Analysis Code: 37

Severity: Warning

Analysis Code: 37

What is Interrupt Coalescing?

"Interrupt coalescing" (IntCoal) allows the device driver to delay delivery of the interrupt to the kernel by waiting a short while (around 0.2 ms by default) in order to coalesce other interrupts.

The advantage of IntCoal

The advantage of this is that the kernel can then handle multiple interrupts at once, saving additional context-switches and other such overhead. This results in better throughput when there are multiple parallel IO threads from the hosts.

The disadvantage of IntCoal

Unfortunately, interrupt coalescing improves throughput at the expense of latency for the single-threaded case since the interrupts are unnecessarily delayed.

HP's Applied Technology Center (ATC) performance team analyzed performance implications of IntCoal on 3PAR host ports and Remote Copy ports and found that better performance was achieved overall with the setting disabled. As a result of their testing, host port Intcoal is disabled by default since 3PAR OS version 3.1.2 GA. In addition, if the setting is enabled for host ports, a subcommand to Controlport is available to disable interrupt coalescing on host (target) ports.

NOTE: This also applies to Remote Copy ports…Remote Copy ports IntCoal should be disabled for the same reasons stated above.

WARNING

Care should be taken when modifying port parameters with the Controlport CLI command or thru the Management Console. The affected port will be reset and you should allow enough time for the array to process the event caused by this action and to also process any failovers/failbacks as a result of the port reset. Therefore, it is recommended to wait 10-15 minutes before effecting a change on another port via the Controlport CLI command or IMC/SSMC.

IMPORTANT:

The 10-15 minute delay allows the array to recover as a result of the port reset.

This port reset may cause fail-overs and fail-backs depending on the configuration.

Also, keep in mind with a port reset that a very large number of events will occur

that the OS must process. If multiple resets occur rapidly there is a risk of an outage

due to overwhelming the array.

**NOTE (as of June 2015): While the default has been changed at the 3PAR OS level, the Management Console (MC) still shows Interrupt Coalescence Enabled as the default. This will be changed in a future release of MC.

To view IntCoal port settings via CLI:

cli% showport -par

N:S:P Connmode ConnType CfgRate MaxRate Class2 UniqNodeWwn VCN IntCoal

0:0:1 disk point 6Gbps 6Gbps n/a n/a n/a enabled

0:0:2 disk point 6Gbps 6Gbps n/a n/a n/a enabled

0:1:1 host point auto 8Gbps disabled disabled disabled disabled

0:1:2 host point auto 8Gbps disabled disabled disabled disabled

1:0:1 disk point 6Gbps 6Gbps n/a n/a n/a enabled

1:0:2 disk point 6Gbps 6Gbps n/a n/a n/a enabled

1:1:1 host point auto 8Gbps disabled disabled disabled disabled

1:1:2 host point auto 8Gbps disabled disabled disabled disabled

------------------------------------------------------------------------------

8

To disable IntCoal via CLI:

WARNING

This is disruptive!! After disabling a host port, wait 10-15 minutes for the port to stabilize and verify the port is back online prior to taking action on another host port.

cli% controlport intcoal disable N:S:P

WARNING: Port <N:S:P> has active hosts that may be disrupted.

Are you sure you want to run controlport intcoal disable on port <N:S:P>?
select q=quit y=yes n=no: y