Re: [PATCH 0/7] Per-bdi writeback flusher threads v20

[Chris Mason]

On Tue, Sep 22, 2009 at 09:18:32PM +0800, Wu Fengguang wrote:
> On Tue, Sep 22, 2009 at 07:30:55PM +0800, Chris Mason wrote:

[ using a very large MAX_WRITEBACK_PAGES ]

> > > > I'm starting to rethink the 128MB MAX_WRITEBACK_PAGES.  128MB is the
> > > > right answer for the flusher thread on sequential IO, but definitely not
> > > > on random IO.  We don't want the flusher to get bogged down on random
> > > > writeback and start ignoring every other file.
> > > 
> > > Hmm, I'd think a larger MAX_WRITEBACK_PAGES shall never increase the
> > > writeback randomness.
> > 
> > It doesn't increase the randomness, but if we have a file full of
> > buffered random IO (say from bdb or rpm), the 128MB max will mean that
> > one file dominates the flusher thread writeback completely.
> 
> What if we add a bdi->max_segments quota? A segment is a continuous
> run of dirty pages in the inode address space. SSD or fast RAID could
> set it to a large enough value.

I'd rather play with timeslice ideas first ;)  But, don't let me stop
you from trying interesting things.

> 
> > > 
> > > > My btrfs performance branch has long had a change to bump the
> > > > nr_to_write up based on the size of the delayed allocation that we're
> > > > doing.  It helped, but not as much as I really expected it too, and a
> > > > similar patch from Christoph for XFS was good but not great.
> > > > 
> > > > It turns out the problem is in write_cache_pages.  It processes a whole
> > > > pagevec at a time, something like this:
> > > > 
> > > > while(!done) {
> > > > 	for each page in the pagegvec {
> > > > 		writepage()
> > > > 		if (wbc->nr_to_write <= 0)
> > > > 			done = 1;
> > > > 	}
> > > > }
> > > > 
> > > > If the filesystem decides to bump nr_to_write to cover a whole
> > > > extent (or a max reasonable size), the new value of nr_to_write may
> > > > be ignored if nr_to_write had already gone done to zero.
> > > > 
> > > > I fixed btrfs to recheck nr_to_write every time, and the results are
> > > > much smoother.  This is what it looks like to write out all the .o files
> > > > in the kernel.
> > > > 
> > > > http://oss.oracle.com/~mason/seekwatcher/btrfs-nr-to-write.png
> > > > 
> > > > In this graph, Btrfs is writing the full extent or 8192 pages, whichever
> > > > is smaller.  The write_cache_pages change is here, but it is local to
> > > > the btrfs copy of write_cache_pages:
> > > > 
> > > > http://git.kernel.org/?p=linux/kernel/git/mason/btrfs-unstable.git;a=commit;h=f85d7d6c8f2ad4a86a1f4f4e3791f36dede2fa76
> > > 
> > > It seems you tried to an upper limit of 32-64MB:
> > > 
> > > +               if (wbc->nr_to_write < delalloc_to_write) {
> > > +                       int thresh = 8192;
> > > +
> > > +                       if (delalloc_to_write < thresh * 2)
> > > +                               thresh = delalloc_to_write;
> > > +                       wbc->nr_to_write = min_t(u64, delalloc_to_write,
> > > +                                                thresh);
> > > +               }
> > > 
> > > However it is possible that btrfs bumps up nr_to_write for each inode, 
> > > so that the accumulated bump ups are too large to be acceptable for
> > > balance_dirty_pages().
> > 
> > We bump up to a limit of 64MB more than the original nr_to_write. This
> > is because when we do bump we know we'll write the whole amount, and
> > then write_cache_pages will end.
> 
> Imagine this scenario. There are inodes A, B, C, ...
> 
> A) delalloc_to_write=3000 but only 1000 pages dirty.

The part that isn't clear from the code you're reading is that if
delalloc_to_write is 3000, then there must be 3000 pages dirty.  The
count of delalloc bytes to go down always reflects IO that must be done.

So, once my writepage call bumps nr_to_write, that IO will happen.  The
only exception is if someone else jumps in and writes the pages, which
won't happen unless there is synchronous writeback.


> > > Yes a more general solution would help. I'd like to propose one which
> > > works in the other way round. In brief,
> > > (1) the VFS give a large enough per-file writeback quota to btrfs;
> > > (2) btrfs tells VFS "here is a (seek) boundary, stop voluntarily",
> > >     before exhausting the quota and be force stopped.
> > > 
> > > There will be two limits (the second one is new):
> > > 
> > > - total nr to write in one wb_writeback invocation
> > > - _max_ nr to write per file (before switching to sync the next inode)
> > > 
> > > The per-invocation limit is useful for balance_dirty_pages().
> > > The per-file number can be accumulated across successive wb_writeback
> > > invocations and thus can be much larger (eg. 128MB) than the legacy
> > > per-invocation number. 
> > > 
> > > The file system will only see the per-file numbers. The "max" means
> > > if btrfs find the current page to be the last page in the extent,
> > > it could indicate this fact to VFS by setting wbc->would_seek=1. The
> > > VFS will then switch to write the next inode.
> > > 
> > > The benefit of early voluntarily yield is, it reduced the possibility
> > > to be force stopped half way in an extent. When next time VFS returns
> > > to sync this inode, it will again be honored the full 128MB quota,
> > > which should be enough to cover a big fresh extent.
> > 
> > This is interesting, but it gets into a problem with defining what a
> > seek is.  On some hardware they are very fast and don't hurt at all.  It
> > might be more interesting to make timeslices.
> 
> We could have quotas for max pages, page segments and submission time.
> Will they be good enough? The first two quotas could be made per-bdi
> to reflect hardware capabilities.

The reason I prefer the timeslice idea is that we don't need the
hardware to tell us how fast it is.  We just write for a while and move
on.

-chris

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