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

[Alexei Starovoitov]

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?
nice. I guess it makes snese that overwrite is faster.
I guess than moving the header to the end will have the same
performance in this benchmark, since RAWOVWRT is the same as well.