Re: [PATCH v4 0/4] Sending kernel pathrecord query to user cache server

Hal Rosenstock
2015-06-10T16:46:55Z

On 6/10/2015 11:49 AM, Wan, Kaike wrote:
>>>>>>> A SA cache is undeniably critical for fabric scalability and performance.
>>>>>>> In user space, the ibacm application provides a good example of
>>>>>>> pathrecord cache for address and route resolution. With the recent
>>>>>>> implementation of the provider architecture, ibacm offers more
>>>>>> extensibility as a SA cache.
>>>>>>> In kernel, ipoib implements its own small cache for pathrecords,
>>>>>>> which is however not available for general use. Furthermore, the
>>>>>>> implementation of a SA cache in user space offers better
>>>>>>> flexibility, larger capacity, and more robustness for the system.
>>>>>>>
>>>>>>> In this patch series, a mechanism is implemented to allow ib_sa to
>>>>>>> send pathrecord query to a user application (eg ibacm) through netlink.
>>>>>>
>>>>>> While this appears to address the current upstream use model for
>>>>>> ACM with it's multicast overlay backend where PRs are static, it
>>>>>> does not appear to address PR changes.
>>>>>> Should aging/revalidation of PRs be supported ? If so, would this
>>>>>> make PRs similar at "high" level to IP neighbor cache in kernel ?
>>>>>
>>>>> Even for the default provider acmp, PRs will time out and the length
>>>>> of
>>>> timeout is configurable. For other providers (eg ibssa), the PR
>>>> change could be managed correctly and promptly, and this capability
>>>> is beyond ibacm core itself.
>>>>
>>>> That deals with the update of PR in user space (ACM). Doesn't kernel
>>>> need some way of knowing PR was updated ?
>>>
>>> Maybe. If a kernel client is interested in PR changes, it can register
>>> for notification
>>
>> Wouldn't the notification mechanism be via netlink ? What would the
>> granularity be for the registrations and notifications ?
>
> Not necessarily. AFAIK, SA notification is not currently supported in kernel. One could register notification through netlink, as long as the user space application (eg: ibacm) supports it. But this would be a new feature, as Sean pointed out in previous e-mail.

Such notifications are not necessarily tied to SA event reporting. In
fact, Todd made argument at 2014 OFA Devcon that such mechanism is
anathema to (exa)scalability.

-- Hal

> Kaike