Re: [patch 4/5] mm: writeback: throttle __GFP_WRITE on per-zone dirty limits

[Johannes Weiner <jweiner@redhat.com>]

On Tue, Jul 26, 2011 at 08:40:59AM +0900, Minchan Kim wrote:
> Hi Andi,
> 
> On Tue, Jul 26, 2011 at 5:37 AM, Andi Kleen <ak@linux.intel.com> wrote:
> >> The global dirty limits are put in proportion to the respective zone's
> >> amount of dirtyable memory and the allocation denied when the limit of
> >> that zone is reached.
> >>
> >> Before the allocation fails, the allocator slowpath has a stage before
> >> compaction and reclaim, where the flusher threads are kicked and the
> >> allocator ultimately has to wait for writeback if still none of the
> >> zones has become eligible for allocation again in the meantime.
> >>
> >
> > I don't really like this. It seems wrong to make memory
> > placement depend on dirtyness.
> >
> > Just try to explain it to some system administrator or tuner: her
> > head will explode and for good reasons.
> >
> > On the other hand I like doing round-robin in filemap by default
> > (I think that is what your patch essentially does)
> > We should have made  this default long ago. It avoids most of the
> > "IO fills up local node" problems people run into all the time.
> >
> > So I would rather just change the default in filemap allocation.

It's not only a problem that exists solely on a node-level but also on
a zone-level.  Round-robin over the nodes does not fix the problem
that a small zone can fill up with dirty pages before the global dirty
limit kicks in.

> Just out of curiosity.
> Why do you want to consider only filemap allocation, not IO(ie,
> filemap + sys_[read/write]) allocation?

I guess Andi was referring to the page cache (mapping file offsets to
pages), rather than mmaps (mapping virtual addresses to pages).

mm/filemap.c::__page_cache_alloc()

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