Ready for review: https://github.com/apache/ignite/pull/8870
On Sun, Mar 21, 2021 at 8:09 PM Pavel Tupitsyn <ptupit...@apache.org> wrote: > Simple benchmark added - see JmhCacheAsyncListenBenchmark in the PR. > There is a 6-8% drop (1 client, 2 servers, 1 machine, int key/val). > I expect this difference to become barely observable on real-world > workloads. > > On Thu, Mar 18, 2021 at 12:35 PM Pavel Tupitsyn <ptupit...@apache.org> > wrote: > >> 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_wil...@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.kuliche...@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....@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.kopti...@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 <ptupit...@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.kopti...@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 < >>> ptupit...@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.kopti...@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.kopti...@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 < >>> > > > ptupit...@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_wil...@trimble.com >>> >>> < >>> https://worksos.trimble.com/?utm_source=Trimble&utm_medium=emailsign&utm_campaign=Launch >>> > >>> >>
