On 10/24/2014 6:30 AM, Or Gerlitz wrote:
On Fri, Oct 24, 2014 at 3:39 AM, Eli Cohen <eli@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxx> 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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