Michael Gibney created SOLR-16932:
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Summary: Http2Client should have configurable
`maxOutstandingRequests`, to support parallel requests in high-shard-count
contexts
Key: SOLR-16932
URL: https://issues.apache.org/jira/browse/SOLR-16932
Project: Solr
Issue Type: Improvement
Security Level: Public (Default Security Level. Issues are Public)
Affects Versions: 9.3, main (10.0)
Reporter: Michael Gibney
Http2SolrClient is asynchronous, but it only allows for a
[hardcoded|https://github.com/apache/solr/blob/88990d640a89091a8f7b0b2493377ac24118afe8/solr/solrj/src/java/org/apache/solr/client/solrj/impl/Http2SolrClient.java#L964]
max number (1000) of outstanding requests. Thus, under sufficient load,
intra-cluster communication is not fully concurrent/asynchronous, and the
top-level coordinator node can become a bottleneck. This is especially
problematic for high-shard-count collections (>1k shards) where a single
top-level request easily generates sufficient load to hit this throttling,
effectively guaranteeing a near doubling of top-level (client-side) request
latency.
It should be possible to configure this {{maxOutstandingConnections}} threshold
via the HttpShardHandlerFactory config. If I understand correctly the
implications of this limit, it should be reasonable to scale it roughly
according to the number of nodes in the cluster (consider, e.g.: 1k outstanding
requests to 2 nodes is a very different situation than 1k outstanding requests
to 128 nodes).
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