Re: [PATCH 0/6] perf core: Read from overwrite ring buffer

["Wangnan (F)" <wangnan0@huawei.com>]

On 2016/1/22 11:21, Alexei Starovoitov wrote:
> On Fri, Jan 22, 2016 at 10:21:19AM +0800, Wangnan (F) wrote:
>>
>> On 2016/1/21 14:51, Wangnan (F) wrote:
>>>
>>> On 2016/1/20 10:20, Alexei Starovoitov wrote:
>>>> On Wed, Jan 20, 2016 at 09:37:42AM +0800, Wangnan (F) wrote:
>>>>> On 2016/1/20 1:42, Alexei Starovoitov wrote:
>>>>>> On Tue, Jan 19, 2016 at 11:16:44AM +0000, Wang Nan wrote:
>>>>>>> This patchset introduces two methods to support reading from
>>>>>>> overwrite.
>>>>>>>
>>>>>>>   1) Tailsize: write the size of an event at the end of it
>>>>>>>   2) Backward writing: write the ring buffer from the end of it to
>>>>>>> the
>>>>>>>      beginning.
>>>>>> what happend with your other idea of moving the whole header to the
>>>>>> end?
>>>>>> That felt better than either of these options.
>>>>> I'll try it today. However, putting all of the three together is
>>>>> not as easy as this patchset.
>>>> I'm missing something. Why all three in one set?
>>> Can't implement all three in one, but implement two of them make
>>> benchmarking simpler :)
>>>
>>> Here comes some numbers.
>>>
>>> I attach a target program at the end of this mail. It calls
>>> close(-1) for 3000000 times, and use gettimeofday to check
>>> how many us it takes.
>>>
>>> Following cases are tested:
>>>
>>>
>>> BASE    : ./a.out
>>> RAWPERF : ./perf record -o /dev/null -e raw_syscalls:* ./a.out
>>> WRTBKWRD: ./perf record -o /dev/null -e raw_syscalls:* ./a.out
>>> TAILSIZE: ./perf record --no-has-write-backward -o /dev/null -e
>>> raw_syscalls:*/overwrite/ ./a.out
>>> RAWOVWRT: ./perf record --no-has-write-backward --no-has-tailsize -o
>>> /dev/null -e raw_syscalls:*/overwrite/ ./a.out
>>>
>>> With this script:
>>>
>>> func() {
>>>         for x in `seq 1 100` ; do $1; done | tee data_$2
>>> }
>>>
>>> func ./a.out base
>>> func "./perf record -o /dev/null -e raw_syscalls:* ./a.out" rawperf
>>> func "./perf record -o /dev/null -e raw_syscalls:*/overwrite/ ./a.out"
>>> wrtbkwrd
>>> func "./perf record -o /dev/null --no-has-write-backward -e
>>> raw_syscalls:*/overwrite/ ./a.out" tailsize
>>> func "./perf record -o /dev/null --no-has-write-backward --no-has-tailsize
>>> -o /dev/null -e raw_syscalls:*/overwrite/ ./a.out" rawovwrt
>>>
>>> Result:
>>>
>>>             MEAN           STDVAR
>>> BASE    :  879870.81      11913.13
>>> RAWPERF : 2603854.7      706658.4
>>> WRTBKWRD: 2313301.220      6727.957
>>> TAILSIZE: 2383051.860      5248.061
>>> RAWOVWRT: 2315273.180      5221.025
>> Add a number: I tested original perf overwrite ring buffer in pure v4.4
>> on the same machine:
>>
>>                      MEAN          STDVAR
>> RAWOVWRT(original): 2323970.45    5103.39
>>
>> So I think backward writing method doesn't add extra overhead into
>> fastpath.
>>
>> I will send this patchset again with several bugs fixed. After that
>> I'll start working on tail-header if it is still required.
> interesting.
> did I read the numbers correctly that 'write backwards' method
> is actually the fastest? even faster than no-overwrite?

Yes. But notice STDVAR, we can't say 'WRTBKWRD' outperform 'RAWOVWRT'. 
However,
at least 'WRTBKWRD' should be as fast as 'RAWOVWRT'.

> nice. I guess it makes snese that overwrite is faster.

In no-overwrite case perf itself wakes up many times to collect data,
I guess it is the source of high stdvar.

> I guess than moving the header to the end will have the same
> performance in this benchmark, since RAWOVWRT is the same as well.
>
Yes.

Do you want to test it by yourself? The code is ready.

Thank you.