Mongo socketTimeoutMS不起作用

I perform "findOne()" query from PHP 5.4 to Mongo 2.4.10
PHP mongo driver version is 1.5.5
Mongo DB is replicaset

To limit waiting for results of this "findOne()" query I use "socketTimeoutMS" option
According to documentation in case of exceeding "socketTimeoutMS" function "findOne()" must throw "MongoCursorTimeoutException" exception
But I don't see any exception in logs

Also I measure excecution time of this "findOne()" query --- often it's more (sometimes in several times) than "socketTimeoutMS"

So looks like that "socketTimeoutMS" option doesn't work
Need to mention that our mongo DB is under very high load

Also need to mention that on my local machine "socketTimeoutMS" works OK

So what can be reasons for "socketTimeoutMS" option ignorance on production ?

2 --- 4   Using 50 from default with
2 --- 4   found connection 192.168.2.172:27017;gmt;admin/gmt/fb5f987e48d27c093e233d3cda7758e2;11843 (looking for 192.168.2.172:27017;gmt;admin/gmt/fb5f987e48d27c093e233d3cda7758e2;11843)
2 --- 4   is_ping: skipping: last ran at 1407943807, now: 1407943807, time left: 5
2 --- 4   found connection 192.168.2.92:27017;gmt;admin/gmt/fb5f987e48d27c093e233d3cda7758e2;11843 (looking for 192.168.2.92:27017;gmt;admin/gmt/fb5f987e48d27c093e233d3cda7758e2;11843)
2 --- 4   is_ping: skipping: last ran at 1407943807, now: 1407943807, time left: 5
2 --- 4   found connection 192.168.2.94:27017;gmt;admin/gmt/fb5f987e48d27c093e233d3cda7758e2;11843 (looking for 192.168.2.94:27017;gmt;admin/gmt/fb5f987e48d27c093e233d3cda7758e2;11843)
2 --- 4   is_ping: skipping: last ran at 1407943807, now: 1407943807, time left: 5
2 --- 4   discover_topology: checking ismaster for 192.168.2.172:27017;gmt;admin/gmt/fb5f987e48d27c093e233d3cda7758e2;11843
2 --- 4   discover_topology: checking ismaster for 192.168.2.92:27017;gmt;admin/gmt/fb5f987e48d27c093e233d3cda7758e2;11843
2 --- 4   found connection 192.168.2.92:27017;gmt;admin/gmt/fb5f987e48d27c093e233d3cda7758e2;11843 (looking for 192.168.2.92:27017;gmt;admin/gmt/fb5f987e48d27c093e233d3cda7758e2;11843)
2 --- 4   ismaster: skipping: last ran at 1407943798, now: 1407943807, time left: 6
2 --- 4   discover_topology: ismaster got skipped
2 --- 4   discover_topology: checking ismaster for 192.168.2.94:27017;gmt;admin/gmt/fb5f987e48d27c093e233d3cda7758e2;11843
2 --- 4   found connection 192.168.2.94:27017;gmt;admin/gmt/fb5f987e48d27c093e233d3cda7758e2;11843 (looking for 192.168.2.94:27017;gmt;admin/gmt/fb5f987e48d27c093e233d3cda7758e2;11843)
2 --- 4   ismaster: skipping: last ran at 1407943798, now: 1407943807, time left: 6
2 --- 4   discover_topology: ismaster got skipped
2 --- 4   No timeout changes for 192.168.2.92:27017;gmt;admin/gmt/fb5f987e48d27c093e233d3cda7758e2;11843
2 --- 4   No timeout changes for 192.168.2.92:27017;gmt;admin/gmt/fb5f987e48d27c093e233d3cda7758e2;11843