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From Pavel Tupitsyn <ptupit...@apache.org>
Subject Re: IEP-70: Async Continuation Executor
Date Thu, 18 Mar 2021 09:35:26 GMT
Denis,

For a reproducer, please see CacheAsyncContinuationExecutorTest.java in the
linked PoC [1]

[1]
https://github.com/apache/ignite/pull/8870/files#diff-c788c12013622093df07d8f628a6e8c5fb0c15007f0787f2d459dbb3e377fc5aR54

On Thu, Mar 18, 2021 at 1:58 AM Raymond Wilson <raymond_wilson@trimble.com>
wrote:

> We implemented the ContinueWith() suggestion from Pavel, and it works well
> so far in testing, though we do not have data to support if there is or is
> not a performance penalty in our use case..
>
> To lend another vote to the 'Don't do continuations on the striped thread
> pool' line of thinking: Deadlocking is an issue as is starvation. In some
> ways starvation is more insidious because by the time things stop working
> the cause and effect distance may be large.
>
> I appreciate the documentation does make statements about not performing
> cache operations in a continuation due to deadlock possibilities, but that
> statement does not reveal why this is an issue. It is less a case of a
> async cache operation being followed by some other cache operation (an
> immediate issue), and more a general case of the continuation of
> application logic using a striped pool thread in a way that might mean that
> thread is never given back - it's now just a piece of the application
> infrastructure until some other application activity schedules away from
> that thread (eg: by ContinueWith or some other async operation in the
> application code that releases the thread). To be fair, beyond structures
> like ContinueWith(), it is not obvious how that continuation thread should
> be handed back. This will be the same behaviour for dedicated continuation
> pools, but with far less risk in the absence of ContinueWith() constructs.
>
> In the .Net world this is becoming more of an issue as fewer .Net use cases
> outside of UI bother with synchronization contexts by default.
>
> Raymond.
>
>
> On Thu, Mar 18, 2021 at 9:56 AM Valentin Kulichenko <
> valentin.kulichenko@gmail.com> wrote:
>
> > Hi Denis,
> >
> > I think Pavel's main point is that behavior is unpredictable. For
> example,
> > AFAIK, putAsync can be executed in the main thread instead of the striped
> > pool thread if the operation is local. The listener can also be executed
> in
> > the main thread - this happens if the future is completed prior to
> listener
> > invocation (this is actually quite possible in the unit test environment
> > causing the test to pass). Finally, I'm pretty sure there are many cases
> > when a deadlock does not occur right away, but instead it will reveal
> > itself under high load due to thread starvation. The latter type of
> issues
> > is the most dangerous because they are often reproduced only in
> production.
> > Finally, there are performance considerations as well - cache operations
> > and listeners share the same fixed-sized pool which can have negative
> > effects.
> >
> > I'm OK with the change. Although, it might be better to introduce a new
> > fixed-sized pool instead of ForkJoinPool for listeners, simply because
> this
> > is the approach taken throughout the project. But this is up to a debate.
> >
> > -Val
> >
> > On Wed, Mar 17, 2021 at 11:31 AM Denis Garus <garus.d.g@gmail.com>
> wrote:
> >
> > > Pavel,
> > > I tried this:
> > >
> > > @Test
> > > public void test() throws Exception {
> > >     IgniteCache<Integer, String> cache =
> > > startGrid().getOrCreateCache("test_cache");
> > >
> > >     cache.putAsync(1, "one").listen(f -> cache.replace(1, "two"));
> > >
> > >     assertEquals("two", cache.get(1));
> > > }
> > >
> > > and this test is green.
> > > I believe that an user can make listener that leads to deadlock, but
> > > the example in the IEP does not reflect this.
> > >
> > >
> > >
> > > ср, 17 мар. 2021 г. в 17:36, Вячеслав Коптилин <
> slava.koptilin@gmail.com
> > >:
> > >
> > > > Hi Pavel,
> > > >
> > > > > Not a good excuse really. We have a usability problem, you have to
> > > admit
> > > > it.
> > > > Fair enough. I agree that this is a usability issue, but I have
> doubts
> > > that
> > > > the proposed approach to overcome it is the best one.
> > > >
> > > > > Documentation won't help - no one is going to read the Javadoc for
> a
> > > > trivial method like putAsync
> > > > That is sad... However, I don't think that this is a strong argument
> > > here.
> > > >
> > > > This is just my opinion. Let's see what other community members have
> to
> > > > say.
> > > >
> > > > Thanks,
> > > > S.
> > > >
> > > >
> > > > ср, 17 мар. 2021 г. в 17:01, Pavel Tupitsyn <ptupitsyn@apache.org>:
> > > >
> > > > > > the user should use the right API
> > > > >
> > > > > Not a good excuse really. We have a usability problem, you have to
> > > admit
> > > > > it.
> > > > > "The brakes did not work on your car - too bad, you should have
> known
> > > > that
> > > > > on Sundays only your left foot is allowed on the pedal"
> > > > >
> > > > > This particular use case is too intricate.
> > > > > Even when you know about that, it is difficult to decide what can
> run
> > > on
> > > > > the striped pool,
> > > > > and what can't. It is too easy to forget.
> > > > > And most people don't know, even among Ignite developers.
> > > > >
> > > > > Documentation won't help - no one is going to read the Javadoc for
> a
> > > > > trivial method like putAsync.
> > > > >
> > > > >
> > > > > So I propose to have a safe default.
> > > > > Then document the performance tuning opportunity on [1].
> > > > >
> > > > > Think about how many users abandon a product because it
> mysteriously
> > > > > crashes and hangs.
> > > > >
> > > > > [1]
> > > > >
> > > > >
> > > >
> > >
> >
> https://ignite.apache.org/docs/latest/perf-and-troubleshooting/general-perf-tips
> > > > >
> > > > >
> > > > > On Wed, Mar 17, 2021 at 4:21 PM Вячеслав Коптилин
<
> > > > > slava.koptilin@gmail.com>
> > > > > wrote:
> > > > >
> > > > > > Hi Pavel,
> > > > > >
> > > > > > Well, I think that the user should use the right API instead
of
> > > > > introducing
> > > > > > uncontested overhead for everyone.
> > > > > > For instance, the code that is provided by IEP can changed as
> > > follows:
> > > > > >
> > > > > > IgniteFuture fut = cache.putAsync(1, 1);
> > > > > > fut.listenAync(f -> {
> > > > > >     // Executes on Striped pool and deadlocks.
> > > > > >     cache.replace(1, 2);
> > > > > > }, ForkJoinPool.commonPool());
> > > > > >
> > > > > > Of course, it does not mean that this fact should not be properly
> > > > > > documented.
> > > > > > Perhaps, I am missing something.
> > > > > >
> > > > > > Thanks,
> > > > > > S.
> > > > > >
> > > > > > ср, 17 мар. 2021 г. в 16:01, Pavel Tupitsyn <
> ptupitsyn@apache.org
> > >:
> > > > > >
> > > > > > > Slava,
> > > > > > >
> > > > > > > Your suggestion is to keep things as is and discard the
IEP,
> > right?
> > > > > > >
> > > > > > > >  this can lead to significant overhead
> > > > > > > Yes, there is some overhead, but the cost of accidentally
> > starving
> > > > the
> > > > > > > striped pool is worse,
> > > > > > > not to mention the deadlocks.
> > > > > > >
> > > > > > > I believe that we should favor correctness over performance
in
> > any
> > > > > case.
> > > > > > >
> > > > > > >
> > > > > > > On Wed, Mar 17, 2021 at 3:34 PM Вячеслав Коптилин
<
> > > > > > > slava.koptilin@gmail.com>
> > > > > > > wrote:
> > > > > > >
> > > > > > > > Well, the specified method already exists :)
> > > > > > > >
> > > > > > > >     /**
> > > > > > > >      * Registers listener closure to be asynchronously
> notified
> > > > > > whenever
> > > > > > > > future completes.
> > > > > > > >      * Closure will be processed in specified executor.
> > > > > > > >      *
> > > > > > > >      * @param lsnr Listener closure to register. Cannot
be
> > {@code
> > > > > > null}.
> > > > > > > >      * @param exec Executor to run listener. Cannot
be {@code
> > > > null}.
> > > > > > > >      */
> > > > > > > >     public void listenAsync(IgniteInClosure<? super
> > > > IgniteFuture<V>>
> > > > > > > lsnr,
> > > > > > > > Executor exec);
> > > > > > > >
> > > > > > > > Thanks,
> > > > > > > > S.
> > > > > > > >
> > > > > > > > ср, 17 мар. 2021 г. в 15:25, Вячеслав
Коптилин <
> > > > > > slava.koptilin@gmail.com
> > > > > > > >:
> > > > > > > >
> > > > > > > > > Hello Pavel,
> > > > > > > > >
> > > > > > > > > I took a look at your IEP and pool request. I
have the
> > > following
> > > > > > > > concerns.
> > > > > > > > > First of all, this change breaks the contract
of
> > > > > > > > IgniteFuture#listen(lsnr)
> > > > > > > > >
> > > > > > > > >     /**
> > > > > > > > >      * Registers listener closure to be asynchronously
> > notified
> > > > > > > whenever
> > > > > > > > > future completes.
> > > > > > > > >      * Closure will be processed in thread that
completes
> > this
> > > > > future
> > > > > > > or
> > > > > > > > > (if future already
> > > > > > > > >      * completed) immediately in current thread.
> > > > > > > > >      *
> > > > > > > > >      * @param lsnr Listener closure to register.
Cannot be
> > > {@code
> > > > > > > null}.
> > > > > > > > >      */
> > > > > > > > >     public void listen(IgniteInClosure<? super
> > IgniteFuture<V>>
> > > > > > lsnr);
> > > > > > > > >
> > > > > > > > >     In your pull request, the listener is always
called
> from
> > a
> > > > > > > specified
> > > > > > > > > thread pool (which is fork-join by default)
> > > > > > > > >     even though the future is already completed
at the
> moment
> > > the
> > > > > > > listen
> > > > > > > > > method is called.
> > > > > > > > >     In my opinion, this can lead to significant
overhead -
> > > > > submission
> > > > > > > > > requires acquiring a lock and notifying a pool
thread.
> > > > > > > > >
> > > > > > > > >     It seems to me, that we should not change
the current
> > > > behavior.
> > > > > > > > > However, thread pool executor can be added as
an optional
> > > > parameter
> > > > > > of
> > > > > > > > > listen() method as follows:
> > > > > > > > >
> > > > > > > > >         public void listen(IgniteInClosure<?
super
> > > > IgniteFuture<V>>
> > > > > > > lsnr,
> > > > > > > > > Executor exec);
> > > > > > > > >
> > > > > > > > > Thanks,
> > > > > > > > > S.
> > > > > > > > >
> > > > > > > > > пн, 15 мар. 2021 г. в 19:24, Pavel Tupitsyn
<
> > > > ptupitsyn@apache.org
> > > > > >:
> > > > > > > > >
> > > > > > > > >> Igniters,
> > > > > > > > >>
> > > > > > > > >> Please review the IEP [1] and let me know
your thoughts.
> > > > > > > > >>
> > > > > > > > >> [1]
> > > > > > > > >>
> > > > > > > > >>
> > > > > > > >
> > > > > > >
> > > > > >
> > > > >
> > > >
> > >
> >
> https://cwiki.apache.org/confluence/display/IGNITE/IEP-70%3A+Async+Continuation+Executor
> > > > > > > > >>
> > > > > > > > >
> > > > > > > >
> > > > > > >
> > > > > >
> > > > >
> > > >
> > >
> >
>
>
> --
> <http://www.trimble.com/>
> Raymond Wilson
> Solution Architect, Civil Construction Software Systems (CCSS)
> 11 Birmingham Drive | Christchurch, New Zealand
> raymond_wilson@trimble.com
>
> <
> https://worksos.trimble.com/?utm_source=Trimble&utm_medium=emailsign&utm_campaign=Launch
> >
>

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