Re: [dpdk-dev] [PATCH v4 2/3] test/distributor: replace sync builtins with atomic builtins

["Phil Yang (Arm Technology China)" <Phil.Yang@arm.com>]

> -----Original Message-----
> From: Hunt, David <david.hunt@intel.com>
> Sent: Wednesday, April 10, 2019 10:06 PM
> To: Phil Yang (Arm Technology China) <Phil.Yang@arm.com>; dev@dpdk.org;
> thomas@monjalon.net
> Cc: reshma.pattan@intel.com; Gavin Hu (Arm Technology China)
> <Gavin.Hu@arm.com>; Honnappa Nagarahalli
> <Honnappa.Nagarahalli@arm.com>; nd <nd@arm.com>
> Subject: Re: [PATCH v4 2/3] test/distributor: replace sync builtins with atomic
> builtins
> 
> Hi Phil,
> 
> On 8/4/2019 4:02 AM, Phil Yang wrote:
> > '__sync' built-in functions are deprecated, should use the '__atomic'
> > built-in instead. the sync built-in functions are full barriers, while
> > atomic built-in functions offer less restrictive one-way barriers,
> > which help performance.
> >
> > Here is the example test result on TX2:
> > sudo ./arm64-armv8a-linuxapp-gcc/app/test -l 112-139 \ -n 4
> > --socket-mem=1024,1024 -- -i
> > RTE>>distributor_perf_autotest
> >
> > *** distributor_perf_autotest without this patch *** ==== Cache line
> > switch test === Time for 33554432 iterations = 1519202730 ticks Ticks
> > per iteration = 45
> >
> > *** distributor_perf_autotest with this patch *** ==== Cache line
> > switch test === Time for 33554432 iterations = 1251715496 ticks Ticks
> > per iteration = 37
> >
> > Less ticks needed for the cache line switch test. It got 17% of
> > performance improvement.
> 
> 
Hi, Dave

Thanks for your input.

> I'm seeing about an 8% performance degradation on my platform for the

I'd tested this patch on our x86 server (E5-2640 v3 @ 2.60GHz) several rounds. However, I didn't found performance degradation. Please check the test result below.
$ sudo  ./x86_64-native-linuxapp-gcc/app/test -l 8-15 -n 4 --socket-mem=1024,1024 -- -i
RTE>>distributor_perf_autotest

####  without this patch ####
==== Cache line switch test ===
Time for 33554432 iterations = 12379399910 ticks
Ticks per iteration = 368

=== Performance test of distributor (single mode) ===
Time per burst:  5815
Time per packet: 90

=== Performance test of distributor (burst mode) ===
Time per burst:  3487
Time per packet: 54

####  with this patch ####
==== Cache line switch test ===
Time for 33554432 iterations = 12388791845 ticks
Ticks per iteration = 369

=== Performance test of distributor (single mode) ===
Time per burst:  5796
Time per packet: 90

=== Performance test of distributor (burst mode) ===
Time per burst:  3477
Time per packet: 54

From my test, there was a little bit of performance improvement (You can also think of it as a measurement bias) on x86. 

> cache line switch test with the patch, however the single mode and burst
> mode tests area showing no difference, which are the more important tests.
> What kind of differences are you seeing in the single/burst mode tests?

Actually, I found no difference in the single mode and burst mode on aarch64 neither. I think it means this test case is not the hotspot for those two mode's performance. 

Just like the __sync_xxx builtins, the __atomic_xxx builtins are atomic operations, which elide the memory barrier. So I think it should benefit all platform.

Thanks,
Phil
> 
> Rgds,
> Dave.
> 
> 
> ---snip---
> 
>