Poor multi-threaded indexing performance

[superkelvint]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

I wrote a multithreaded indexer in Java for Lucene 4.8 and one for Lucene.net 4.8. I tried to do pretty much a line-for-line port. However, there is a substantial difference in the performance characteristics of the two. In Java, I used a ThreadPoolExecutor with CallerRunsPolicy, and in C# I used Parallel.ForEach.

1. The Java program scales more or less linearly to the number of CPUs.
2. The C# program doesn't seem to fundamentally speed up when increasing the number of threads.
3. The Java program completes in about 8 minutes with 4 threads, the Lucene.net version takes about 65 minutes with 4, 10 or even 20 threads.
4. It was very easy to max out CPU utilization in Java and pretty much impossible to do so in C#.

I can provide both Java and .NET files, but before doing so, I wanted to understand if this is expected behavior and therefore not a bug.

I am observing identical behavior in .NET 8 as well as .NET 6, so I don't think its related to #933

Expected Behavior

Multi-threaded indexing performance scales with the number of threads used.

Steps To Reproduce

No response

Exceptions (if any)

No response

Lucene.NET Version

4.8

.NET Version

8.0.204 and 6.0.29

Operating System

Windows 10

Anything else?

No response

[paulirwin]

I would say this is very much not expected behavior. Having your benchmark code would be very helpful! Thanks for the report.

[eladmarg]

this sounds too much difference between java / c#.
do you add the documents and commit only in the end?

a test program would be great to analyze this.
thanks

[NightOwl888]

Concurrent document writing support is currently broken due to a bug mentioned in #886 (comment). No objections if someone wants to contribute a solution to that issue, but this isn't considered a blocker for the Lucene.NET 4.8.0 release.

[eladmarg]

@NightOwl888 I remember this was an old bug relevant only to older dotnet core.
so if its not reproducible on the new dotnet, and only happen on versions that out of support anyway, maybe we shouldn't too

[jeme]

@NightOwl888 If you apply the changes to DocumentsWriterFlushControl from that branch on top of the latest can you still reproduce it? I have no luck on 5.0, 6.0 or 7.0 - but asking because maybe I am doing something wrong >.<

(I have not run the entire test suite yet, there is allot of test and I have problems compiling the IKVM stuff)

@superkelvint Sharing the source code would certainly be relevant, Parallel.ForEach would not be my first choice here.

[superkelvint]

@superkelvint Sharing the source code would certainly be relevant, Parallel.ForEach would not be my first choice here.

Yes, I just got approval to do so, and will be providing that shortly.

[superkelvint]

Here is the MultithreadedIndexer.cs file.
MultithreadedIndexer.cs.zip

I'm not a C# programmer so perhaps I am doing something silly that's slowing things down.

[NightOwl888]

@NightOwl888 I remember this was an old bug relevant only to older dotnet core. so if its not reproducible on the new dotnet, and only happen on versions that out of support anyway, maybe we shouldn't too

Nope. This happens on all operating systems and .NET versions. I got it to fail on .NET 7 after that post.

We cannot rule out that the problem lies in the test or the test framework, but it almost certainly is in our codebase. We have a configuration that never existed in Lucene. We are somewhere between 4.8.0 and 4.8.1 on the documents writer stuff. We should go through the JIRA issues in the Changes.txt to determine if we have all of the changes that are relevant to the documents writer. That may or may not be contributing to this.

@NightOwl888 If you apply the changes to DocumentsWriterFlushControl from that branch on top of the latest can you still reproduce it? I have no luck on 5.0, 6.0 or 7.0 - but asking because maybe I am doing something wrong >.<

(I have not run the entire test suite yet, there is allot of test and I have problems compiling the IKVM stuff)

IKVM can be tricky to get to compile the first time because it doesn't properly clean the build. I suggest closing the IDE and then doing the following:

• Delete all of your /bin and /obj folders (be careful not to delete the objects folder below /.git)
• Clear the IKVM cache in the ~\AppData\Local\Temp\ikvm\cache folder
• Clear the ~\.m2 cache

If upon reopening you still have issues compiling, report the issue to ikvm-maven. The team is pretty responsive.

---

On Azure DevOps, I was able to reproduce the test failure, but this time it took 60 runs and it only happened once: https://dev.azure.com/lucene-net-temp4/Main/_build/results?buildId=266&view=ms.vss-test-web.build-test-results-tab&runId=191370&resultId=100202&paneView=debug

I was also able to get it to fail locally with the seed from #886. Using the [Repeat(1000)] attribute on the test, it failed on the first run on net6.0 on Windows.

I just put the attribute in the TestRollingUpdates.cs file, but be sure to remove it when you are done debugging.

It also failed on the first run with the seed from the above Azure DevOps failure on net6.0 on Windows.

[assembly: Lucene.Net.Util.RandomSeed("0x9f28125320b5e6c3")]
[assembly: NUnit.Framework.SetCulture("lrc-IR")]

I also got it to fail on net7.0 with that seed on the first run, but it took significantly longer.

[pc-LMS]

@jeme thoughts on the included code within MultithreadedIndexer.cs file?

[jeme]

@pc-LMS I have actually not looked further into it as I have used the time I have for Lucene (here and there) to attempt to reproduce the problem linked by @NightOwl888 locally (as that would ease things hugely for correcting it) or just trying to unravel the code in an attempt to get a theoretical idea of what might be wrong, so far no luck on reproducing it though.

[NightOwl888]

@jeme - I am a bit perplexed why you cannot reproduce this. While it is not trivial to do, it also is not that hard and I have reproduced it on multiple machines.

It is possible that if the problem is a timing issue, perhaps the particular machine that you are experimenting with is somehow perfectly synchronized to miss the error, too fast/slow, or the timing accuracy of the underlying OS is sufficient to where it doesn't happen. Have you attempted to reproduce on more than one machine and/or OS?

[jeme]

@NightOwl888 No, I have not tried other machines or OS'es for now, I have tried running the tests under various .NET versions with the seeds provided, I would have to install VM's for trying other OS'es for now (or dust off my old 2012 Mac book PRO and boot that into OSX, again it would take some time).

But I could perhaps find a few other machines to run it on, my machine is a bit dated (i9-7900X, 128GB Ram and M.2 SSD's). If you can reproduce it locally, what are you running on? (OS etc).

Also, are you running all the test suites, just that project containing that test, just that suite (file) or can you reproduce it with just running that one test alone?

Because if this is timing and mine just runs slower pr. thread (10 cores means lower clock speed after all + the IPC has increased a bit since then I guess).

[NightOwl888]

I am on Windows 10 x64 running with an I7-8850H, 2.60Ghz, 6 Cores, 12 Logical Processors, 32GB RAM, and M.2 SSD's.

I am unable to reproduce the failure locally without the [Repeat(1000)] attribute on the TestRollingUpdates.TestUpdateSameDoc() method since the failure happens so rarely. It usually fails within the first 3 runs with that attribute applied. And I am explicitly running just that one test using Visual Studio 2022. But I have also had it fail when running the entire test project that it is in.

FYI - I did a new manual revert that is up to date with master here: https://github.com/NightOwl888/lucenenet/tree/fix/GH-935-documentswriter-concurrency

[jeme]

Ok, I was actually in the thinking that this was because my CPU was a slow old one, but maybe it's the other way around. It passes just fine, takes a while with repeat:1000 obviously, but since it at max starts 6 threads I tried upping that seeing as it would never occupy all Cores(10)/Logical(20) on my system. Thinking that it could also be if the system was put under stress, even with a random between 32 to 64 threads, my system is just not stressed at all, but I still wonder if stressing things could make it easier to reproduce, so my plan is to do 2 things after work today.

A) Move the code to my old MacBook (with windows) and run the tests there, that would run on a Core i7 i7 3820QM.
B) Write a stress test suite that tries to push things as high as I can.

Thanks for the updated source/branch.

So status is that the test passes for me with:

[assembly: Lucene.Net.Util.RandomSeed("0x9f28125320b5e6c3")]
[assembly: NUnit.Framework.SetCulture("lrc-IR")]
[Repeat(1000)]

(what you can reproduce it with)

And it also passes with in addition to the above:

int numThreads = TestUtil.NextInt32(Random, 32, 64);

[NightOwl888]

Interesting. I tried with net8.0 (which has been changed as the default IDE test platform since the last time I checked) with the same configuration you used and it doesn't fail there. But then I changed the test target to net6.0 and it failed on the first run. So, that probably explains why you are not reproducing it.

I think I worked out what is happening, though.

Issue 1

The error indicates that dwpt is null on this line:

https://github.com/NightOwl888/lucenenet/blob/878261fa920a961288639863da28d7c9f5d0d800/src/Lucene.Net/Index/DocumentsWriterFlushControl.cs#L471

Digging into the DocumentsWriterPerThreadPool, there is only 1 way it could return null, and that is if this method is called: https://github.com/NightOwl888/lucenenet/blob/878261fa920a961288639863da28d7c9f5d0d800/src/Lucene.Net/Index/DocumentsWriterPerThreadPool.cs#L91

We cannot pass this line unless perThread.IsInitialized returns true. And that can only happen if dwpt is not null.

Therefore, to fail in this way, DocumentsWriterPerThreadPool.Reset(perThread, closed) must be called on a different thread while these lines are executing. This explains why it is an exceedingly rare event.

I checked and there are 4 calls to DocumentsWriterPerThreadPool.Reset(perThread, closed). All of them are called within UninterruptableMonitor.Enter(this) except for this one:

https://github.com/NightOwl888/lucenenet/blob/878261fa920a961288639863da28d7c9f5d0d800/src/Lucene.Net/Index/DocumentsWriterFlushControl.cs#L818

So, moving the UninterruptableMonitor.Enter(this) outside of that call should fix the problem.

Issue 2

After that fix, we still got some failures. But it was also clear from the stack trace what was happening. If TryLock() returns false, we end up on this line:

https://github.com/NightOwl888/lucenenet/blob/fix/GH-935-documentswriter-concurrency/src/Lucene.Net/Index/DocumentsWriterFlushControl.cs#L484

The documentation for ReentrantLock.tryLock() states:

Even when this lock has been set to use a fair ordering policy, a call to tryLock() will immediately acquire the lock if it is available, whether or not other threads are currently waiting for the lock. This "barging" behavior can be useful in certain circumstances, even though it breaks fairness. If you want to honor the fairness setting for this lock, then use tryLock(0, TimeUnit.SECONDS) which is almost equivalent (it also detects interruption).

If the current thread already holds this lock then the hold count is incremented by one and the method returns true.

If the lock is held by another thread then this method will return immediately with the value false.

So, it can return false, but I don't think it ever does in practice since a null return value from InternalTryCheckOutForFlush() will cause an error (assert in test, NullReferenceException in production). So, apparently Lucene relies on the "barging" behavior of this method to be put in the front of the queue.

.NET has no such behavior, but I don't think it really matters which order the locks are taken in as long as ReentrantLock.TryLock() returns true. So, I changed it to call UninterruptableMonitor.Enter(object, ref bool) instead of UninterruptableMonitor.TryEnter(object), which will always acquire the lock and return true.

This probably won't perform as well because the method is supposed to return immediately and may end up deadlocking, but so far it seems to perform better and never deadlock.

---

I am running the tests now to see if the above changes work on all platforms, but locally this seems promising.

[NightOwl888]

I have just submitted #940 with a patch. Well, there were 3 different attempts.

While identifying the problem wasn't too difficult, finding a solution for Issue 2 turned out to be quite challenging due to how performance sensitive of an area this is.

If interested parties could run some benchmarks and/or review the PR to provide some feedback on how to improve this approach, suggestions would be welcomed. I have already attempted Thread.Sleep(1), Thread.SpinWait(1) and the SpinWait struct as part of the retry loop, but all of these seem to slow the tests down a lot. I don't think it would work right if Thread.Yield() were not also used, but it seems to perform best with only Thread.Yield() and nothing else.

I also gave SpinLock a try. Although it has all of the API members we need, it is not a reentrant implementation, so no dice.

[jeme]

Interesting. I tried with net8.0 (which has been changed as the default IDE test platform since the last time I checked) with the same configuration you used and it doesn't fail there. But then I changed the test target to net6.0 and it failed on the first run. So, that probably explains why you are not reproducing it.

Not really as I actually tested on the various .net versions, other things came up as we had a break down in one of our systems due to a windows update, so I have sadly been busy elsewhere.

But I am actually having another timing related bug in another codebase which I can easily reproduce at work with a single run of a test (always fails after ~max 2min, a run is 10min), but it's far harder to reproduce the same bug at home on the same PC as I am having trouble reproducing this issue (takes 2-3 runs). So I don't know.

Running the Lucene tests at work has proven to be a challenge though, I don't know if it's because I use ReSharper that the Lucene solution seems to be very slugish for me to work with, or it's something else. But I always have challenges getting started working on anything.

But great if you found a fix.