Re: [patch 1/1] Writeback fix for concurrent large and small filewrites

[Fengguang Wu]

On Thu, Nov 29, 2007 at 12:16:36PM -0800, Michael Rubin wrote:
> Due to my faux pas of top posting (see
> http://www.zip.com.au/~akpm/linux/patches/stuff/top-posting.txt) I am
> resending this email.
> 
> On Nov 28, 2007 4:34 PM, Fengguang Wu <wfg@xxxxxxxxxxxxxxxx> wrote:
> > Could you demonstrate the situation? Or if I guess it right, could it
> > be fixed by the following patch? (not a nack: If so, your patch could
> > also be considered as a general purpose improvement, instead of a bug
> > fix.)
> >
> > diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> > index 0fca820..62e62e2 100644
> > --- a/fs/fs-writeback.c
> > +++ b/fs/fs-writeback.c
> > @@ -301,7 +301,7 @@ __sync_single_inode(struct inode *inode, struct writeback_control *wbc)
> >                          * Someone redirtied the inode while were writing back
> >                          * the pages.
> >                          */
> > -                       redirty_tail(inode);
> > +                       requeue_io(inode);
> >                 } else if (atomic_read(&inode->i_count)) {
> >                         /*
> >                          * The inode is clean, inuse
> >
> 
> By testing the situation I can confirm that the one line patch above
> fixes the problem.
> 
> I will continue testing some other cases to see if it cause any other
> issues but I don't expect it to.

One major concern could be whether a continuous writer dirting pages
at the 'right' pace will generate a steady flow of write I/Os which are
_tiny_hence_inefficient_.

I have gathered some timing info about writeback speed in
http://lkml.org/lkml/2007/10/4/468. For ext3, it takes wb_kupdate()
~15ms to submit 4MB. Whereas one disk I/O typically takes ~5ms. So if
there are too many tiny write I/Os, they will simply get delayed and
merged into bigger ones.

So it's not a problem in *theory* :-)

> I will post this change for 2.6.24 and list Feng as author. If that's
> ok with Feng.

Thank you.

> As for the original patch I will resubmit it for 2.6.25 as a general
> purpose improvement.

There are some discussions and patches on inode number based writeback
clustering which you may want to reference/compare with:
                http://lkml.org/lkml/2007/8/21/396
                http://lkml.org/lkml/2007/8/27/45

Cheers,
Fengguang

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