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From Simon Willnauer <simon.willna...@googlemail.com>
Subject Re: large scale indexing issues / single threaded bottleneck
Date Fri, 28 Oct 2011 19:27:36 GMT
On Fri, Oct 28, 2011 at 9:17 PM, Simon Willnauer
<simon.willnauer@googlemail.com> wrote:
> Hey Roman,
>
> On Fri, Oct 28, 2011 at 8:38 PM, Roman Alekseenkov
> <ralekseenkov@gmail.com> wrote:
>> Hi everyone,
>>
>> I'm looking for some help with Solr indexing issues on a large scale.
>>
>> We are indexing few terabytes/month on a sizeable Solr cluster (8
>> masters / serving writes, 16 slaves / serving reads). After certain
>> amount of tuning we got to the point where a single Solr instance can
>> handle index size of 100GB without much issues, but after that we are
>> starting to observe noticeable delays on index flush and they are
>> getting larger. See the attached picture for details, it's done for a
>> single JVM on a single machine.
>>
>> We are posting data in 8 threads using javabin format and doing commit
>> every 5K documents, merge factor 20, and ram buffer size about 384MB.
>> From the picture it can be seen that a single-threaded index flushing
>> code kicks in on every commit and blocks all other indexing threads.
>> The hardware is decent (12 physical / 24 virtual cores per machine)
>> and it is mostly idle when the index is flushing. Very little CPU
>> utilization and disk I/O (<5%), with the exception of a single CPU
>> core which actually does index flush (95% CPU, 5% I/O wait).
>>
>> My questions are:
>>
>> 1) will Solr changes from real-time branch help to resolve these
>> issues? I was reading
>> http://blog.mikemccandless.com/2011/05/265-indexing-speedup-with-lucenes.html
>> and it looks like we have exactly the same problem
>
> did you also read http://bit.ly/ujLw6v - here I try to explain the
> major difference between Lucene 3.x and 4.0 and why 3.x has these long
> idle times. In Lucene 3.x a full flush / commit is a single threaded
> process, as you observed there is only one thread making progress. In
> Lucene 4 there is still a single thread executing the commit but other
> threads are not blocked anymore. Depending on how fast the thread can
> flush other threads might help flushing segments for that commit
> concurrently or simply index into new documents writers. So basically
> 4.0 won't have this problem anymore. The realtime branch you talk
> about is already merged into 4.0 trunk.
>
>>
>> 2) what would be the best way to port these (and only these) changes
>> to 3.4.0? I tried to dig into the branching and revisions, but got
>> lost quickly. Tried something like "svn diff
>> […]realtime_search@r953476 […]realtime_search@r1097767", but I'm not
>> sure if it's even possible to merge these into 3.4.0
>
> Possible yes! Worth the trouble, I would say no!
> DocumentsWriterPerThread (DWPT) is a very big change and I don't think
> we should backport this into our stable branch. However, this feature
> is very stable in 4.0 though.
>>
>> 3) what would you recommend for production 24/7 use? 3.4.0?
>
> I think 3.4 is a safe bet! I personally tend to use trunk in
> production too the only problem is that this is basically a moving
> target and introduces extra overhead on your side to watch changes and
> index format modification which could basically prevent you from
> simple upgrades
>
>>
>> 4) is there a workaround that can be used? also, I listed the stack trace below
>>
>> Thank you!
>> Roman
>>
>> P.S. This single "index flushing" thread spends 99% of all the time in
>> "org.apache.lucene.index.BufferedDeletesStream.applyDeletes", and then
>> the merge seems to go quickly. I looked it up and it looks like the
>> intent here is deleting old commit points (we are keeping only 1
>> non-optimized commit point per config). Not sure why is it taking that
>> long.
>
> in 3.x there is no way to apply deletes without doing a flush (afaik).
> In 3.x a flush means single threaded again - similar to commit just
> without syncing files to disk and writing a new segments file. In 4.0
> you have way more control over this via
> IndexWriterConfig#setMaxBufferedDeleteTerms which are also applied
> without blocking other threads. In trunk we hijack indexing threads to
> do all that work concurrently so you get better cpu utilization and
> due to concurrent flushing better and usually continuous IO
> utilization.
>
> hope that helps.
>
> simon
>>
>> pool-2-thread-1 [RUNNABLE] CPU time: 3:31
>> java.nio.Bits.copyToByteArray(long, Object, long, long)
>> java.nio.DirectByteBuffer.get(byte[], int, int)
>> org.apache.lucene.store.MMapDirectory$MMapIndexInput.readBytes(byte[], int, int)
>> org.apache.lucene.index.TermBuffer.read(IndexInput, FieldInfos)
>> org.apache.lucene.index.SegmentTermEnum.next()
>> org.apache.lucene.index.TermInfosReader.<init>(Directory, String,
>> FieldInfos, int, int)
>> org.apache.lucene.index.SegmentCoreReaders.<init>(SegmentReader,
>> Directory, SegmentInfo, int, int)
>> org.apache.lucene.index.SegmentReader.get(boolean, Directory,
>> SegmentInfo, int, boolean, int)
>> org.apache.lucene.index.IndexWriter$ReaderPool.get(SegmentInfo,
>> boolean, int, int)
>> org.apache.lucene.index.IndexWriter$ReaderPool.get(SegmentInfo, boolean)
>> org.apache.lucene.index.BufferedDeletesStream.applyDeletes(IndexWriter$ReaderPool,
>> List)
>> org.apache.lucene.index.IndexWriter.doFlush(boolean)
>> org.apache.lucene.index.IndexWriter.flush(boolean, boolean)
>> org.apache.lucene.index.IndexWriter.closeInternal(boolean)
>> org.apache.lucene.index.IndexWriter.close(boolean)
>> org.apache.lucene.index.IndexWriter.close()
>> org.apache.solr.update.SolrIndexWriter.close()
>> org.apache.solr.update.DirectUpdateHandler2.closeWriter()
>> org.apache.solr.update.DirectUpdateHandler2.commit(CommitUpdateCommand)
>> org.apache.solr.update.DirectUpdateHandler2$CommitTracker.run()
>> java.util.concurrent.Executors$RunnableAdapter.call()
>> java.util.concurrent.FutureTask$Sync.innerRun()
>> java.util.concurrent.FutureTask.run()
>> java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$101(ScheduledThreadPoolExecutor$ScheduledFutureTask)
>> java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run()
>> java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor$Worker)
>> java.util.concurrent.ThreadPoolExecutor$Worker.run()
>> java.lang.Thread.run()

one more thing, after somebody (thanks robert) pointed me at the
stacktrace it seems kind of obvious what the root cause of your
problem is. Its solr :) Solr closes the IndexWriter on commit which is
very wasteful since you basically wait until all merges are done. Solr
trunk has solved this problem.

We should maybe try to fix this in 3.x too?

simon
>>
>

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