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

[Wangnan (F)]

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.

Thank you.