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The HBAnyware Utility User Manual Page 80
Server Performance (Windows)
I/O Coalescing
I/O Coalescing is enabled and controlled by two driver parameters: CoalesceMsCnt and
CoalesceRspCnt. The effect of I/O Coalescing will depend on the CPU resources available on the
server. With I/O Coalescing turned on, interrupts are batched, reducing the number of interrupts and
maximizing the number of commands processed with each interrupt. For heavily loaded systems, this
will provide better throughput.
With I/O Coalescing turned off (the default), each I/O processes immediately, one CPU interrupt per I/O.
For systems not heavily loaded, the default will provide better throughput. The following table shows
recommendations based upon the number of I/Os per HBA.
CoalesceMsCnt
The CoalesceMsCnt parameter controls the maximum elapsed time in milliseconds that the HBA waits
before it generates a CPU interrupt. The value range is 0 - 63 (decimal) or 0x0 - 0x3F (hex). The default
is 0 and disables I/O Coalescing.
CoalesceRspCnt
The CoalesceRspCnt parameter controls the maximum number of responses to batch before an
interrupt generates. If CoalesceRspCnt expires, an interrupt generates for all responses collected up to
that point. With CoalesceRspCnt set to less than 2, response coalescing is disabled and an interrupt
triggers for each response. The value range for CoalesceRspCnt is 1 - 255 (decimal) or 0x1 - 0xFF
(hex). The default value is 8.
Performance Testing
There are four driver parameters that must be considered (and perhaps changed from the default) for
better performance testing: QueueDepth, NumFcpContext, 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.
Table 9: Recommended Settings for I/O Coalescing
I/Os per Second
Suggested
CoalesceMsCnt
Suggested
CoalesceRspCnt
I/Os < 10000 0 8
10000 < I/Os < 18000 1 8
18000 < I/Os < 26000 1 16
!/Os > 26000 1 24
Note: A system restart is required to make changes to CoalesceMsCnt and/or CoalesceRspCnt.
Note: Parameter values recommended in this topic are for performance testing only and not
for general operation.