Re: [PATCH] ext4: optimize orphan_list handling for ext4_setattr

Andreas Dilger
2010-11-03T09:06:43Z

On 2010-10-25, at 01:06, Dmitry Monakhov wrote:
> Surprisingly chown() on ext4 is not SMP scalable operation.
> Due to unconditional orphan_del(NULL, inode) in ext4_setattr()
> result in significant performance overhead because of global orphan
> mutex, especially in no-journal mode (where orphan_add() is noop).
> It is possible to skip explicit orphan_del if possible.
>
> Results of fchown() micro-benchmark in no-journal mode
> while (1) {
> iteration++;
> fchown(fd, uid, gid);
> fchown(fd, uid + 1, gid + 1)
> }
> measured: iterations per millisecond
> | nr_tasks | w/o patch | with patch |
> | 1 | 142 | 185 |
> | 4 | 109 | 642 |

AFAICS, both ext4_orphan_add() and ext4_orphan_del() already check right at the top of the function whether the handle is valid, so I can't really understand how this patch would make any difference for no-journal mode.

> @@ -5636,7 +5640,7 @@ int ext4_setattr(struct dentry *dentry, struct iattr

> + if (ext4_handle_valid(handle)) {
> + error = ext4_orphan_add(handle, inode);
> + orphan = 1;
> + }
> EXT4_I(inode)->i_disksize = attr->ia_size;
> rc = ext4_mark_inode_dirty(handle, inode);
> if (!error)

> @@ -5636,7 +5640,7 @@ int ext4_setattr(struct dentry *dentry, struct iattr
> * If the call to ext4_truncate failed to get a transaction handle at
> * all, we need to clean up the in-core orphan list manually.
> */
> - if (inode->i_nlink)
> + if (orphan && inode->i_nlink)
> ext4_orphan_del(NULL, inode);
>
> if (!rc && (ia_valid & ATTR_MODE))

Basically, the first hunk is moving the ext4_handle_valid() into the caller, and the second is avoiding a call to ext4_orphan_del->ext4_handle_valid(), which would likely be an unmeasurable performance difference. No locks are taken (or avoided) before ext4_handle_valid() is checked.

I think the actual fix would be to always set "orphan = 1" after ext4_orphan_add() is called, and only call ext4_orphan_del() in that case. This is essentially what your patch does for with-journal mode, and the non-journal mode is a red-herring due to the early exit from ext4_orphan_del().

Even better would be a no-lock check of list_empty(&ei->i_orphan()) before getting s_orphan_lock, since it shouldn't be possible to have two threads adding/removing the same inode to the orphan list (otherwise the inode may not be on the orphan list at all since add/del is not refcounted). The only reason for s_orphan_lock is to prevent corruption of the global list.

Cheers, Andreas