Re: [RFC 1/7] cpuset write dirty map

[Mel Gorman]

On (27/06/07 05:44), Christoph Lameter didst pronounce:
> On Wed, 27 Jun 2007, Andrew Morton wrote:
> 
> > I'm more concerned about all of Mel's code in -mm actually.  I don't recall
> > anyone doing a full review recently and I'm still not sure that this is the
> > overall direction in which we wish to go.  Last time I asked this everyone
> > seemed a bit waffly and non-committal.
> 
> I have looked over this several times lately and it all seems quite okay. 
> Still cannot find a justification for the movable zone (never had to 
> use it in testing) but it seems that it makes memory unplug easier.

As well as helping memory hot-remove, it provides a known lower-limit that
the hugepage pool can be resized to at runtime. Grouping pages by mobility
on its own does not give a known number of pageblocks that could be used
for hugepage allocation. On it's own, it provides a high probability that
the hugepage pool can be grown but how large depends on the workload. With
the zone, an administrator can set aside X amount of memory that can be used
for hugepages if necessary and base pages otherwise.

> The 
> antifrag patchset together with a page migration patch simplifies the 
> unplug patchset significantly.
> 
> I think the antifrag code is a significant step forward and will enable 
> lots of other features (memory unplug, larger page use in SLUB, huge page 
> allocs after boot). It may be useful to put memory compaction and memory 
> unplug in at the same time (I think we can get there even for .23) so that 
> we have a full package.

The memory unplug patches already migrate pages without the help of memory
compaction. Memory compaction needs a bit more work before I try sending
it towards -mm for wider testing. It's important to get right because
with compaction, the migration code is getting a lot more exercise than it
does today. However, memory compaction needing more work should not hold
back memory unplug nor does it impact on the utility of grouping pages
by mobility or the movable zone.

> With compaction we can finally recover from loads 
> that typically cause memory to be split in a lot of disjoint pieces and 
> get to a sitaution were we can dynamically reconfigure the number of huge 
> pages at run time (Our customers currently reboot to do this which is a 
> pain). Compaction increases the chance of I/O controllers being able to 
> merge I/O requests since contiguous pages can be served by the page 
> allocator again. Antifrag almost gets there but I can still construct 
> allocation scenarios that fragment memory significantly.
> 

The number of scenarios that cause fragmentation have dropped over time.
It's something I see as constantly improving over time.

> Also compaction is a requirement if we ever want to support larger 
> blocksizes. That would allow the removal of various layers that are now 
> needed to compensate for not supporting larger pages.
> 
> The whole approach is useful to increase performance. We have seen 
> several percentage points of performance wins with SLUB when allowing 
> larger pages sizes. The use of huge pages is also mainly done for 
> performance reasons. The large blocksize patch has shown a 50% performance 
> increase even in its prototype form where we certainly have not solved 
> server performance issues.
> 
> Even without large blocksize: The ability to restore the capability of the 
> page allocator to serve pages that are in sequence can be used to shorten
> the scatter gather lists in the I/O layer speeding up I/O.
> 
> I think this is an important contribution that will move a lot of other 
> issues forward.
> 

Thanks Christoph for your reviewing of the code, your comments here and the
quite rigorous exercise of the anti-fragmentation code. I too believe
there are performance gains to be had through the use of larger pages
and working on this does not preclude work on improving the use of base
pages.

For larger pages to be used though, external fragmentation has to be
addressed in some fashion and no other viable implementation exists
besides anti-fragmentation right now.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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