Re: your mail

Wu Fengguang
2010-06-22T14:12:37Z

On Tue, Jun 22, 2010 at 09:54:58PM +0800, Jan Kara wrote:
> On Tue 22-06-10 10:59:41, Wu Fengguang wrote:
> > > - use tagging also for WB_SYNC_NONE writeback - there's problem with an
> > > interaction with wbc->nr_to_write. If we tag all dirty pages, we can
> > > spend too much time tagging when we write only a few pages in the end
> > > because of nr_to_write. If we tag only say nr_to_write pages, we may
> > > not have enough pages tagged because some pages are written out by
> > > someone else and so we would have to restart and tagging would become
> >
> > This could be addressed by ignoring nr_to_write for the WB_SYNC_NONE
> > writeback triggered by sync(). write_cache_pages() already ignored
> > nr_to_write for WB_SYNC_ALL.
> We could do that but frankly, I'm not very fond of adding more special
> cases to writeback code than strictly necessary...

So do me. However for this case we only need to broaden the special case test:

if (nr_to_write > 0) {
nr_to_write--;
if (nr_to_write == 0 &&
- wbc->sync_mode == WB_SYNC_NONE) {
+ !wbc->for_sync) {

> > > essentially useless. So my option is - switch to tagging for WB_SYNC_NONE
> > > writeback if we can get rid of nr_to_write. But that's a story for
> > > a different patch set.
> >
> > Besides introducing overheads, it will be a policy change in which the
> > system loses control to somehow "throttle" writeback of huge files.
> Yes, but if we guarantee we cannot livelock on a single file, do we care?
> Memory management does not care because it's getting rid of dirty pages
> which is what it wants. User might care but actually writing out files in
> the order they were dirtied (i.e., the order user written them) is quite
> natural so it's not a "surprising" behavior. And I don't think we can
> assume that data in those small files are more valuable than data in the
> large file and thus should be written earlier...

It could be a surprising behavior when, there is a 4GB dirty file and
100 small dirty files. The user may expect the 100 small dirty files
be synced to disk after 30s. However without nr_to_write, they could
be delayed by the 4GB file for 40 more seconds. Now if something goes
wrong in between and the small dirty files happen to include some
.c/.tex/.txt/... files.

Small files are more likely your precious documents that are typed in
word-by-word and perhaps the most important data you want to protect.
Naturally you'll want them enjoy more priority than large files.

> With the overhead you are right that tagging is more expensive than
> checking nr_to_write limit...

Thanks,
Fengguang