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

Trond Myklebust
2009-12-24T12:05:32Z

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?

Trond