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SubjectRe: [PATCH 29/35] nfs: in-commit pages accounting and wait queue
On Tue, Dec 14, 2010 at 11:57:25PM +0800, Trond Myklebust wrote:
> On Tue, 2010-12-14 at 23:40 +0800, Wu Fengguang wrote:
> > On Tue, Dec 14, 2010 at 05:15:51AM +0800, Trond Myklebust wrote:
> > > On Mon, 2010-12-13 at 22:47 +0800, Wu Fengguang wrote:
> > > > plain text document attachment (writeback-nfs-in-commit.patch)
> > > > When doing 10+ concurrent dd's, I observed very bumpy commits submission
> > > > (partly because the dd's are started at the same time, and hence reached
> > > > 4MB to-commit pages at the same time). Basically we rely on the server
> > > > to complete and return write/commit requests, and want both to progress
> > > > smoothly and not consume too many pages. The write request wait queue is
> > > > not enough as it's mainly network bounded. So add another commit request
> > > > wait queue. Only async writes need to sleep on this queue.
> > > >
> > >
> > > I'm not understanding the above reasoning. Why should we serialise
> > > commits at the per-filesystem level (and only for non-blocking flushes
> > > at that)?
> >
> > I did the commit wait queue after seeing this graph, where there is
> > very bursty pattern of commit submission and hence completion:
> >
> >
> >
> > leading to big fluctuations, eg. the almost straight up/straight down
> > lines below
> >
> >
> >
> >
> > A commit wait queue will help wipe out the "peaks". The "fixed" graph
> > is
> >
> >
> >
> > Blocking flushes don't need to wait on this queue because they already
> > throttle themselves by waiting on the inode commit lock before/after
> > the commit. They actually should not wait on this queue, to prevent
> > sync requests being unnecessarily blocked by async ones.
> OK, but isn't it better then to just abort the commit, and have the
> relevant async process retry it later?

I'll drop this patch. I vaguely remember that bursty commit graph
mentioned below

> > I did the commit wait queue after seeing this graph, where there is
> > very bursty pattern of commit submission and hence completion:
> >
> >

is caused by this condition in nfs_should_commit():

/* big enough */
if (to_commit >= MIN_WRITEBACK_PAGES)
return true;

It's because the 100 dd's accumulated 4MB dirty pages at roughly the
same time. Then I added the in_commit accounting (for the below test)
and wait queue. It seems that the below condition is good enough to
smooth out the commit distribution.

/* active commits drop low: kick more IO for the server disk */
if (to_commit > in_commit / 2)
return true;

And I'm going further remove the above two conditions, and do a much
more simple change:

- if (nfsi->ncommit <= (nfsi->npages >> 1))
+ if (nfsi->ncommit <= (nfsi->npages >> 4))
goto out_mark_dirty;

The change to ">> 4" helps reduce the fluctuation to the acceptable
level: balance_dirty_page() is now doing soft dirty throttling in a
small range of bdi_dirty_limit/8. The above change guarantees that
when an NFS commit completes, the bdi_dirty won't suddenly drop out
of the soft throttling region. On my mem=3GB test box and 1-dd case,
npages/16 ~= 32MB is still a large size.

Basic tests show that it achieves roughly the same effect with these
two patches

[PATCH 29/35] nfs: in-commit pages accounting and wait queue
[PATCH 30/35] nfs: heuristics to avoid commit

It would not only be simpler, but also be able to do larger commits in
the case of "fast and memory bounty server/client connected by slow
network". In this case, the above two patches will do 4MB commits,
while the simpler change can do much larger.

> This is a code path which is followed by kswapd, for instance. It seems
> dangerous to be throttling that instead of allowing it to proceed (and
> perhaps being able to free up memory on some other partition in the mean
> time).

It seems pageout() calls nfs_writepage(), the latter does unstable
write and also won't commit the page. This means pageout() cannot
guarantee free of the page at all.. so NFS dirty pages are virtually


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