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

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Date:	Tue, 18 Mar 2014 12:26:40 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Dave Hansen <dave.hansen@...el.com>,
	lsf@...ts.linux-foundation.org, Linux-MM <linux-mm@...ck.org>,
	Wu Fengguang <fengguang.wu@...el.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [Lsf] [LSF/MM TOPIC] Testing Large-Memory Hardware

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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