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SubjectRe: [Linuxarm] Re: [PATCH net-next v2 2/2] ptr_ring: make __ptr_ring_empty() checking more reliable
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On 2021/6/27 14:07, Michael S. Tsirkin wrote:
> On Fri, Jun 25, 2021 at 05:20:10PM +0800, Yunsheng Lin wrote:
>> On 2021/6/25 14:39, Michael S. Tsirkin wrote:
>>> On Fri, Jun 25, 2021 at 11:18:56AM +0800, Yunsheng Lin wrote:
>>>> Currently r->queue[] is cleared after r->consumer_head is moved
>>>> forward, which makes the __ptr_ring_empty() checking called in
>>>> page_pool_refill_alloc_cache() unreliable if the checking is done
>>>> after the r->queue clearing and before the consumer_head moving
>>>> forward.
>>>>
>>>> Move the r->queue[] clearing after consumer_head moving forward
>>>> to make __ptr_ring_empty() checking more reliable.
>>>>
>>>> As a side effect of above change, a consumer_head checking is
>>>> avoided for the likely case, and it has noticeable performance
>>>> improvement when it is tested using the ptr_ring_test selftest
>>>> added in the previous patch.
>>>>
>>>> Using "taskset -c 1 ./ptr_ring_test -s 1000 -m 0 -N 100000000"
>>>> to test the case of single thread doing both the enqueuing and
>>>> dequeuing:
>>>>
>>>> arch unpatched patched delta
>>>> arm64 4648 ms 4464 ms +3.9%
>>>> X86 2562 ms 2401 ms +6.2%
>>>>
>>>> Using "taskset -c 1-2 ./ptr_ring_test -s 1000 -m 1 -N 100000000"
>>>> to test the case of one thread doing enqueuing and another thread
>>>> doing dequeuing concurrently, also known as single-producer/single-
>>>> consumer:
>>>>
>>>> arch unpatched patched delta
>>>> arm64 3624 ms + 3624 ms 3462 ms + 3462 ms +4.4%
>>>> x86 2758 ms + 2758 ms 2547 ms + 2547 ms +7.6%
>>>
>>> Nice but it's small - could be a fluke.
>>> How many tests did you run? What is the variance?
>>> Did you try pinning to different CPUs to observe numa effects?
>>> Please use perf or some other modern tool for this kind
>>> of benchmark. Thanks!
>>
>> The result is quite stable, and retest using perf stat:
>
> How stable exactly? Try with -r so we can find out.

Retest with "perf stat -r":

For unpatched one:
Performance counter stats for './ptr_ring_test -s 1000 -m 1 -N 100000000' (100 runs):

6780.97 msec task-clock # 2.000 CPUs utilized ( +- 5.36% )
73 context-switches # 0.011 K/sec ( +- 5.07% )
0 cpu-migrations # 0.000 K/sec ( +-100.00% )
81 page-faults # 0.012 K/sec ( +- 0.76% )
17629544748 cycles # 2.600 GHz ( +- 5.36% )
25496488950 instructions # 1.45 insn per cycle ( +- 0.26% )
<not supported> branches
11489031 branch-misses ( +- 1.69% )

3.391 +- 0.182 seconds time elapsed ( +- 5.35% )

For patched one:
Performance counter stats for './ptr_ring_test_opt -s 1000 -m 1 -N 100000000' (100 runs):

6567.83 msec task-clock # 2.000 CPUs utilized ( +- 5.53% )
71 context-switches # 0.011 K/sec ( +- 5.26% )
0 cpu-migrations # 0.000 K/sec
82 page-faults # 0.012 K/sec ( +- 0.85% )
17075489298 cycles # 2.600 GHz ( +- 5.53% )
23861051578 instructions # 1.40 insn per cycle ( +- 0.07% )
<not supported> branches
10473776 branch-misses ( +- 0.60% )

3.284 +- 0.182 seconds time elapsed ( +- 5.53% )


The result is more stable when using taskset to limit the running cpu, but I suppose
the above data is stable enough to justify the performance improvement?










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