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https://issues.apache.org/jira/browse/SOLR-17419?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17878128#comment-17878128
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Jason Gerlowski commented on SOLR-17419:
----------------------------------------
I opened a PR for this ticket but accidentally swapped two digits in the JIRA
number, so it hasn't been linked here yet. Would love reviews on the code
[here|https://github.com/apache/solr/pull/2681] if anyone has a few moments.
Rather than trying to improve HttpShardHandler, the linked PR introduces an
alternate implementation: ParallelHttpShardHandler[Factory]. The new
implementation extends HttpShardHandler[Factory] and reuses most of its code.
The only difference: ParallelHttpShardHandler.submit uses the executor to both
send the request and await the response. This allows the "submit" call to
return sooner, and for SearchHandler (or other callers) to iterate through a
series of "submit" calls more quickly. The tradeoff of course is potentially
higher CPU utilization, depending on the number of shards in play, etc.
----
I've run some perf tests (details below), and the results look pretty promising
in certain scenarios!
!shardhandler-perf-graph.png!
To summarize:
# With auth disabled HttpShardHandler outperforms the new implementation even
in collections with many shards.
# With auth enabled though, ParallelShardHandler outperforms HttpShardHandler
by a factor of nearly "3x" regardless of the number of shards.
At first glance, it's surprising that "auth" has such a huge impact. The
cause, according to some profiling, is PKI auth. Generating and signing the
"PKI" header consumes a massive amount of CPU and drives the cost of
HttpShardHandler.submit. Without PKI, request-sending is cheap enough that
doing it serially outperforms the thread-creation overhead incurred by the
"Parallel" implementation. As soon as PKI is needed though, it dwarfs any
thread-creation overhead and ParallelHttpShardHandler wins out.
P.S: With PKI being so expensive, should we stress the security.json
"forwardCredentials=true" option more strongly in the docs? Or make it a
default in the AuthPlugins that support it?
*Appendix: Perf Test Details*
The results discussed above come from a series of perf experiments I ran on a
local 4-node Solr cluster created via "bin/solr start -e cloud -m 2g". "Avg
QTime" measurements were taken by (1) creating a collection with a particular
'numShard' value and using a particular 'ShardHandlerFactory', (2) loading in
some data, (3) warming the JVM and collection with an initial round of queries,
and (4) running 'numQueries' from each of 'numThreads' QueryRunner threads.
As a data set, I used emails from our "dev@" list exported from
"lists.apache.org". (I have a little script to export and clean up this data
that I'm considering saving somewhere - it's been an interesting dataset to
work with and might be useful in our benchmark module or elsewhere?) For
queries, the perf-test code uses the "/terms" handler to fetch common terms and
then creates single-term queries from those.
Other details:
* *Solr Version:* 10.0.0-SNAPSHOT (a branch based off of 'main', with the
commit: 1818841868d70adebde364165d60114f164952de)
* *JVM:* openjdk version "17.0.8" 2023-07-18
* *OS:* MacOS Sonoma 14.6.1
* *Hardware:* 2017 iMac Pro, 2.5GHz 14-core Intel Xeon, 128gb ram
> Improve HttpShardHandler performance in many-shard collections
> --------------------------------------------------------------
>
> Key: SOLR-17419
> URL: https://issues.apache.org/jira/browse/SOLR-17419
> Project: Solr
> Issue Type: Bug
> Security Level: Public(Default Security Level. Issues are Public)
> Components: SolrCloud
> Affects Versions: 9.0, 9.6.1
> Reporter: Jason Gerlowski
> Priority: Major
> Attachments: shardhandler-perf-graph.png
>
>
> In Solr 8, HttpShardHandler sends shard-requests by submitting Callables to
> an ExecutorService. As a result, both the "request-sending" and
> "response-awaiting" happened asynchronous to the original request-thread.
> {code:java}
> @Override
> public void submit(final ShardRequest sreq, final String shard, final
> ModifiableSolrParams params) {
> ShardRequestor shardRequestor = new ShardRequestor(sreq, shard, params,
> this); // Callable
> try {
> shardRequestor.init();
> pending.add(completionService.submit(shardRequestor));
> } finally {
> shardRequestor.end();
> }
> }
> {code}
> However, in Solr 9.x HttpShardHandler ditched the
> ExecutorService/per-request-thread approach in favor of [sending all requests
> serially using
> "SolrClient.requestAsync"|https://github.com/apache/solr/blob/main/solr/core/src/java/org/apache/solr/handler/component/HttpShardHandler.java#L163].
> SOLR-14354, which made this change, did this in an effort to avoid
> unnecessary thread and CPU context-switching. As Dat described in SOLR-14354:
> {quote}after sending a request that thread basically do nothing just waiting
> for response from other side. That thread will be swapped out and CPU will
> try to handle another thread (this is called context switch, CPU will save
> the context of the current thread and switch to another one). When some data
> (not all) come back, that thread will be called to parsing these data, then
> it will wait until more data come back. So there will be lots of context
> switching in CPU. That is quite inefficient
> {quote}
> This approach comes with a downside though - all the shard requests are sent
> serially. If sending each request takes ~1ms, then a user is unlikely to
> notice this in their collection with 5 or 10 shards. But the cost here
> scales linearly, so in *a collection with 50 shards - this approach would
> bake a ~50ms delay into the critical path of every single query!*
> This issue is intended to reevaluate whether there's a better way to balance
> these concerns. Ideally we can come up with an approach that improves all
> scenarios. Lacking that, maybe Solr could choose between one of several
> approaches semi-intelligently based on the number of shards or other factors?
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