On Wed, 2007-01-17 at 12:05 -0800, Christoph Lameter wrote:
> On Tue, 16 Jan 2007, Peter Zijlstra wrote:
>
> > The current NFS client congestion logic is severely broken, it marks the
> > backing device congested during each nfs_writepages() call and implements
> > its own waitqueue.
>
> This is the magic bullet that Andrew is looking for to fix the NFS issues?
Dunno if its magical, but it does solve a few issues I ran into.
> > Index: linux-2.6-git/include/linux/nfs_fs_sb.h
> > ===================================================================
> > --- linux-2.6-git.orig/include/linux/nfs_fs_sb.h 2007-01-12
> > 08:03:47.000000000 +0100
> > +++ linux-2.6-git/include/linux/nfs_fs_sb.h 2007-01-12 08:53:26.000000000
> > +0100
> > @@ -82,6 +82,8 @@ struct nfs_server {
> > struct rpc_clnt * client_acl; /* ACL RPC client handle */
> > struct nfs_iostats * io_stats; /* I/O statistics */
> > struct backing_dev_info backing_dev_info;
> > + atomic_t writeback; /* number of writeback pages */
> > + atomic_t commit; /* number of commit pages */
> > int flags; /* various flags */
>
> I think writeback is frequently incremented? Would it be possible to avoid
> a single global instance of an atomic_t here? In a busy NFS system
> with lots of processors writing via NFS this may cause a hot cacheline
> that limits write speed.
This would be per NFS mount, pretty global indeed. But not different
that other backing_dev_info's. request_queue::nr_requests suffers a
similar fate.
> Would it be possible to use NR_WRITEBACK? If not then maybe add another
> ZVC counter named NFS_NFS_WRITEBACK?
Its a per backing_dev_info thing. So using the zone counters will not
work.
> > Index: linux-2.6-git/mm/page-writeback.c
> > ===================================================================
> > --- linux-2.6-git.orig/mm/page-writeback.c 2007-01-12 08:03:47.000000000
> > +0100
> > +++ linux-2.6-git/mm/page-writeback.c 2007-01-12 08:53:26.000000000
> > +0100
> > @@ -167,6 +167,12 @@ get_dirty_limits(long *pbackground, long
> > *pdirty = dirty;
> > }
> >
> > +int dirty_pages_exceeded(struct address_space *mapping)
> > +{
> > + return dirty_exceeded;
> > +}
> > +EXPORT_SYMBOL_GPL(dirty_pages_exceeded);
> > +
>
> Export the variable instead of adding a new function? Why does it take an
> address space parameter that is not used?
>
Yeah, that function used to be larger.
>
> > Index: linux-2.6-git/fs/inode.c
> > ===================================================================
> > --- linux-2.6-git.orig/fs/inode.c 2007-01-12 08:03:47.000000000 +0100
> > +++ linux-2.6-git/fs/inode.c 2007-01-12 08:53:26.000000000 +0100
> > @@ -81,6 +81,7 @@ static struct hlist_head *inode_hashtabl
> > * the i_state of an inode while it is in use..
> > */
> > DEFINE_SPINLOCK(inode_lock);
> > +EXPORT_SYMBOL_GPL(inode_lock);
>
> Hmmm... Commits to all NFS servers will be globally serialized via the
> inode_lock?
Hmm, right, thats not good indeed, I can pull the call to
nfs_commit_list() out of that loop.
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