Re: [PATCH RFC 0/5] xprtrdma Send completion batching

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> On Sep 5, 2017, at 4:06 PM, Jason Gunthorpe <jgunthorpe@xxxxxxxxxxxxxxxxxxxx> wrote:
> 
> On Tue, Sep 05, 2017 at 01:00:10PM -0400, Chuck Lever wrote:
> 
>> - Send completions are batched to reduce interrupts, but still
>>  provide a periodic heartbeat signal for SQ housekeeping
> 
> I would scrub this commentary, it is very misleading.
> 
> The idea of a periodic completion does not match how verbs works at
> all, it was an incomplete root cause analysis from a HCA that uses
> different rules for freeing space in the SQ.

I think it does bear mentioning that, given this diagnosis, it is
still safe to remove the ib_post_send counting mechanism in 5/5,
which has been in xprtrdma for as long as I can recall, and has
been effective (with a few minor adjustments) at preventing SQ
overflow.

I'm not able to test this change with every HCA the Linux kernel
currently supports, unfortunately. The best I can do is offer a
"proof of correctness" and hope that vendors will jump on this
and try it out.


> Instead, I would say this series creates strong SQ accounting and
> properly guarentees the SQ can never overflow by only releasing SQ's
> back into the pool when the HCA has confirmed they are completed via a
> CQ.

I will adjust the cover letter (and patch descriptions as necessary)
next time I post this series. Thanks for your suggestions and review.


--
Chuck Lever



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