Re: Large Pages - Linux Foundation HPC

[Balbir Singh]

[Fix my email address to balbir@xxxxxxxxxxxxxxxxxx]

* Dave Hansen <dave@xxxxxxxxxxxxxxxxxx> [2009-04-21 09:57:05]:
> On Tue, 2009-04-21 at 09:32 -0700, Badari Pulavarty wrote:
> > Hi Dave,
> > 
> > On the Linux foundation HPC track summary, I saw:
> > 
> > -- Memory and interface to it - mapping memory into apps
> >      - large pages important - current state not good enough
> 
> I'm not sure exactly what this means.  But, there was continuing concern
> about large page interfaces.  hugetlbfs is fine, but it still requires
> special tools, planning, and requires some modification of the app.  We
> can modify it with linker tricks or with LD_PRELOAD, but those certainly
> don't work everywhere.  I was told over and over again that hugetlbfs
> isn't a sufficient interface for large pages, no matter how much
> userspace we try to stick in front of it.
> 
> Some of their apps get a 6-7x speedup from large pages!
> 
> Fragmentation also isn't an issue for a big chunk of the users since
> they reboot between each job.
> 
> > nodes going down due to memory exhaustion
> 
> Virtually all the apps in an HPC environment start up try to use all the
> memory they can get their hands on.  With strict overcommit on, that
> probably means brk() or mmap() until they fail.  They also usually
> mlock() anything they're able to allocate.  Swapping is the devil to
> them. :)
> 
> Basically, what all the apps do is a recipe for stressing the VM and
> triggering the OOM killer.  Most of the users simply hack the kernel and
> replace the OOM killer with one that fits their needs.  Some have an
> attitude that "the user's app should never die" and others "the user
> caused this, so kill their app".  Basically, there's no way to make
> everyone happy since they have conflicting requirements.  But, this is
> true of the kernel in general... nothing special here.

OOM killer has been a hot topic. Have you seen Dan Malek's patches at
http://lkml.org/lkml/2009/4/13/276.

> 
> The split LRU should help things.  It will at least make our memory
> scanning more efficient and ensure we're making more efficient reclaim
> progress.  I'm not sure that anyone there knew about the oom_adjust and
> oom_score knobs in /proc.  They do now. :)

:-)

> 
> One of my suggestions was to use the memory resource controller.  They
> could give each app 95% (or whatever) of the system.  This should let
> them keep their current "consume all memory" behavior, but stop them at
> sane limits.
> 

Soft limits should help as well, basically we are trying to allow
unrestricted memory access until there is contention. The patches are
still under development.

> That leads into another issue, which is the "wedding cake" software
> stack.  There are a lot of software dependencies both in and out of the
> kernel.  It is hard to change individual components, especially in the
> lower levels.  This leads many of the users to use old (think 2.6.9)
> kernels.  Nobody runs mainline, of course.
> 
> Then, there's Lustre.  Everybody uses it, it's definitely a big hunk of
> the "wedding cake".  I haven't seen any LKML postings on it in years and
> I really wonder how it interacts with the VM.  No idea.
> 
> There's a "Hyperion cluster" which is for testing new HPC software on a
> decently sized cluster.  One suggestion of ours was to try and get
> mainline tested on this every so often to look for regressions since
> we're not able to glean feedback from 2.6.9 kernel users.  We'll see
> where that goes. 
> 
> > checkpoint/restart
> 
> Many of the MPI implementations have mechanisms in userspace for
> checkpointing of user jobs.  Most cluster administrators instruct their
> users to use these mechanisms.  Some do.  Most don't.
>

Good inputs and summary. Thanks! 

-- 
	Balbir
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