Message-ID: <Pine.LNX.4.64.0911271704410.12136@sleet.zeugmasystems.local>
Date:	Fri, 27 Nov 2009 17:19:12 -0800 (PST)
From:	Anirban Sinha <asinha@...gmasystems.com>
To:	Buck <charles.krasic@...il.com>
cc:	Stijn Devriendt <highguy@...il.com>, linux-kernel@...r.kernel.org,
	krasic@...ubc.ca
Subject: Re: observe and act upon workload parallelism: PERF_TYPE_PARALLELISM
   (Was: [RFC][PATCH] sched_wait_block: wait for blocked threads)


Hi Stijn:

Let me chime in a little bit ...

In one of your past responses, you've written:

> One solution would be to let Thread 2 call sched_yield, but the
> question then is "when" and "how much". Every time a lightweight
> thread yields, you'll incur context switches. Because you don't
> know when or how much, you'll be penalized for context switching
> even when not needed. (Consider 1 blocked thread and 4 extra threads
> sched_yield'ing every 5 work items)


You see, this is where the coop_poll() interface is so useful. Everytime you
'wake up' from coop_poll(), the kernel gives you a future time value at which
you'd be calling coop_poll() again. Now the beauty of this approach is that
this time value is calculated by the kernel to be just the amount that is
necessary for maintaining a fair CPU sharing among all the runnable tasks. So
you don't have to worry about the when part. Further, the fair sharing itself
will determine how long you actually block within coop_poll(). As long as you
maintain the cooperative polling semantics (by properly honoring the time
value provided to you by the kernel), you are guranteed that only one of the
threads within the cooperative thread pool will be ever run by the kernel.

Let me know if this makes any sense and me and Buck will try to answer your
(or any other person's) doubts.

Cheers,

Ani


 On Fri, 27 Nov 2009, Buck wrote:

> Hi Stijn,
>
> Thanks for replying!
>
> I agree that our goals are slightly different, but I think they are
> not so different as one might think on first glance.
>
> Using the CPU to the fullest is certainly our goal also, to be "work
> conserving" is a long-standing feature of any good scheduler.
>
> I assert that our interface could certainly be used by a
> thread-pool to achieve the "only run one of us at a time/don't
> pre-empt us unless we misbehave" semantic.
>
> I think it is not true however that our approach only works when
> timing is available from applications.  It's true that the evaluation
> in our paper is focussed on those cases, but we do also have
> non-timesensitive tasks in our workloads (e.g. gcc).
>
> The trick is that even if the application doesn't provide timing, our
> kernel scheduler will.  When a task uses our coop_poll call, it
> receives a time value as a return value.  This may be interpreted as
> "you may run until this time without fear of pre-emption, but you must
> voluntarily yield back to the kernel by this time, and you may not
> block before such yield".  I'll discuss the "you must not block"
> below.
>
> The time value returned by coop_poll is calculated by the kernel based
> on release times of other coop_poll tasks *and* the fairshare
> scheduling timeslice set by the kernel scheduling algorithm.  So even
> if there are no other coop_poll release times (because the
> applications did not provide times to coop poll), the interface will
> still default to the kernel's fairshare timeslice based time.
>
> Back to your case.  Suppose the threads of a pool are running using
> coop_poll, but none of them provide times.  If one of them is running
> and it blocks, the kernel removes it from the pool (coop group).  The
> remaining members of the pool remain eligible for execution.  Which
> task runs next depends on fair sharing criteria (maybe a task from
> another pool).  Later when the blocked thread awakes, it will resume
> as a "best-effort" thread (still not in a thread pool), until it again
> yields by calling coop_poll, at that point it "re-joins" it's thread
> pool.
>
> What does it mean?
>
>  1) if CPU intensive tasks avoid blocking (by isolating blocking calls
>     to helper threads), and they honor coop_poll times, they will
>     never be pre-empted.  [ this is maybe where we differ, we assume
>     that time sensitive applications have good reasons to isolate
>     their time-sensitive and CPU intensive code from synchronous IO ]
>
>  2) if IO tasks are not CPU intensive, they will likely never be
>     pre-empted either, even though they lose their "cooperative"
>     status in the period between blocking on IO, waking up, until
>     calling coop_poll again.
>
>  3) only "best-effort" tasks, i.e. tasks that don't use coop_poll
>     at all, are pre-empted, in the normal way they would under
>     regular fair share scheduling.
>
> -- Buck
>
>
>
> On Nov 27, 4:20 am, Stijn Devriendt <high...@...il.com> wrote:
> > Hi Buck,
> >
> > Thanks for taking the time to inform me about your research.
> > I've just read up on it, but as far as I can tell there's still
> > quite some difference between our goals. My goal was to utilize the
> > CPU to it's fullest, regardless of blocking calls, in the context of a
> > threadpool. Your approach seems to try and do away with scheduling
> > latency and is better suited when timing information is available.
> >
> > A generic threadpool doesn't have timing information about the work-items
> > it is processing, so it cannot take part in this type of cooperative scheduling.
> >
> > Secondly you're using coop_poll() to wait for timeout and/or fds. In my case
> > I'm trying to let every blocking syscall cause the wake-up of the next
> > thread. This means that the threadpool should make no assumptions
> > about work-items and threadpool users can write any code they want.
> >
> > Regards,
> > Stijn
> >
> >
> >
> > On Thu, Nov 26, 2009 at 10:08 PM, Buck <charles.kra...@...il.com> wrote:
> > > Hi,
> >
> > > I think our research on scheduling is somewhat related to this topic.
> >
> > > I posted to the kernel list a while back, but I'm not sure anyone
> > > noticed.   Here is a link to
> > > that post, which contains links to slides and our paper.
> >
> > >http://lwn.net/Articles/358295/
> >
> > > -- Buck
> >
> > > On Nov 21, 3:27 am, Stijn Devriendt <high...@...il.com> wrote:
> > >> > Think of it like a classic user-level threading package, where one process
> > >> > implements multiple threads entirely in user space, and switches between
> > >> > them. Except we'd do the exact reverse: create multiple threads in the
> > >> > kernel, but only run _one_ of them at a time. So as far as the scheduler
> > >> > is concerned, it acts as just a single thread - except it's a single
> > >> > thread that has multiple instances associated with it.
> >
> > >> > And every time the "currently active" thread in that group runs out of CPU
> > >> > time - or any time it sleeps - we'd just go on to the next thread in the
> > >> > group.
> >
> > >> Without trying to sound selfish: after some thinking I can't see how this
> > >> solves my problem. This is fine for the case you mentioned later on,
> > >> like UI threads, but it's not powerful enough for what I'm trying to achieve.
> >
> > >> Let's make the round-trip for the thread pool case and start with an empty
> > >> thread pool queue:
> > >> - All threads are blocked on the queue condition variable untill new work
> > >> is queued.
> > >> - Thread 1 happily wakes up and runs the work item untill it's blocked.
> > >> - A new work item arrives and Thread 2 is woken to handle the new work
> > >>   item.
> > >> - As long as new work arrives and Thread 2 is not blocked (regardless
> > >>   of preemption because the deal was that they will not preempt each
> > >>   other) Thread 2 keeps running this work.
> > >>   Even when Thread 1 is woken, it will not preempt Thread 1.
> >
> > >> One solution would be to let Thread 2 call sched_yield, but the
> > >> question then is "when" and "how much". Every time a lightweight
> > >> thread yields, you'll incur context switches. Because you don't
> > >> know when or how much, you'll be penalized for context switching
> > >> even when not needed. (Consider 1 blocked thread and 4 extra threads
> > >> sched_yield'ing every 5 work items)
> >
> > >> Another option is to have a group-leader. Non-leader threads will call
> > >> sched_yield once in a while in order to try and jump back to the group-leader.
> > >> The group-leader will always continue work without sched_yield'ing.
> > >> There's no preemption between these threads.
> > >> The down-side is that in case multiple of these threads are waiting for
> > >> an event, wake-ups must wake the group leader rather than the other
> > >> coop-scheduled threads for this model to work.
> > >> Another down-side is that when a non-leader thread is blocked and the
> > >> group-leader is run, the non-leader thread is treated unfair.
> >
> > >> Either solution's end-result is a very unfair threadpool where one cannot
> > >> guarantee even a loose FIFO-model where items are handled more or
> > >> less in the order they are queued and a library that needs to make
> > >> trade-offs in performance to get this behaviour back.
> >
> > >> The solution is great when the threads are blocked most of the time
> > >> and have little CPU processing to do (like UI threads), but doesn't
> > >> scale beyond that.
> >
> > >> As ever, enlighten me when you have a great solution to this problem.
> >
> > >> Stijn
> > >> --
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