Re: [PATCH 7/8] xprtrdma: Split the completion queue

Chuck Lever
2014-04-17T13:55:04Z

On Apr 17, 2014, at 3:06 AM, Sagi Grimberg <sagig-LDSdmyG8hGV8YrgS2mwiifqBs+8SCbDb@public.gmane.org> wrote:

> On 4/16/2014 9:21 PM, Chuck Lever wrote:
>> Passing a small array to ip_poll_cq() is actually easy to do, and is
>> exactly equivalent to a poll budget. The struct ib_wc should be taken
>> off the stack anyway, IMO.
>>
>> The only other example I see in 3.15 right now is IPoIB, which seems
>> to do exactly this.
>>
>> I’m testing a patch now. I’d like to start simple and make it more
>> complex only if we need to.
>
> What array size are you using? Note that if you use a small array it may be an overkill since
> a lot more interrupts are invoked (-> more latency). I found that for a high workload a budget
> of 256/512/1024 keeps fairness and doesn't increase latency.

My array size is currently 4. It’s a macro that can be changed easily.

By a very large majority, my workloads see only one WC per completion
upcall. However, I’m using an older card with simple synthetic benchmarks.

I don’t want to make the array large because struct ib_wc is at least
64 bytes on my systems — each WC array would be enormous and hardly ever
used. But we can dial it in over time.

--
Chuck Lever
chuck[dot]lever[at]oracle[dot]com