Re: regression in page writeback

Wu Fengguang
2009-10-02T03:28:16Z

On Fri, Oct 02, 2009 at 06:17:39AM +0800, Jan Kara wrote:
> On Wed 30-09-09 13:32:23, Wu Fengguang wrote:
> > writeback: bump up writeback chunk size to 128MB
> >
> > Adjust the writeback call stack to support larger writeback chunk size.
> >
> > - make wbc.nr_to_write a per-file parameter
> > - init wbc.nr_to_write with MAX_WRITEBACK_PAGES=128MB
> > (proposed by Ted)
> > - add wbc.nr_segments to limit seeks inside sparsely dirtied file
> > (proposed by Chris)
> > - add wbc.timeout which will be used to control IO submission time
> > either per-file or globally.
> >
> > The wbc.nr_segments is now determined purely by logical page index
> > distance: if two pages are 1MB apart, it makes a new segment.
> >
> > Filesystems could do this better with real extent knowledges.
> > One possible scheme is to record the previous page index in
> > wbc.writeback_index, and let ->writepage compare if the current and
> > previous pages lie in the same extent, and decrease wbc.nr_segments
> > accordingly. Care should taken to avoid double decreases in writepage
> > and write_cache_pages.
> >
> > The wbc.timeout (when used per-file) is mainly a safeguard against slow
> > devices, which may take too long time to sync 128MB data.
> >
> > The wbc.timeout (when used globally) could be useful when we decide to
> > do two sync scans on dirty pages and dirty metadata. XFS could say:
> > please return to sync dirty metadata after 10s. Would need another
> > b_io_metadata queue, but that's possible.
> >
> > This work depends on the balance_dirty_pages() wait queue patch.
> I don't know, I think it gets too complicated... I'd either use the
> segments idea or the timeout idea but not both (unless you can find real
> world tests in which both help).

Maybe complicated, but nr_segments and timeout each has their target
application. nr_segments serves two major purposes:
- fairness between two large files, one is continuously dirtied,
another is sparsely dirtied. Given the same amount of dirty pages,
it could take vastly different time to sync them to the _same_
device. The nr_segments check helps to favor continuous data.
- avoid seeks/fragmentations. To give each file fair chance of
writeback, we have to abort a file when some nr_to_write or timeout
is reached. However they are both not good abort conditions.
The best is for filesystem to abort earlier in seek boundaries,
and treat nr_to_write/timeout as large enough bottom lines.
timeout is mainly a safeguard in case nr_to_write is too large for
slow devices. It is not necessary if nr_to_write is auto-computed,
however timeout in itself serves as a simple throughput adapting
scheme.

> Also when we'll assure fairness via
> timeout, maybe nr_to_write isn't needed anymore? WB_SYNC_ALL writeback
> doesn't use nr_to_write. WB_SYNC_NONE writeback either sets it to some
> large value (like LONG_MAX) or number of dirty pages (to effectively write
> back as much as possible) or to MAX_WRITEBACK_PAGES to assure fairness
> in kupdate style writeback. There are a few exceptions in btrfs but I
> belive nr_to_write isn't really needed there either...

Totally agreed. I'd rather remove the top level nr_page/nr_to_write
parameters. They are simply redundant ones.. The meaningful ones are
background threshold, dirty expireness or global timeout, depending on
the mission of the writeback work.

Thanks,
Fengguang