Re: [PATCH] improve the performance of large sequential write NFS workloads

Wu Fengguang
2009-12-25T05:56:23Z

On Thu, Dec 24, 2009 at 08:04:41PM +0800, Trond Myklebust wrote:
> On Thu, 2009-12-24 at 10:52 +0800, Wu Fengguang wrote:
> > Trond,
> >
> > On Thu, Dec 24, 2009 at 03:12:54AM +0800, Trond Myklebust wrote:
> > > On Wed, 2009-12-23 at 19:05 +0100, Jan Kara wrote:
> > > > On Wed 23-12-09 15:21:47, Trond Myklebust wrote:
> > > > > @@ -474,6 +482,18 @@ writeback_single_inode(struct inode *inode, struct writeback_control *wbc)
> > > > > }
> > > > >
> > > > > spin_lock(&inode_lock);
> > > > > + /*
> > > > > + * Special state for cleaning NFS unstable pages
> > > > > + */
> > > > > + if (inode->i_state & I_UNSTABLE_PAGES) {
> > > > > + int err;
> > > > > + inode->i_state &= ~I_UNSTABLE_PAGES;
> > > > > + spin_unlock(&inode_lock);
> > > > > + err = commit_unstable_pages(inode, wait);
> > > > > + if (ret == 0)
> > > > > + ret = err;
> > > > > + spin_lock(&inode_lock);
> > > > > + }
> > > > I don't quite understand this chunk: We've called writeback_single_inode
> > > > because it had some dirty pages. Thus it has I_DIRTY_DATASYNC set and a few
> > > > lines above your chunk, we've called nfs_write_inode which sent commit to
> > > > the server. Now here you sometimes send the commit again? What's the
> > > > purpose?
> > >
> > > We no longer set I_DIRTY_DATASYNC. We only set I_DIRTY_PAGES (and later
> > > I_UNSTABLE_PAGES).
> > >
> > > The point is that we now do the commit only _after_ we've sent all the
> > > dirty pages, and waited for writeback to complete, whereas previously we
> > > did it in the wrong order.
> >
> > Sorry I still don't get it. The timing used to be:
> >
> > write 4MB ==> WRITE block 0 (ie. first 512KB)
> > WRITE block 1
> > WRITE block 2
> > WRITE block 3 ack from server for WRITE block 0 => mark 0 as unstable (inode marked need-commit)
> > WRITE block 4 ack from server for WRITE block 1 => mark 1 as unstable
> > WRITE block 5 ack from server for WRITE block 2 => mark 2 as unstable
> > WRITE block 6 ack from server for WRITE block 3 => mark 3 as unstable
> > WRITE block 7 ack from server for WRITE block 4 => mark 4 as unstable
> > ack from server for WRITE block 5 => mark 5 as unstable
> > write_inode ==> COMMIT blocks 0-5
> > ack from server for WRITE block 6 => mark 6 as unstable (inode marked need-commit)
> > ack from server for WRITE block 7 => mark 7 as unstable
> >
> > ack from server for COMMIT blocks 0-5 => mark 0-5 as clean
> >
> > write_inode ==> COMMIT blocks 6-7
> >
> > ack from server for COMMIT blocks 6-7 => mark 6-7 as clean
> >
> > Note that the first COMMIT is submitted before receiving all ACKs for
> > the previous writes, hence the second COMMIT is necessary. It seems
> > that your patch does not improve the timing at all.
>
> That would indicate that we're cycling through writeback_single_inode()
> more than once. Why?

Yes. The above sequence can happen for a 4MB sized dirty file.
The first COMMIT is done by L547, while the second COMMIT will be
scheduled either by __mark_inode_dirty(), or scheduled by L583
(depending on the time ACKs for L543 but missed L547 arrives:
if an ACK missed L578, the inode will be queued into b_dirty list,
but if any ACK arrives between L547 and L578, the inode will enter
b_more_io_wait, which is a to-be-introduced new dirty list).

537 dirty = inode->i_state & I_DIRTY;
538 inode->i_state |= I_SYNC;
539 inode->i_state &= ~I_DIRTY;
540
541 spin_unlock(&inode_lock);
542
==> 543 ret = do_writepages(mapping, wbc);
544
545 /* Don't write the inode if only I_DIRTY_PAGES was set */
546 if (dirty & (I_DIRTY_SYNC | I_DIRTY_DATASYNC)) {
==> 547 int err = write_inode(inode, wait);
548 if (ret == 0)
549 ret = err;
550 }
551
552 if (wait) {
553 int err = filemap_fdatawait(mapping);
554 if (ret == 0)
555 ret = err;
556 }
557
558 spin_lock(&inode_lock);
559 inode->i_state &= ~I_SYNC;
560 if (!(inode->i_state & (I_FREEING | I_CLEAR))) {
561 if (mapping_tagged(mapping, PAGECACHE_TAG_DIRTY)) {
562 /*
563 * We didn't write back all the pages. nfs_writepages()
564 * sometimes bales out without doing anything.
565 */
566 inode->i_state |= I_DIRTY_PAGES;
567 if (wbc->nr_to_write <= 0) {
568 /*
569 * slice used up: queue for next turn
570 */
571 requeue_io(inode);
572 } else {
573 /*
574 * somehow blocked: retry later
575 */
576 requeue_io_wait(inode);
577 }
==> 578 } else if (inode->i_state & I_DIRTY) {
579 /*
580 * At least XFS will redirty the inode during the
581 * writeback (delalloc) and on io completion (isize).
582 */
==> 583 requeue_io_wait(inode);

Thanks,
Fengguang