Re: [Lse-tech] Re: a quest for a better scheduler

Re: [Lse-tech] Re: a quest for a better scheduler

[Hubertus Franke]

Kanoj, our cpu-pooling + loadbalancing allows you to do that.
The system adminstrator can specify at runtime through a
/proc filesystem interface the cpu-pool-size, whether loadbalacing
should take place.
We can put limiting to the local cpu-set during reschedule_idle
back into the code, to make it complete and compatible with
the approach that Andrea has taken.
This way, one can fully isolate or combine cpu-sets.

here is the code for the pooling.
http://lse.sourceforge.net/scheduling/LB/2.4.1-MQpool

loadbalancing and /proc system combined in this module.
http://lse.sourceforge.net/scheduling/LB/loadbalance.c

a writeup explaining this concept is available under
http://lse.sourceforge.net/scheduling/LB/poolMQ.html

Prerequisite is the MQ scheduler...
http://lse.sourceforge.net/scheduling/2.4.1.mq1-sched

We need to update these for 2.4.3 .... (coming)

Hubertus Franke
Enterprise Linux Group (Mgr),  Linux Technology Center (Member Scalability)
, OS-PIC (Chair)
email: frankeh@us.ibm.com
(w) 914-945-2003    (fax) 914-945-4425   TL: 862-2003



Kanoj Sarcar <kanoj@google.engr.sgi.com>@lists.sourceforge.net on
04/04/2001 12:50:58 PM

Sent by:  lse-tech-admin@lists.sourceforge.net


To:   andrea@suse.de (Andrea Arcangeli)
cc:   mingo@elte.hu (Ingo Molnar), Hubertus Franke/Watson/IBM@IBMUS,
      mkravetz@sequent.com (Mike Kravetz), fabio@chromium.com (Fabio
      Riccardi), linux-kernel@vger.kernel.org (Linux Kernel List),
      lse-tech@lists.sourceforge.net
Subject:  Re: [Lse-tech] Re: a quest for a better scheduler



>
> I didn't seen anything from Kanoj but I did something myself for the
wildfire:
>
>
ftp://ftp.us.kernel.org/pub/linux/kernel/people/andrea/kernels/v2.4/2.4.3aa1/10_numa-sched-1

>
> this is mostly an userspace issue, not really intended as a kernel
optimization
> (however it's also partly a kernel optimization). Basically it splits the
load
> of the numa machine into per-node load, there can be unbalanced load
across the
> nodes but fairness is guaranteed inside each node. It's not extremely
well
> tested but benchmarks were ok and it is at least certainly stable.
>

Just a quick comment. Andrea, unless your machine has some hardware
that imply pernode runqueues will help (nodelevel caches etc), I fail
to understand how this is helping you ... here's a simple theory though.
If your system is lightly loaded, your pernode queues are actually
implementing some sort of affinity, making sure processes stick to
cpus on nodes where they have allocated most of their memory on. I am
not sure what the situation will be under huge loads though.

As I have mentioned to some people before, percpu/pernode/percpuset/global
runqueues probably all have their advantages and disadvantages, and their
own sweet spots. Wouldn't it be really neat if a system administrator
or performance expert could pick and choose what scheduler behavior he
wants, based on how the system is going to be used?

Kanoj

_______________________________________________
Lse-tech mailing list
Lse-tech@lists.sourceforge.net
http://lists.sourceforge.net/lists/listinfo/lse-tech

Re: [Lse-tech] Re: a quest for a better scheduler

[Kanoj Sarcar]

> 
> 
> 
> Kanoj, our cpu-pooling + loadbalancing allows you to do that.
> The system adminstrator can specify at runtime through a
> /proc filesystem interface the cpu-pool-size, whether loadbalacing
> should take place.

Yes, I think this approach can support the various requirements
put on the scheduler.

I think there are two degrees of freedom that are needed in the
scheduler. One, as you say, for the sysadmin to be able to specify
what overall scheduler behavior he wants. 

Secondly, from the kernel standpoint, there needs to be perarch
hooks, to be able to utilize nodelevel/multilevel caches, NUMA
aspects etc.

Kanoj

Re: [Lse-tech] Re: a quest for a better scheduler

[alad]

This concept I think is used in Solaris .. as they have dynamic loadable
schedulers..




Zdenek Kabelac <kabi@i.am> on 04/05/2001 05:43:15 PM

To:   Andrea Arcangeli <andrea@suse.de>
cc:    (bcc: Amol Lad/HSS)

Subject:  Re: [Lse-tech] Re: a quest for a better scheduler





Hello

Just dump idea - why not make scheduler switchable with modules - so
users
could select any scheduler they want ?

This should not be that hard and would make it easy to replace scheduler
at runtime so everyone could easily try what's the best for him/her.

kabi@i.am

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

Re: [Lse-tech] Re: a quest for a better scheduler

[Zdenek Kabelac]

Hello

Just dump idea - why not make scheduler switchable with modules - so
users
could select any scheduler they want ?

This should not be that hard and would make it easy to replace scheduler
at runtime so everyone could easily try what's the best for him/her.

kabi@i.am

Re: [Lse-tech] Re: a quest for a better scheduler

[Andrea Arcangeli]

On Wed, Apr 04, 2001 at 10:49:04AM -0700, Kanoj Sarcar wrote:
> Imagine that most of the program's memory is on node 1, it was scheduled
> on node 2 cpu 8 momentarily (maybe because kswapd ran on node 1, other
> higher priority processes took over other cpus on node 1, etc). 
> 
> Then, your patch will try to keep the process on node 2, which is not
> neccessarily the best solution. Of course, as I mentioned before, if
> you have a node local cache on node 2, that cache might have been warmed
> enough to make scheduling on node 2 a good option. 

Exactly it made it a good option, and more important this heuristic can
only improve performance if compared to the mainline scheduler.

Infact I tried to reschedule the task back to its original node and it dropped
performance, anyways I cannot say to have done an extensive research on that, I
believe if we take care of some more history of the node migration we may
understand it's right time to push it back to its original node but that was
not obvious and I wanted a simple solution to boost the performance under CPU
bound load to start with.

Andrea

Re: [Lse-tech] Re: a quest for a better scheduler

[Kanoj Sarcar]

> 
> It helps by keeping the task in the same node if it cannot keep it in
> the same cpu anymore.
> 
> Assume task A is sleeping and it last run on cpu 8 node 2. It gets a wakeup
> and it gets running and for some reason cpu 8 is busy and there are other
> cpus idle in the system. Now with the current scheduler it can be moved in any
> cpu in the system, with the numa sched applied we will try to first reschedule
> it in the idles cpus of node 2 for example. The per-node runqueue are mainly
> necessary to implement the heuristic.
>

Yes. But this is not the best solution, if I can add on to the example
and make some assumptions.

Imagine that most of the program's memory is on node 1, it was scheduled
on node 2 cpu 8 momentarily (maybe because kswapd ran on node 1, other
higher priority processes took over other cpus on node 1, etc). 

Then, your patch will try to keep the process on node 2, which is not
neccessarily the best solution. Of course, as I mentioned before, if
you have a node local cache on node 2, that cache might have been warmed
enough to make scheduling on node 2 a good option. 

I am not saying there is a wrong or right answer, there are so many
possibilities, everything probably works and breaks under different
circumstances. 

Btw, while we are swapping patches, the patch at

	http://oss.sgi.com/projects/numa/download/sched242.patch

tries to implement per-arch scheduling. The current scheduler behavior
is smp_arch_goodness() and smp_pick_cpu(), but the patch allows the
possibility for a specific platform to change that to something else. 

Linus has seen this patch, and agrees to it in principle. He does not
consider this 2.4 material though. Of course, I am completely open to
Ingo (or someone else) coming up with a different way of providing the
same freedom to arch specific code.

Kanoj

Re: [Lse-tech] Re: a quest for a better scheduler

[Paul McKenney]

> Just a quick comment. Andrea, unless your machine has some hardware
> that imply pernode runqueues will help (nodelevel caches etc), I fail
> to understand how this is helping you ... here's a simple theory though.
> If your system is lightly loaded, your pernode queues are actually
> implementing some sort of affinity, making sure processes stick to
> cpus on nodes where they have allocated most of their memory on. I am
> not sure what the situation will be under huge loads though.

Exactly.  If a given task has run on a particular nodes for a while,
its memory will tend to be allocated on that node.  So preferentially
running it on another CPU on that same node should get you better
memory latency than would running it on some other node's CPUs.

In addition, continuing to run the task on a particular node means
that more of that task's memory is from that node, which again means
good memory latency.  In contrast, if you move a task back and forth
between nodes, it can end up with its memory spread over many nodes,
which means that it does not get good memory latency no matter where
you run it.

                              Thanx, Paul

> As I have mentioned to some people before,
percpu/pernode/percpuset/global
> runqueues probably all have their advantages and disadvantages, and their
> own sweet spots. Wouldn't it be really neat if a system administrator
> or performance expert could pick and choose what scheduler behavior he
> wants, based on how the system is going to be used?
>
> Kanoj

Re: [Lse-tech] Re: a quest for a better scheduler

[Hubertus Franke]

Correct, that's true.

Our patch does various things.
(a) limit search for a task to a admin specified set of cpu's
    during schedule()..
(b) limits search for a preemptable task to another set of cpu's
    during reschedule_idle()
     <need to reactivate this functionality 10 lines of code>
(c) loadbalancing, i.e. moving from queue to queue.
    Currently we balance within a set and across sets.

Obviously in NUMA one could specify
     (a) such that multiple sets fall into the same node
         no node crossings.
     (b) specify this set to at least span a node
     (c) do some intelligent moving based on memory maps
          etc.

I guess (c) would be first instance on where to plug architecture
dependent information, e.g. how much memory footprint does a task
have on a particular node and how much would the moving cost.
The loadbalance we provide is a simple sceleton to tickle you mind,
not a solution. Nevertheless, one can see it can have some impact.

See for results for various combinations of poolsizes and balancings:
http://lse.sourceforge.net/scheduling/results012501/status.html#Load%20Balancing


Hubertus Franke
Enterprise Linux Group (Mgr),  Linux Technology Center (Member Scalability)

email: frankeh@us.ibm.com
(w) 914-945-2003    (fax) 914-945-4425   TL: 862-2003



Kanoj Sarcar <kanoj@google.engr.sgi.com> on 04/04/2001 01:14:28 PM

To:   Hubertus Franke/Watson/IBM@IBMUS
cc:   linux-kernel@vger.kernel.org (Linux Kernel List),
      lse-tech@lists.sourceforge.net
Subject:  Re: [Lse-tech] Re: a quest for a better scheduler



>
>
>
> Kanoj, our cpu-pooling + loadbalancing allows you to do that.
> The system adminstrator can specify at runtime through a
> /proc filesystem interface the cpu-pool-size, whether loadbalacing
> should take place.

Yes, I think this approach can support the various requirements
put on the scheduler.

I think there are two degrees of freedom that are needed in the
scheduler. One, as you say, for the sysadmin to be able to specify
what overall scheduler behavior he wants.

Secondly, from the kernel standpoint, there needs to be perarch
hooks, to be able to utilize nodelevel/multilevel caches, NUMA
aspects etc.

Kanoj

Re: [Lse-tech] Re: a quest for a better scheduler

[Andrea Arcangeli]

On Wed, Apr 04, 2001 at 09:50:58AM -0700, Kanoj Sarcar wrote:
> > 
> > I didn't seen anything from Kanoj but I did something myself for the wildfire:
> > 
> > 	ftp://ftp.us.kernel.org/pub/linux/kernel/people/andrea/kernels/v2.4/2.4.3aa1/10_numa-sched-1
> > 
> > this is mostly an userspace issue, not really intended as a kernel optimization
> > (however it's also partly a kernel optimization). Basically it splits the load
> > of the numa machine into per-node load, there can be unbalanced load across the
> > nodes but fairness is guaranteed inside each node. It's not extremely well
> > tested but benchmarks were ok and it is at least certainly stable.
> >
> 
> Just a quick comment. Andrea, unless your machine has some hardware
> that imply pernode runqueues will help (nodelevel caches etc), I fail 
> to understand how this is helping you ... here's a simple theory though. 

It helps by keeping the task in the same node if it cannot keep it in
the same cpu anymore.

Assume task A is sleeping and it last run on cpu 8 node 2. It gets a wakeup
and it gets running and for some reason cpu 8 is busy and there are other
cpus idle in the system. Now with the current scheduler it can be moved in any
cpu in the system, with the numa sched applied we will try to first reschedule
it in the idles cpus of node 2 for example. The per-node runqueue are mainly
necessary to implement the heuristic.

> cpus on nodes where they have allocated most of their memory on. I am 
> not sure what the situation will be under huge loads though.

after all cpus are busy we try to reschedule only on the cpus of the local
node, that's why it can generate some unbalance yes, but it will tend to
rebalance over the time because some node will end with all tasks with
zero counter first if it's less loaded, and so then it will start
getting tasks with has_cpu 0 in the runqueues out of other nodes.

You may want to give it a try on your machines and see what difference it
makes, I'd be curious to know of course.

Andrea

Re: [Lse-tech] Re: a quest for a better scheduler

[Andrea Arcangeli]

On Wed, Apr 04, 2001 at 09:39:23AM -0700, Kanoj Sarcar wrote:
> example, for NUMA, we need to try hard to schedule a thread on the 
> node that has most of its memory (for no reason other than to decrease
> memory latency). Independently, some NUMA machines build in multilevel 
> caches and local snoops that also means that specific processors on
> the same node as the last_processor are also good candidates to run 
> the process next.

yes. That will probably need to be optional and choosen by the architecture
at compile time too. The probably most important factor to consider is the
penality of accessing remote memory, I think I can say on all recent and future
machines with a small difference between local and remote memory (and possibly
as you say with a decent cache protocol able to snoop cacheline data from the
other cpus even if they're not dirty) it's much better to always try to keep
the task in its last node. My patch is actually assuming recent machines and it
keeps the task in its last node if not in the last cpu and it keeps doing
memory allocation from there and it forgets about its original node where it
started allocating the memory from.  This provided the best performance during
userspace CPU bound load as far I can tell and it also better distribute the load.

Kanoj could you also have a look at the NUMA related common code MM fixes I did
in this patch? I'd like to get them integrated (just skip the arch/alpha/*
include/asm-alpha/* stuff while reading the patch, they're totally orthogonal).

	ftp://ftp.us.kernel.org/pub/linux/kernel/people/andrea/kernels/v2.4/2.4.3aa1/00_alpha-numa-1

If you prefer I can extract them in a more finegrinded patch just dropping
the alpha stuff by hand.

Andrea

Re: [Lse-tech] Re: a quest for a better scheduler

[Kanoj Sarcar]

> 
> I didn't seen anything from Kanoj but I did something myself for the wildfire:
> 
> 	ftp://ftp.us.kernel.org/pub/linux/kernel/people/andrea/kernels/v2.4/2.4.3aa1/10_numa-sched-1
> 
> this is mostly an userspace issue, not really intended as a kernel optimization
> (however it's also partly a kernel optimization). Basically it splits the load
> of the numa machine into per-node load, there can be unbalanced load across the
> nodes but fairness is guaranteed inside each node. It's not extremely well
> tested but benchmarks were ok and it is at least certainly stable.
>

Just a quick comment. Andrea, unless your machine has some hardware
that imply pernode runqueues will help (nodelevel caches etc), I fail 
to understand how this is helping you ... here's a simple theory though. 
If your system is lightly loaded, your pernode queues are actually 
implementing some sort of affinity, making sure processes stick to 
cpus on nodes where they have allocated most of their memory on. I am 
not sure what the situation will be under huge loads though.

As I have mentioned to some people before, percpu/pernode/percpuset/global
runqueues probably all have their advantages and disadvantages, and their
own sweet spots. Wouldn't it be really neat if a system administrator
or performance expert could pick and choose what scheduler behavior he
wants, based on how the system is going to be used?

Kanoj

Re: [Lse-tech] Re: a quest for a better scheduler

[Kanoj Sarcar]

> 
> 
> On Wed, 4 Apr 2001, Hubertus Franke wrote:
> 
> > Another point to raise is that the current scheduler does a exhaustive
> > search for the "best" task to run. It touches every process in the
> > runqueue. this is ok if the runqueue length is limited to a very small
> > multiple of the #cpus. [...]
> 
> indeed. The current scheduler handles UP and SMP systems, up to 32
> (perhaps 64) CPUs efficiently. Agressively NUMA systems need a different
> approach anyway in many other subsystems too, Kanoj is doing some
> scheduler work in that area.

Actually, not _much_ work has been done in this area. Alongwith a bunch
of other people, I have some ideas about what needs to be done. For 
example, for NUMA, we need to try hard to schedule a thread on the 
node that has most of its memory (for no reason other than to decrease
memory latency). Independently, some NUMA machines build in multilevel 
caches and local snoops that also means that specific processors on
the same node as the last_processor are also good candidates to run 
the process next.

To handle a single layer of shared caches, I have tried certain simple
things, mostly as hacks, but am not pleased with the results yet. More
testing needed.

Kanoj

> 
> but the original claim was that the scheduling of thousands of runnable
> processes (which is not equal to having thousands of sleeping processes)
> must perform well - which is a completely different issue.
> 
> 	Ingo
> 
> 
> _______________________________________________
> Lse-tech mailing list
> Lse-tech@lists.sourceforge.net
> http://lists.sourceforge.net/lists/listinfo/lse-tech
> 

Re: [Lse-tech] Re: a quest for a better scheduler

[Christoph Hellwig]

On Wed, Apr 04, 2001 at 09:44:22AM -0600, Khalid Aziz wrote:
> Let me stress that HP scheduler is not meant to be a replacement for the
> current scheduler. The HP scheduler patch allows the current scheduler
> to be replaced by another scheduler by loading a module in special
> cases.

HP also has a simple mq patch that is _not_ integrated into the pluggable
scheduler framework, I have used it myself.

	Christoph

-- 
Of course it doesn't work. We've performed a software upgrade.