Error during Creating Documents through sync gateway

I had a test on Sync Gateway, I set 2 servers for Couchbase and 2 other servers for Sync Gateway. I made a test for inserting documents by Jmeter with below specifications:
Number Of Threads: 500 , Loop Count: 500
I did the test by 4 clients so Totally one Million requests.
When I did this test behind a LoadBalancer,(I connect to LoadBalancer and it automatically dispatches requests to Sync Gateway), everything is quitely ok, of course it occurs some percentage of error, but that’s ok.
the problem is:
when I directy did the test on just one of Sync Gateway servers and one Couchbase Server (because Sync Gateway servers became half, I decreased requests from 1M to 500K ), the test quit in middle and it doesn’t be completed, and I got the below error:
“ Address already in use: connect”
Does any one know the result? I think it should work properly if I create documents only by one of Sync Gateways, but it’s not!

That sounds like your client is running out of sockets and is unable to connect to Sync Gateway.