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Date: Fri, 28 Nov 2014 18:23:23 +0100
From: Jan Kara <jack@...e.cz>
To: Theodore Ts'o <tytso@....edu>
Cc: Ext4 Developers List <linux-ext4@...r.kernel.org>,
Linux Filesystem Development List
<linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH-v5 1/5] vfs: add support for a lazytime mount option
On Fri 28-11-14 01:00:06, Ted Tso wrote:
> Add a new mount option which enables a new "lazytime" mode. This mode
> causes atime, mtime, and ctime updates to only be made to the
> in-memory version of the inode. The on-disk times will only get
> updated when (a) if the inode needs to be updated for some non-time
> related change, (b) if userspace calls fsync(), syncfs() or sync(), or
> (c) just before an undeleted inode is evicted from memory.
>
> This is OK according to POSIX because there are no guarantees after a
> crash unless userspace explicitly requests via a fsync(2) call.
>
> For workloads which feature a large number of random write to a
> preallocated file, the lazytime mount option significantly reduces
> writes to the inode table. The repeated 4k writes to a single block
> will result in undesirable stress on flash devices and SMR disk
> drives. Even on conventional HDD's, the repeated writes to the inode
> table block will trigger Adjacent Track Interference (ATI) remediation
> latencies, which very negatively impact 99.9 percentile latencies ---
> which is a very big deal for web serving tiers (for example).
>
> Google-Bug-Id: 18297052
>
> Signed-off-by: Theodore Ts'o <tytso@....edu>
> ---
> fs/fs-writeback.c | 64 ++++++++++++++++++++++++++++++++++++++++-----
> fs/inode.c | 41 ++++++++++++++++++++++++-----
> fs/libfs.c | 2 +-
> fs/logfs/readwrite.c | 2 +-
> fs/nfsd/vfs.c | 2 +-
> fs/pipe.c | 2 +-
> fs/proc_namespace.c | 1 +
> fs/sync.c | 8 ++++++
> fs/ufs/truncate.c | 2 +-
> include/linux/backing-dev.h | 1 +
> include/linux/fs.h | 11 ++++++--
> include/uapi/linux/fs.h | 1 +
> mm/backing-dev.c | 10 +++++--
> 13 files changed, 126 insertions(+), 21 deletions(-)
>
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index ef9bef1..518f3bb 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -397,7 +397,7 @@ static void requeue_inode(struct inode *inode, struct bdi_writeback *wb,
> * shot. If still dirty, it will be redirty_tail()'ed below. Update
> * the dirty time to prevent enqueue and sync it again.
> */
> - if ((inode->i_state & I_DIRTY) &&
> + if ((inode->i_state & I_DIRTY_WB) &&
> (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages))
> inode->dirtied_when = jiffies;
>
> @@ -428,13 +428,15 @@ static void requeue_inode(struct inode *inode, struct bdi_writeback *wb,
> */
> redirty_tail(inode, wb);
> }
> - } else if (inode->i_state & I_DIRTY) {
> + } else if (inode->i_state & I_DIRTY_WB) {
> /*
> * Filesystems can dirty the inode during writeback operations,
> * such as delayed allocation during submission or metadata
> * updates after data IO completion.
> */
> redirty_tail(inode, wb);
> + } else if (inode->i_state & I_DIRTY_TIME) {
> + list_move(&inode->i_wb_list, &wb->b_dirty_time);
> } else {
> /* The inode is clean. Remove from writeback lists. */
> list_del_init(&inode->i_wb_list);
> @@ -482,11 +484,15 @@ __writeback_single_inode(struct inode *inode, struct writeback_control *wbc)
> /* Clear I_DIRTY_PAGES if we've written out all dirty pages */
> if (!mapping_tagged(mapping, PAGECACHE_TAG_DIRTY))
> inode->i_state &= ~I_DIRTY_PAGES;
> - dirty = inode->i_state & I_DIRTY;
> - inode->i_state &= ~(I_DIRTY_SYNC | I_DIRTY_DATASYNC);
> + dirty = inode->i_state & (I_DIRTY_SYNC | I_DIRTY_DATASYNC);
> + if (dirty && (inode->i_state & I_DIRTY_TIME))
> + dirty |= I_DIRTY_TIME;
> + inode->i_state &= ~dirty;
> spin_unlock(&inode->i_lock);
> + if (dirty & I_DIRTY_TIME)
> + mark_inode_dirty_sync(inode);
Hum, when someone calls fsync() for an inode, you likely want to sync
timestamps to disk even if everything else is clean. I think that doing
what you did in last version:
dirty = inode->i_state & I_DIRTY_INODE;
inode->i_state &= ~I_DIRTY_INODE;
spin_unlock(&inode->i_lock);
if (dirty & I_DIRTY_TIME)
mark_inode_dirty_sync(inode);
looks better to me. IMO when someone calls __writeback_single_inode() we
should write whatever we have...
> /* Don't write the inode if only I_DIRTY_PAGES was set */
> - if (dirty & (I_DIRTY_SYNC | I_DIRTY_DATASYNC)) {
> + if (dirty) {
> int err = write_inode(inode, wbc);
> if (ret == 0)
> ret = err;
> @@ -1162,7 +1168,11 @@ void __mark_inode_dirty(struct inode *inode, int flags)
>
> spin_lock(&inode->i_lock);
> if ((inode->i_state & flags) != flags) {
> - const int was_dirty = inode->i_state & I_DIRTY;
> + const int was_dirty = inode->i_state & I_DIRTY_WB;
> +
> + if ((flags & (I_DIRTY_SYNC | I_DIRTY_DATASYNC)) &&
> + (inode->i_state & I_DIRTY_TIME))
> + inode->i_state &= ~I_DIRTY_TIME;
>
> inode->i_state |= flags;
>
> @@ -1224,6 +1234,24 @@ out_unlock_inode:
> }
> EXPORT_SYMBOL(__mark_inode_dirty);
>
> +void inode_requeue_dirtytime(struct inode *inode)
> +{
> + struct backing_dev_info *bdi = inode_to_bdi(inode);
> +
> + spin_lock(&bdi->wb.list_lock);
> + spin_lock(&inode->i_lock);
> + if ((inode->i_state & I_DIRTY_WB) == 0) {
> + if (inode->i_state & I_DIRTY_TIME)
> + list_move(&inode->i_wb_list, &bdi->wb.b_dirty_time);
> + else
> + list_del_init(&inode->i_wb_list);
> + }
> + spin_unlock(&inode->i_lock);
> + spin_unlock(&bdi->wb.list_lock);
> +
> +}
> +EXPORT_SYMBOL(inode_requeue_dirtytime);
> +
This function has a single call site - update_time(). I'd prefer to
handle this as a special case of __mark_inode_dirty() to have all the dirty
queueing in one place...
> static void wait_sb_inodes(struct super_block *sb)
> {
> struct inode *inode, *old_inode = NULL;
> @@ -1277,6 +1305,29 @@ static void wait_sb_inodes(struct super_block *sb)
> iput(old_inode);
> }
>
> +/*
> + * Take all of the indoes on the dirty_time list, and mark them as
> + * dirty, so they will be written out.
> + */
> +static void flush_sb_dirty_time(struct super_block *sb)
> +{
> + struct bdi_writeback *wb = &sb->s_bdi->wb;
> + LIST_HEAD(tmp);
> +
> + spin_lock(&wb->list_lock);
> + list_cut_position(&tmp, &wb->b_dirty_time, wb->b_dirty_time.prev);
> + while (!list_empty(&tmp)) {
> + struct inode *inode = wb_inode(tmp.prev);
> +
> + list_del_init(&inode->i_wb_list);
> + spin_unlock(&wb->list_lock);
> + if (inode->i_state & I_DIRTY_TIME)
> + mark_inode_dirty_sync(inode);
> + spin_lock(&wb->list_lock);
> + }
> + spin_unlock(&wb->list_lock);
> +}
> +
This shouldn't be necessary when you somewhat tweak what you do in
queue_io().
> /**
> * writeback_inodes_sb_nr - writeback dirty inodes from given super_block
> * @sb: the superblock
> @@ -1388,6 +1439,7 @@ void sync_inodes_sb(struct super_block *sb)
> return;
> WARN_ON(!rwsem_is_locked(&sb->s_umount));
>
> + flush_sb_dirty_time(sb);
> bdi_queue_work(sb->s_bdi, &work);
> wait_for_completion(&done);
>
> diff --git a/fs/inode.c b/fs/inode.c
> index 26753ba..1ec0629 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -30,7 +30,7 @@
> * inode_sb_list_lock protects:
> * sb->s_inodes, inode->i_sb_list
> * bdi->wb.list_lock protects:
> - * bdi->wb.b_{dirty,io,more_io}, inode->i_wb_list
> + * bdi->wb.b_{dirty,io,more_io,dirty_time}, inode->i_wb_list
> * inode_hash_lock protects:
> * inode_hashtable, inode->i_hash
> *
> @@ -1430,11 +1430,19 @@ static void iput_final(struct inode *inode)
> */
> void iput(struct inode *inode)
> {
> - if (inode) {
> - BUG_ON(inode->i_state & I_CLEAR);
> -
> - if (atomic_dec_and_lock(&inode->i_count, &inode->i_lock))
> - iput_final(inode);
> + if (!inode)
> + return;
> + BUG_ON(inode->i_state & I_CLEAR);
> +retry:
> + if (atomic_dec_and_lock(&inode->i_count, &inode->i_lock)) {
> + if (inode->i_nlink && (inode->i_state & I_DIRTY_TIME)) {
> + atomic_inc(&inode->i_count);
> + inode->i_state &= ~I_DIRTY_TIME;
> + spin_unlock(&inode->i_lock);
> + mark_inode_dirty_sync(inode);
> + goto retry;
> + }
> + iput_final(inode);
How about my suggestion from previous version to avoid the retry loop by
checking I_DIRTY_TIME before atomic_dec_and_lock()?
Honza
--
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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