Re: adaptive spinning: fall back to sleep / block?

From: Konstantin Belousov <kostikbel_at_gmail.com>
Date: Tue, 18 May 2021 19:48:15 +0300
On Mon, May 17, 2021 at 02:44:34PM +0300, Andriy Gapon wrote:
> On 17/05/2021 13:23, Mateusz Guzik wrote:
> > On 4/9/21, Andriy Gapon <avg_at_freebsd.org> wrote:
> > > 
> > > 
> > > Until I recently looked at the actual code I was under an impression that
> > > the adaptive spinning is bounded and that after some time / number of spins
> > > a
> > > thread would go to a sleep queue or a turnstile.  But it looks that the
> > > spinning
> > > is actually unbounded as long as its conditions hold (some other thread owns
> > > the
> > > lock and that thread is running, the owner could be changing too).
> > > 
> > > In my opinion, it does not make sense to spin for "too long".
> > > If there was not an opportunity to take a lock quickly, then it's better to
> > > block waiting for it rather than keep occupying a processor.  For instance,
> > > the
> > > spinning can prevent another runnable thread from running.
> > > 
> > > I think that if a lock is heavily contended or its hold times are on the
> > > longer
> > > side (or both), then the adaptive spinning can make the system behavior
> > > (performance, responsiveness) worse.
> > > 
> > > Finally, I was under an impression that 'adaptive' meant some heuristic on
> > > whether and when to do the spinning.  _A lock owner is running_ seems to be
> > > too
> > > simple to qualify as 'adaptive'.
> > > 
> > > As an example, this looks like a relatively sophisticated implementation of
> > > the
> > > "adaptiveness":
> > > http://hg.openjdk.java.net/jdk8/jdk8/hotspot/file/87ee5ee27509/src/share/vm/runtime/objectMonitor.cpp#l1919
> > > But, JIMHO, simply having a hard limit on the spin count would be better
> > > than
> > > what we have now.
> > > 
> > 
> > There is no clear cut answer to this that I'm aware of. Ultimately all
> > behavior in face of contention is about damage control.
> > 
> > It's not hard to make a counter point to going off cpu after a timeout:
> > 1. going off cpu is also a serializing operation, that is threads can
> > content on doing so, albeit less than on the original lock
> > 2. existence of blocked waiters makes it more expensive to release the
> > lock, making contention worse and even then it is unclear what wake up
> > policy you would like to enact (e.g., do you hand off the lock to the
> > oldest sleeper? do you wake everyone and let them fight it out? all
> > choices suffer problems)
> > 3. consider a thread which holds foo_lock and now contends on bar_lock
> > and decides to go off cpu. if there is a thread which waits on
> > foo_lock, it will also go off cpu. This kind of behavior easily leads
> > to dramatic collapse in throughput, even on top of whatever problems
> > which are present due to contention to begin with.
> > 
> > Now, locking primitives in the FreeBSD kernel are problematic in at
> > least 3 ways:
> > 1. there are no fairness guarantees, for example a constant stream of
> > threads can end up excluding some threads for a long time
> > 2. there is no support for locking under kvm (as in the kernel does
> > not take advantage of it)
> > 3. rw locking is using cas loops instead of add
> > 
> > imo, if doing anything serious with locks, the 3 above problems needs
> > to be solved, in that order.
> > 
> > I can't stress enough the lack of fairness, which arbitrary going to
> > sleep will only exacerbate. As noted earlier, I don't know if timeout
> > sleep is an inherently good or bad idea, but I'm confident playing
> > with it in the situation is on the bad side.
> 
> I agree with you.  And it looks like we do not disagree in general
> .
> I just want to make a clarification, maybe redundant, that you seem to be
> mostly concerned about effects on threads that contend on same locks or
> related lock chains.  I am more concerned about effects on completely
> unrelated threads.
> 
> Yes, sleeping and waking introduces overhead (and not only) on threads doing
> them and threads depending on those threads.  But spinning, when it happens
> on a large share of CPUs (e.g., ~ 100% of them), introduces a penalty on the
> whole system.
> 
> At work, where a substantial part of our application lives in kernel, I
> regularly debug issues that seem to happen because of CPUs starvation.  And
> a common theme between them is that many CPUs are tied in the lock spinning.
> FreeBSD as a general purpose OS does not seem to suffer from the same issues
> (at least, that I know of), but we have quite a lot of additional kernel
> threads and kernel locks.
> 
> So, maybe instead of trying to experiment with FreeBSD I should try to
> experiment with our derivative product first.
> 
> Thank you for the feedback and additional information.

BTW, the unfairness and scheduling randomness are quite fundamental
for locking to work.  Minor scheduling inconsistencies and timing drifts
due to external events and internal machine operations allow to avoid
lock convoys, which would otherwise plague us.

The priorities mechanism is the partial compensation for lack of fairness.
Critical sections give you the highest priority execution, factually.
They will probably become more common as more lock-less algorithms start
appear in the kernel.

I saw some references that Windows tried to use the completely fair queued
spinlocks, and either limited their use to very specific places, or get
rid of them.  They were never advertised as generic facility.

Also, I know that Solaris experimented with stuff like direct transfer of
the lock from unlocking thread to (some) lock contender.  They were very
much dissatisfied with the results.
Received on Tue May 18 2021 - 16:48:15 UTC

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