Performance Tuning with SQL Server Dynamic Management Views

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Chapter 7: OS and Hardware Interaction
and CPU-intensive than we would hope. In this particular case, the max degree of
parallelism option is set to 2, in an attempt to maximize throughput but give those
poorly performing queries a bit of a performance boost. Ideally, of course, these queries
would be optimized, but they are the product of a third-party system that we can't touch,
and it has a habit of generating queries that are ten pages long.
CXPACKET waits and latch classes
CXPACKET waits are often associated with the occurrence of latches of the type ACCESS_METHODS_
SCAN_RANGE_GENERATOR. We'll discuss this further in a later section, Investigating Latching.
The third wait type on our list ASYNC_NETWORK_IO, is described in
Books Online as:
Occurs on network writes when the task is blocked behind the network. Verify that the client is processing
data from the server.
This wait type occurs when sending data back to the client over the network. Often,
these waits simply mean that the client is slowly fetching rows as it works on them,
rather than fetching all rows into cache and disconnecting (at least logically, using
connection pooling). It can also indicate a slow/unreliable network that needs to be
upgraded. Although this wait type caused an order of magnitude fewer waits than
CXPACKET, it will certainly merit further investigation.
Not included in our output here, but worth looking out for nevertheless, are
SOS_SCHEDULER_YIELD waits, described in Books Online as:
Occurs when a task voluntarily yields the scheduler for other tasks to execute. During this wait the task is
waiting for its quantum to be renewed.