Re: [Lhms-devel] [PATCH 0/7] Fragmentation Avoidance V19

[Yasunori Goto <y-goto@jp.fujitsu.com>]

Hello.
Nick-san.

I posted patches to make ZONE_REMOVABLE to LHMS.
I don't say they are better than Mel-san's patch.
I hope this will be base of good discussion.


There were 2 types.
One was just add ZONE_REMOVABLE.
This patch came from early implementation of memory hotplug VA-Linux
team. 
http://sourceforge.net/mailarchive/forum.php?thread_id=5969508&forum_id=223

ZONE_HIGHMEM is used for this purpose at early implementation.
We thought ZONE_HIGHMEM is easier removing than other zone.
But some of archtecture don't use it. That is why ZONE_REMOVABLE
was born.
(And I remember that ZONE_DMA32 was defined after this patch.
 So, number of zone became 5, and one more bit was necessary in
 page->flags. (I don't know recent progress of ZONE_DMA32)).


Another one was a bit similar than Mel-san's one.
One of motivation of this patch was to create orthogonal relationship
between Removable and DMA/Normal/Highmem. I thought it is desirable.
Because, ppc64 can treat that all of memory is same (DMA) zone.
I thought that new zone spoiled its good feature.

http://sourceforge.net/mailarchive/forum.php?thread_id=5345977&forum_id=223
http://sourceforge.net/mailarchive/forum.php?thread_id=5345978&forum_id=223
http://sourceforge.net/mailarchive/forum.php?thread_id=5345979&forum_id=223
http://sourceforge.net/mailarchive/forum.php?thread_id=5345980&forum_id=223


Thanks.

P.S. to Mel-san.
 I'm sorry for late writing of this. This threads was mail bomb for me
 to read with my poor English skill. :-(


> Martin J. Bligh wrote:
> 
> >>But let's move this to another thread if it is going to continue. I
> >>would be happy to discuss scheduler problems.
> > 
> > 
> > My point was that most things we do add complexity to the codebase,
> > including the things you do yourself ... I'm not saying the we're worse
> > off for the changes you've made, by any means - I think they've been
> > mostly beneficial.
> 
> Heh - I like the "mostly" ;)
> 
> > I'm just pointing out that we ALL do it, so let us
> > not be too quick to judge when others propose adding something that does ;-)
> > 
> 
> What I'm getting worried about is the marked increase in the
> rate of features and complexity going in.
> 
> I am almost certainly never going to use memory hotplug or
> demand paging of hugepages. I am pretty likely going to have
> to wade through this code at some point in the future if it
> is merged.
> 
> It is also going to slow down my kernel by maybe 1% when
> doing kbuilds, but hey let's not worry about that until we've
> merged 10 more such slowdowns (ok that wasn't aimed at you or
> Mel, but my perception of the status quo).
> 
> > 
> >>You can't what? What doesn't work? If you have no hard limits set,
> >>then the frag patches can't guarantee anything either.
> >>
> >>You can't have it both ways. Either you have limits for things or
> >>you don't need any guarantees. Zones handle the former case nicely,
> >>and we currently do the latter case just fine (along with the frag
> >>patches).
> > 
> > 
> > I'll go look through Mel's current patchset again. I was under the
> > impression it didn't suffer from this problem, at least not as much
> > as zones did.
> > 
> 
> Over time, I don't think it can offer any stronger a guarantee
> than what we currently have. I'm not even sure that it would be
> any better at all for problematic workloads as time -> infinity.
> 
> > Nothing is guaranteed. You can shag the whole machine and/or VM in
> > any number of ways ... if we can significantly improve the probability 
> > of existing higher order allocs working, and new functionality has
> > an excellent probability of success, that's as good as you're going to 
> > get. Have a free "perfect is the enemy of good" Linus quote, on me ;-)
> > 
> 
> I think it falls down if these higher order allocations actually
> get *used* for anything. You'll simply be going through the process
> of replacing your contiguous, easy-to-reclaim memory with pinned
> kernel memory.
> 
> However, for the purpose of memory hot unplug, a new zone *will*
> guarantee memory can be reclaimed and unplugged.
> 
> -- 
> SUSE Labs, Novell Inc.
> 
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> 
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-- 
Yasunori Goto 

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