inode_dirty adds inode to wb dirty list in random order. If a disk has
    > several partitions, writeback might keep spindle moving between partitions.
    > To reduce the move, better write big chunk of one partition and then move to
    > another. Inodes from one fs usually are in one partion, so idealy move indoes
    > from one fs together should reduce spindle move. This patch tries to address
    > this. Before per-bdi writeback is added, the behavior is write indoes
    > from one fs first and then another, so the patch restores previous behavior.
    > The loop in the patch is a bit ugly, should we add a dirty list for each
    > superblock in bdi_writeback?
    >
    > Test in a two partition disk with attached fio script shows about 3% ~ 6%
    > improvement.

    Reviewed-by: Wu Fengguang <fengguang.wu@intel.com>

    Good idea! The optimization looks good to me, it addresses one
    weakness of per-bdi writeback.

    But one problem is, Jan Kara and me are planning to remove b_io and
    hence this move_expired_inodes() function. Not sure how to do this
    optimization without b_io.

    > Signed-off-by: Shaohua Li <shaohua.li@intel.com>
    >
    > diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
    > index 8e1e5e1..fc87730 100644
    > --- a/fs/fs-writeback.c
    > +++ b/fs/fs-writeback.c
    > @@ -324,13 +324,29 @@ static void move_expired_inodes(struct list_head *delaying_queue,
    > struct list_head *dispatch_queue,
    > unsigned long *older_than_this)
    > {
    > + LIST_HEAD(tmp);
    > + struct list_head *pos, *node;
    > + struct super_block *sb;
    > + struct inode *inode;
    > +
    > while (!list_empty(delaying_queue)) {
    > - struct inode *inode = list_entry(delaying_queue->prev,
    > - struct inode, i_list);
    > + inode = list_entry(delaying_queue->prev, struct inode, i_list);
    > if (older_than_this &&
    > inode_dirtied_after(inode, *older_than_this))
    > break;
    > - list_move(&inode->i_list, dispatch_queue);
    > + list_move(&inode->i_list, &tmp);
    > + }
    > +
    > + /* Move indoes from one superblock together */
    > + while (!list_empty(&tmp)) {
    > + inode = list_entry(tmp.prev, struct inode, i_list);
    > + sb = inode->i_sb;
    > + list_for_each_prev_safe(pos, node, &tmp) {

    We are in spin lock, so not necessary to use the safe version?

    > + struct inode *inode = list_entry(pos,

    Could just reuse inode.

    Thanks,
    Fengguang

    > + struct inode, i_list);
    > + if (inode->i_sb == sb)
    > + list_move(&inode->i_list, dispatch_queue);
    > + }
    > }
    > }
    >
    >

    Content-Description: newfio
    > [global]
    > runtime=120
    > ioscheduler=cfq
    > size=2G
    > ioengine=sync
    > rw=write
    > file_service_type=random:256
    > overwrite=1
    >
    > [sdb1]
    > directory=/mnt/b1
    > nrfiles=10
    > numjobs=4
    >
    > [sdb2]
    > directory=/mnt/b2
    > nrfiles=10
    > numjobs=4



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