Re: [PATCH 0/5] [RFC] transfer ASYNC vmscan writeback IO to theflusher threads

From: Wu Fengguang
Date: Fri Jul 30 2010 - 01:34:18 EST

On Fri, Jul 30, 2010 at 12:09:47AM +0800, Jan Kara wrote:
> On Thu 29-07-10 19:51:42, Wu Fengguang wrote:
> > Andrew,
> >
> > It's possible to transfer ASYNC vmscan writeback IOs to the flusher threads.
> > This simple patchset shows the basic idea. Since it's a big behavior change,
> > there are inevitably lots of details to sort out. I don't know where it will
> > go after tests and discussions, so the patches are intentionally kept simple.
> >
> > sync livelock avoidance (need more to be complete, but this is minimal required for the last two patches)
> > [PATCH 1/5] writeback: introduce wbc.for_sync to cover the two sync stages
> > [PATCH 2/5] writeback: stop periodic/background work on seeing sync works
> > [PATCH 3/5] writeback: prevent sync livelock with the sync_after timestamp
> Well, essentially any WB_SYNC_NONE writeback is still livelockable if you
> just grow a file constantly. So your changes are a step in the right
> direction but won't fix the issue completely.

Right. We have complementary patches to prevent livelocks both inside
file and among files.

> But what we could do to fix
> the issue completely would be to just set wbc->nr_to_write to LONG_MAX
> before writing inode for sync use my livelock avoidance using page-tagging
> for this case (it wouldn't have the possible performance issue because we
> are going to write all the inode anyway).

Yeah your patches are good to avoid livelocking in one single busy file.
I didn't forgot them :)

> I can write the patch but frankly there are so many patches floating
> around that I'm not sure what I should base it on...

Me confused too. It may take some time to quiet down..

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