ConcurrentUpdateSolrServer includes a Spin Lock

ConcurrentUpdateSolrServer.blockUntilFinished() includes a Spin Lock. This causes an extremely high amount of CPU to be used on the Cloud Leader during indexing.

Here is a summary of our system testing.

Importing data on Solr4.5.0:
Throughput gets as high as 240 documents per second.

[tomcat@solr-stg01 logs]$ uptime
09:53:50 up 310 days, 23:52, 1 user, load average: 3.33, 3.72, 5.43

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
9547 tomcat 21 0 6850m 1.2g 16m S 86.2 5.0 1:48.81 java

Importing data on Solr4.7.0 with no replicas:
Throughput peaks at 350 documents per second.

[tomcat@solr-stg01 logs]$ uptime
10:03:44 up 311 days, 2 min, 1 user, load average: 4.57, 2.55, 4.18

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
9728 tomcat 23 0 6859m 2.2g 28m S 62.3 9.0 2:20.20 java

Importing data on Solr4.7.0 with replicas:
Throughput peaks at 30 documents per second because the Solr machine is out of CPU.

[tomcat@solr-stg01 logs]$ uptime
09:40:04 up 310 days, 23:38, 1 user, load average: 30.54, 12.39, 4.79

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
9190 tomcat 17 0 7005m 397m 15m S 198.5 1.6 7:14.87 java