Re: ib_post_send execution time

[Steve Wise <swise-7bPotxP6k4+P2YhJcF5u+vpXobYPEAuW@public.gmane.org>]

On 10/24/2014 6:30 AM, Or Gerlitz wrote:
> On Fri, Oct 24, 2014 at 3:39 AM, Eli Cohen <eli-LDSdmyG8hGV8YrgS2mwiifqBs+8SCbDb@public.gmane.org> wrote:
>> On Thu, Oct 23, 2014 at 11:45:05AM -0700, Roland Dreier wrote:
>>> On Thu, Oct 23, 2014 at 10:21 AM, Evgenii Smirnov
>>> <evgenii.smirnov-EIkl63zCoXaH+58JC4qpiA@public.gmane.org> wrote:
>>>> I am trying to achieve high packet per second throughput with 2-byte
>>>> messages over Infiniband from kernel using IB_SEND verb. The most I
>>>> can get so far is 3.5 Mpps. However, ib_send_bw utility from perftest
>>>> package is able to send 2-byte packets with rate of 9 Mpps.
>>>> After some profiling I found that execution of ib_post_send function
>>>> in kernel takes about 213 ns in average, for the user-space function
>>>> ibv_post_send takes only about 57 ns.
>>>> As I understand, these functions do almost same operations. The work
>>>> request fields and queue pair parameters are also the same. Why do
>>>> they have such big difference in execution times?
>>>
>>> Interesting.  I guess it would be useful to look at perf top / and or
>>> get a perf report with "perf report -a -g" when running your high PPS
>>> workload, and see where the time is wasted.
>>>
>> I assume ib_send_bw uses inline with blueflame so it may be part of
>> the explanation to the differences you see.
> I think it should be the other way around... when we use inline we
> consume more CPU cycles and here we see notable different (213ns --
> kernel 57ns user) in favor of libmlx4
>

Inline may consume more cpu cycles but should reduce latency because the 
IO is completed with only 1 DMA transaction, the WR fetch, which 
includes the data.  Non-inline requires 2 DMA transactions, the WR fetch 
and the data fetch.


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