since you refuse to actually learn about what's going on before arguing about it, i'll just leave this here, lifted directly from 10gen's official page on concurrency[1] (nevermind the fact that this is directly addressed in TFA)
mongod threads yield their lock (read or write) in two classes of situations:
* yield-on-page-fault – v2.0 implements a yield-on-page-fault feature which results in much more concurrency than one would achieve with a pure reader/writer lock. For common operational cases, file system page faults are detected in advanced and handled outside of any lock, then the lock is resumed. Not all fault situations yield, but many do. This results in v2.0 having much better concurrency in practice than v1.8.
* yield-on-long-operation – mongod also yields periodically on common operations that are extremely long running. The goal here is to allow interleaving so that other operations which are quick-running can execute soon.
mongod threads yield their lock (read or write) in two classes of situations:
* yield-on-page-fault – v2.0 implements a yield-on-page-fault feature which results in much more concurrency than one would achieve with a pure reader/writer lock. For common operational cases, file system page faults are detected in advanced and handled outside of any lock, then the lock is resumed. Not all fault situations yield, but many do. This results in v2.0 having much better concurrency in practice than v1.8.
* yield-on-long-operation – mongod also yields periodically on common operations that are extremely long running. The goal here is to allow interleaving so that other operations which are quick-running can execute soon.
http://www.mongodb.org/display/DOCS/How+does+concurrency+wor...