The behavior of the write-to-read transfer is determined by the _FAIRNESS_THRESHOLD parameter,which was introduced in Oracle 8.1.5 and defaults to 4. Prior to the introduction of this parameter,when Instance A held a block in exclusive mode and Instance B requested a read-only copy of that block, Instance A would downgrade its exclusive lock to a shared lock and send the block to Instance B,which would also set a shared lock on the block. However, if Instance A is performing frequent updates
on the block, it will need to reacquire the block and set an exclusive lock again. If this process is repeated frequently, then Instance A will be continually interrupted, as it has to downgrade the exclusive lock to a shared lock and wait until Instance B has finished reading the block before it can convert the shared lock back into an exclusive lock.
The _FAIRNESS_THRESHOLD parameter modifies this behavior. When this parameter is set,Instance A will no longer downgrade the exclusive lock. Instead, it sends a null lock to Instance B,and then it can continue processing. However, if instance B requests the block _FAIRNESS_THRESHOLD times, by default 4, then Instance A will revert to the original behavior—it will downgrade the exclusive lock to a shared lock and ship the block to Instance B, which will also set a shared lock on the block.

From:Pro Database 10g RAC on Linux