Tutorial :“Work stealing” vs. “Work shrugging”?



Question:

Why is it that I can find lots of information on "work stealing" and nothing on "work shrugging" as a dynamic load-balancing strategy?

By "work-shrugging" I mean pushing surplus work away from busy processors onto less loaded neighbours, rather than have idle processors pulling work from busy neighbours ("work-stealing").

I think the general scalability should be the same for both strategies. However I believe that it is much more efficient, in terms of latency & power consumption, to wake an idle processor when there is definitely work for it to do, rather than having all idle processors periodically polling all neighbours for possible work.

Anyway a quick google didn't show up anything under the heading of "Work Shrugging" or similar so any pointers to prior-art and the jargon for this strategy would be welcome.

Clarification

I actually envisage the work submitting processor (which may or may not be the target processor) being responsible for looking around the immediate locality of the preferred target processor (based on data/code locality) to decide if a near neighbour should be given the new work instead because they don't have as much work to do.

I dont think the decision logic would require much more than an atomic read of the immediate (typically 2 to 4) neighbours' estimated q length here. I do not think this is any more coupling than implied by the thieves polling & stealing from their neighbours. (I am assuming "lock-free, wait-free" queues in both strategies).

Resolution

It seems that what I meant (but only partially described!) as "Work Shrugging" strategy is in the domain of "normal" upfront scheduling strategies that happen to be smart about processor, cache & memory loyality, and scaleable.

I find plenty of references searching on these terms and several of them look pretty solid. I will post a reference when I identify one that best matches (or demolishes!) the logic I had in mind with my definition of "Work Shrugging".


Solution:1

Load balancing is not free; it has a cost of a context switch (to the kernel), finding the idle processors, and choosing work to reassign. Especially in a machine where tasks switch all the time, dozens of times per second, this cost adds up.

So what's the difference? Work-shrugging means you further burden over-provisioned resources (busy processors) with the overhead of load-balancing. Why interrupt a busy processor with administrivia when there's a processor next door with nothing to do? Work stealing, on the other hand, lets the idle processors run the load balancer while busy processors get on with their work. Work-stealing saves time.

Example

Consider: Processor A has two tasks assigned to it. They take time a1 and a2, respectively. Processor B, nearby (the distance of a cache bounce, perhaps), is idle. The processors are identical in all respects. We assume the code for each task and the kernel is in the i-cache of both processors (no added page fault on load balancing).

A context switch of any kind (including load-balancing) takes time c.

No Load Balancing

The time to complete the tasks will be a1 + a2 + c. Processor A will do all the work, and incur one context switch between the two tasks.

Work-Stealing

Assume B steals a2, incurring the context switch time itself. The work will be done in max(a1, a2 + c) time. Suppose processor A begins working on a1; while it does that, processor B will steal a2 and avoid any interruption in the processing of a1. All the overhead on B is free cycles.

If a2 was the shorter task, here, you have effectively hidden the cost of a context switch in this scenario; the total time is a1.

Work-Shrugging

Assume B completes a2, as above, but A incurs the cost of moving it ("shrugging" the work). The work in this case will be done in max(a1, a2) + c time; the context switch is now always in addition to the total time, instead of being hidden. Processor B's idle cycles have been wasted, here; instead, a busy processor A has burned time shrugging work to B.


Solution:2

I think the problem with this idea is that it makes the threads with actual work to do waste their time constantly looking for idle processors. Of course there are ways to make that faster, like have a queue of idle processors, but then that queue becomes a concurrency bottleneck. So it's just better to have the threads with nothing better to do sit around and look for jobs.


Solution:3

The basic advantage of 'work stealing' algorithms is that the overhead of moving work around drops to 0 when everyone is busy. So there's only overhead when some processor would otherwise have been idle, and that overhead cost is mostly paid by the idle processor with only a very small bus-synchronization related cost to the busy processor.


Solution:4

Work stealing, as I understand it, is designed for highly-parallel systems, to avoid having a single location (single thread, or single memory region) responsible for sharing out the work. In order to avoid this bottleneck, I think it does introduce inefficiencies in simple cases.

If your application is not so parallel that a single point of work distribution causes scalability problems, then I would expect you could get better performance by managing it explicitly as you suggest.

No idea what you might google for though, I'm afraid.


Solution:5

Some issues... if a busy thread is busy, wouldn't you want it spending its time processing real work instead of speculatively looking for idle threads to offload onto?

How does your thread decide when it has so much work that it should stop doing that work to look for a friend that will help?

How do you know that the other threads don't have just as much work and you won't be able to find a suitable thread to offload onto?

Work stealing seems more elegant, because solves the same problem (contention) in a way that guarantees that the threads doing the load balancing are only doing the load balancing while they otherwise would have been idle.

It's my gut feeling that what you've described will not only be much less efficient in the long run, but will require lots of of tweaking per-system to get acceptable results.

Though in your edit you suggest that you want submitting processor to handle this, not the worker threads as you suggested earlier and in some of the comments here. If the submitting processor is searching for the lowest queue length, you're potentially adding latency to the submit, which isn't really a desirable thing.

But more importantly it's a supplementary technique to work-stealing, not a mutually exclusive technique. You've potentially alleviated some of the contention that work-stealing was invented to control, but you still have a number of things to tweak before you'll get good results, these tweaks won't be the same for every system, and you still risk running into situations where work-stealing would help you.

I think your edited suggestion, with the submission thread doing "smart" work distribution is potentially a premature optimization against work-stealing. Are your idle threads slamming the bus so hard that your non-idle threads can't get any work done? Then comes the time to optimize work-stealing.


Solution:6

So, by contrast to "Work Stealing", what is really meant here by "Work Shrugging", is a normal upfront work scheduling strategy that is smart about processor, cache & memory loyalty, and scalable.

Searching on combinations of the terms / jargon above yields many substantial references to follow up. Some address the added complication of machine virtualisation, which wasn't infact a concern of the questioner, but the general strategies are still relevent.


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