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Subject: Re: [PATCH 3/5] perf core: Prepare writing into ring buffer from end
From: pi3orama <pi3orama@163.com>
In-Reply-To: <20160327152005.GU6356@twins.programming.kicks-ass.net>
Date: Sun, 27 Mar 2016 23:30:19 +0800
Cc: "Wangnan (F)" <wangnan0@huawei.com>, mingo@redhat.com,
        linux-kernel@vger.kernel.org, He Kuang <hekuang@huawei.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Arnaldo Carvalho de Melo <acme@redhat.com>,
        Brendan Gregg <brendan.d.gregg@gmail.com>,
        Jiri Olsa <jolsa@kernel.org>,
        Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
        Namhyung Kim <namhyung@kernel.org>, Zefan Li <lizefan@huawei.com>
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References: <1457949585-191064-1-git-send-email-wangnan0@huawei.com> <1457949585-191064-4-git-send-email-wangnan0@huawei.com> <20160323095007.GW6344@twins.programming.kicks-ass.net> <56F52E83.70409@huawei.com> <56F530C1.9010106@huawei.com> <56F547CC.5000408@huawei.com> <20160327152005.GU6356@twins.programming.kicks-ass.net>
To: Peter Zijlstra <peterz@infradead.org>
Sender: linux-kernel-owner@vger.kernel.org
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> ?? 2016??3??27?գ?????11:20??Peter Zijlstra <peterz@infradead.org> д????
> 
> On Fri, Mar 25, 2016 at 10:14:36PM +0800, Wangnan (F) wrote:
>>>> I think you enabled some unusual config options?
> 
> x86_64-defconfig
> 
>>> You must enabled CONFIG_OPTIMIZE_INLINING. Now I get similar result:
> 
> It has that indeed.
> 
>> After enabling CONFIG_OPTIMIZE_INLINING:
>> 
>> Test its performance by calling 'close(-1)' for 3000000 times and
>> use 'perf record -o /dev/null -e raw_syscalls:* test-ring-buffer' to
>> capture system calls:
>> 
>>                  MEAN         STDVAR
>> BASE            800077.1     23448.13
>> RAWPERF.PRE    2465858.0    603473.70
>> RAWPERF.POST   2471925.0    609437.60
>> 
>> Considering the high stdvar, after applying this patch the performance
>> is not change.
> 
> Why is your variance so immense? And doesn't that render the
> measurements pointless?
> 

For some unknown reason, about
10% of these results raises 2 times of normal
results. Say, "normal results" are about
2200000, but those "outliers" are about
4400000 (I can't access raw data now).
Variance becomes much smaller if I remove
those outliers.

I guess the outliers is caused by some type
of lock stepping? No clue about it.

Thank you.