[Devel] Re: [patch 0/7] cpuset writeback throttling

[Andrew Morton]

On Wed, 5 Nov 2008 14:21:47 -0600 (CST)
Christoph Lameter <cl at linux-foundation.org> wrote:

> On Wed, 5 Nov 2008, Andrew Morton wrote:
> 
> > > > Doable.  lru->page->mapping->host is a good start.
> > >
> > > The block layer has a list of inodes that are dirty. From that we need to
> > > select ones that will improve the situation from the cpuset/memcg. How
> > > does the LRU come into this?
> >
> > In the simplest case, dirty-memory throttling can just walk the LRU
> > writing back pages in the same way that kswapd does.
> 
> That means running reclaim. But we are only interested in getting rid of
> dirty pages. Plus the filesystem guys have repeatedly pointed out that
> page sized I/O to random places in a file is not a good thing to do. There
> was actually talk of stopping kswapd from writing out pages!

They don't have to be reclaimed.

> > There would probably be performance benefits in doing
> > address_space-ordered writeback, so the dirty-memory throttling could
> > pick a dirty page off the LRU, go find its inode and then feed that
> > into __sync_single_inode().
> 
> We cannot call into the writeback functions for an inode from a reclaim
> context. We can write back single pages but not a range of pages from an
> inode due to various locking issues (see discussion on slab defrag
> patchset).

We're not in a reclaim context.  We're in sys_write() context.

> > But _are_ people hitting this problem?  I haven't seen any real-looking
> > reports in ages.  Is there some workaround?  If so, what is it?  How
> > serious is this problem now?
> 
> Are there people who are actually having memcg based solutions deployed?
> No enterprise release includes it yet so I guess that there is not much of
> a use yet.

If you know the answer then please provide it.  If you don't, please
say "I don't know".

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