Re: [ckrm-tech] Re: [Lse-tech] [PATCH] cpusets - big numa cpu and memory placement

[Martin J. Bligh]

>> The way I'm looking at it, which is probably wholly insufficient, if not
>> downright wrong, we have multiple process groups, each of which gets some 
>> set of resources. Those resources may be dedicated to that class (a la 
>> cpusets) or not. One could view this as a set of resource groupings, and
>> set of process groupings, where one or more process groupings is bound to
>> a resource grouping.
>> 
>> The resources are cpus & memory, mainly, in my mind (though I guess IO,
>> etc fit too). The resource sets are more like cpusets, and the process
>> groups a bit more like CKRM, except they seem to overlap (to me) when
>> the sets in cpusets are non-exclusive, or when CKRM wants harder performance
>> guarantees.
> 
> I can understand it far enough to see groups of processes using groups
> of resources (cpus & memory, like cpusets).  Both of the phrases
> containing "CKRM" in them go right past ... whizz.  And I'm a little
> fuzzy on what are the sets, invariants, relations, domains, ranges,
> operations, pre and post conditions and such that could be modeled in a
> more precise manner.
> 
> Keep talking ...  Perhaps an example, along the lines of my "use case
> scenarios", would help.  When we start losing each other trying to
> generalize too fast, it can help to make up an overly concrete example,
> to get things grounded again.

Let me make one thing clear: I don't work on CKRM ;-) So I'm not either
desperately familiar with it, or partial to it. Nor am I desperately
infatuated enough with my employer to believe  that just because they're
involved with it, it must be stunningly brilliant. So I think I'm actually
fairly impartial ... and balanced in ignorance on both sides ;-)

I do think both things are solving perfectly valid problems (that IMO
intersect) ... not sure whether either is doing it the best way though ;-).

>> Whatever we call it, the resource management system definitely needs the 
>> ability to isolate a set of resources (CPUs, RAM) totally dedicated to
>> one class or group of processes.
> 
> Not always "totally isolated and dedicated".
> 
> Here's a scenario that shows up some uses for "non-exclusive" cpusts.
> 
> Let's take my big 256 CPU system, divided into portions of 128, 64 and
> 64. At this level, these are three, mutually exclusive cpusets, and
> interaction between them is minimized.  In the first two portions, the
> 128 and the first 64, a couple of "company jewel" applications run.
> These are highly tuned, highly parallel applications that are sucking up
> 99% of every CPU cycle, bus cycle, cache line and memory page available,
> for hours on end, in a closely synchronized dance.  They cannot tolerate
> anything else interfering in their area.  Frankly, they have little use
> for CKRM, fancy schedulers or sophisticated allocators.  They know
> what's there, it's all their's, and they know exactly what they want to
> do with it.  Get out of the way and let them do their job.  Industrial
> strength computing at its finest.
> 
> Ok that much is as before.
> 
> Now the last portion, the second 64, is more of a general use area. It
> is less fully utilized, and it's job mix more varied and less tightly
> administered.  There's some 64-thread background application that puts a
> fairly light load on things, running day and night (maybe the V.P. of
> the MIS shop is a fan of SETI).
> 
> Since this is a parallel programming shop, people show up with at random
> hours with smaller parallel jobs, carve off temporary cpusets of the
> appropriate size, and run an application in them.  Their threads and
> memory within their temporary cpuset are carefully placed, relative to
> their cpuset, but they are not fully utilizing the nodes on which they
> are running and they tolerate other things happening on the same nodes. 
> Perhaps the other stuff doesn't impact their performance much, or
> perhaps they are too poor to pay for dedicated nodes (grad students
> still looking for a grant?) ... whatever.

OK, the dedicated stuff in cpusets makes a lot of sense to me, for the
reasons you describe above. One screaming problem we have at the moment
is we can easily say "I want to bind myself to CPU X" but no way to say
"kick everyone else off it". That seems like a very real problem.

However, the non-dedicated stuff seems much more debateable, and where
the overlap with CKRM stuff seems possible to me. Do the people showing
up at random with smaller parallel jobs REALLY, REALLY care about the
physical layout of the machine? I suspect not, it's not the highly tuned
syncopated rhythm stuff you describe above. The "give me 1.5 CPUs worth
of bandwidth please" model of CKRM makes much more sense to me.
 
> They may well make good use of a batch manager, to which they submit
> jobs of a specified size (cpus and memory) so that the batch manager can
> smooth out the load. and avoid periods of excess idling or thrashing. 
> The implementation of the batch manager relies heavily on the underlying
> cpuset facility to manage various subsets of CPU and Memory Nodes.  The
> batch manager might own the first 192 CPUs on the system too, but most
> users never get to see that part of the system.
> 
> Within that last 64 portion the current mechanisms, including the per
> task cpus_allowed and mems_allowed, and the current schedulers and
> allocators, may well be doing a pretty good job.  Sure, there is an
> element of chaos and things aren't perfect.  It's the "usual" timeshare
> environment with a varied load mix.
> 
> The enforced placement within the smaller nested non-exclusive cpusets
> probably surprises the scheduler and allocator at times, leading to
> unfair inbalances.  I imagine that if CKRM just had that last 64 portion
> to manage, and this was just a 64 CPU system, not a 256, then CKRM could
> do a pretty good job of managing the systems resources.

Right - exactly. Sounds like we're actually pretty much on the same page
(by the time I'd finished your email ;-)). So whatever the interface we
have, the underlying mechanisms seem to have two fundamentals: dedicated
and non-decicated resources. cpusets seems to do a good job of dedicated
and I'd argue the interface of specifying physical resources is a bit
clunky for non-dedicated stuff. CKRM doesn't seem to tackle the dedicated
at all, but seems to have an easier way of doing the non-dedicated.

So personally what I'd like is to have a unified interface (and I care 
not a hoot which, or a new one altogether), that can specify dedicated
or non-decicated resources for groups of processes, and then have a
"cpusets-style" mechanism for the dedicated, and "CKRM-style" mechanism
for the non-dedicated. Not sure if that's exactly what Andrew was hoping
for, or the rest of you either ;-)

The whole discussion about multiple sched-domains, etc, we had earlier
is kind of just an implementation thing, but is a crapload easier to do
something efficient here if the bits caring about that stuff are only
dealing with dedicated resource partitions.

OK, now my email is getting as long as yours, so I'll stop ;-) ;-)

M.
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