[LSF/MM TOPIC] Testing Large-Memory Hardware

[LSF/MM TOPIC] Testing Large-Memory Hardware

[Dave Hansen]

I have a quick topic that could perhaps be addressed along with the
testing topic that Dave Jones proposed.  I won't be attending, but there
will be a couple of other Intel folks there.  This should be a fairly
quick thing to address.

Topic:

Fengguang Wu who runs the wonderful LKP and 0day build tests was
recently asking if I thought there was value in adding a large-memory
system, say with 1TB of RAM.  LKP is the system that generates these
kinds of automated bug reports and performance tests:

	http://lkml.org/lkml/2014/3/9/201

My gut reaction was that we'd probably be better served by putting
resources in to systems with higher core counts rather than lots of RAM.
 I have encountered the occasional boot bug on my 1TB system, but it's
far from a frequent occurrence, and even more infrequent to encounter
things at runtime.

Would folks agree with that?  What kinds of tests, benchmarks, stress
tests, etc... do folks run that are both valuable and can only be run on
a system with a large amount of actual RAM?

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Re: [Lsf] [LSF/MM TOPIC] Testing Large-Memory Hardware

[Peter Zijlstra]

On Tue, Mar 18, 2014 at 09:32:59AM -0700, Dave Hansen wrote:
> I have a quick topic that could perhaps be addressed along with the
> testing topic that Dave Jones proposed.  I won't be attending, but there
> will be a couple of other Intel folks there.  This should be a fairly
> quick thing to address.
> 
> Topic:
> 
> Fengguang Wu who runs the wonderful LKP and 0day build tests was
> recently asking if I thought there was value in adding a large-memory
> system, say with 1TB of RAM.  LKP is the system that generates these
> kinds of automated bug reports and performance tests:
> 
> 	http://lkml.org/lkml/2014/3/9/201
> 
> My gut reaction was that we'd probably be better served by putting
> resources in to systems with higher core counts rather than lots of RAM.
>  I have encountered the occasional boot bug on my 1TB system, but it's
> far from a frequent occurrence, and even more infrequent to encounter
> things at runtime.
> 
> Would folks agree with that?  What kinds of tests, benchmarks, stress
> tests, etc... do folks run that are both valuable and can only be run on
> a system with a large amount of actual RAM?

We had a sched-numa + kvm fail on really large systems the other day,
but yeah in general such problems tend to be rare. Then again, without
test coverage they will always be rare, for even if there were problems,
nobody would notice :-)

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Re: [Lsf] [LSF/MM TOPIC] Testing Large-Memory Hardware

[Christoph Lameter]

On Tue, 18 Mar 2014, Peter Zijlstra wrote:

> > My gut reaction was that we'd probably be better served by putting
> > resources in to systems with higher core counts rather than lots of RAM.
> >  I have encountered the occasional boot bug on my 1TB system, but it's
> > far from a frequent occurrence, and even more infrequent to encounter
> > things at runtime.
> >
> > Would folks agree with that?  What kinds of tests, benchmarks, stress
> > tests, etc... do folks run that are both valuable and can only be run on
> > a system with a large amount of actual RAM?
>
> We had a sched-numa + kvm fail on really large systems the other day,
> but yeah in general such problems tend to be rare. Then again, without
> test coverage they will always be rare, for even if there were problems,
> nobody would notice :-)

SGI had systems out there up to few PB of RAM. There were a couple of
tricks to get this going. Bootup time was pretty long. I/O has to be done
carefully. The MM subsystem used to work with these sizes (I have not had
a chance to verify that recently).

This was Itanium with 64K page size so you had a factor of 16 less page
structs to process. What I saw there is one of the reasons why I would
like to see larger page support in the kernel. Managing massive amounts of
4k pages is creation far too much overhead.

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