Re: [PATCH 01/45] writeback: reduce calls to global_page_state inbalance_dirty_pages()

[Peter Zijlstra]

On Sun, 2009-10-11 at 10:28 +0800, Wu Fengguang wrote:
> 
> Note that the total limit check itself may not be sufficient. For
> example, there are no nr_writeback limit for NFS (and maybe btrfs)
> after removing the congestion waits.  Therefore it is very possible
> 
>         nr_writeback => dirty_thresh
>         nr_dirty     => 0
> 
> which is obviously undesirable: everything newly dirtied are soon put
> to writeback. It violates the 30s expire time and the background
> threshold rules, and will hurt write-and-truncate operations (ie. temp
> files).
> 
> So the better solution would be to impose a nr_writeback limit for
> every filesystem that didn't already have one (the block io queue).
> NFS used to have that limit with congestion_wait, but now we need
> to do a wait queue for it.
> 
> With the nr_writeback wait queue, it can be guaranteed that once
> balance_dirty_pages() asks for writing 1500 pages, it will be done
> with necessary sleeping in the bdi flush thread. So we can safely
> remove the loop and double checking of global dirty limit in
> balance_dirty_pages().

nr_reclaim = nr_dirty + nr_writeback + nr_unstable, so anything calling
into balance_dirty_pages() would still block on seeing such large
amounts of nr_writeback.

Having the constraint nr_dirty + nr_writeback + nr_unstable <
dirty_thresh should ensure we never have nr_writeback > dirty_thresh,
simply because you cannot dirty more, which then cannot be converted to
more writeback.

Or am I missing something?

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