Dell Emulex Family of Adapters - Manual (Page 588)

Emulex Drivers for Windows User Manual P010077-01A Rev. A
3. Configuration
FC/FCoE Driver Configuration
588
Note: A system restart is required to make changes to CoalesceMsCnt and
CoalesceRspCnt.
Performance Testing
There are three driver parameters that need to be considered (and perhaps changed
from the default) for better performance testing: QueueDepth, CoalesceMsCnt, and
CoalesceRspCnt.
QueueDepth
If the number of outstanding I/Os per device is expected to exceed 32, increase this
parameter to a value greater than the number of expected I/Os per device, up to a
maximum of 254. The QueueDepth parameter defaults to 32. If 32 is set and not a high
enough value, performance degradation may occur due to Storport throttling its device
queue.
CoalesceMsCnt
CoalesceMsCnt defaults to zero. If you are using a performance evaluation tool such as
IOMETER and if you expect the I/O activity to be greater than 8000 I/Os per second,
set CoalesceMsCnt to 1 and reinitialized with an adapter reset or system reboot.
CoalesceRspCnt
CoalesceRspCnt defaults to 8. For all other values up to the maximum of 63, the adapter
does not interrupt the host with a completion until either CoalesceMsCnt milliseconds
has elapsed or CoalesceRspCnt responses are pending. The value of these two driver
parameters reduces the number of interrupts per second which improves overall CPU
utilization. However, there is a point where the number of I/Os per second is small
relative to CoalesceMsCnt and this will slow down the completion process, causing
performance degradation.
Examples
Test Scenario One:

You execute IOMETER with an I/O depth of 1 I/O per device in a small-scale
configuration (16 devices). In this case, the test does not exceed the adapter 's
performance limits and the number of I/Os per second are in the low
thousands.

Recommendation: set CoalesceMsCnt to 0 (or leave the default value).
Test Scenario Two:

You execute IOMETER with an I/O depth of 48 I/Os per device in a small-scale
configuration (16 devices).

Recommendation: set QueueDepth to be greater than 48 (for example, 64).