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

[Wangnan (F)]

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

So it seems backward writing methods is good enough. We don't need to 
consider
tailsize method.

Code for this benchmark can be found from:

https://git.kernel.org/cgit/linux/kernel/git/pi3orama/linux.git/ 
perf/overwrite-benchmark

Thank you.

-------- Test program ----------
#include <unistd.h>
#include <fcntl.h>
#include <sys/time.h>
#include <stdio.h>

int main()
{
        int i;
        struct timeval tv1, tv2;
        long long us1, us2;

        gettimeofday(&tv1, NULL);
        for (i = 0; i < 1000 * 1000 * 3; i++) {
                close(-1);
        }
        gettimeofday(&tv2, NULL);

        us1 = tv1.tv_sec * 1000000 + tv1.tv_usec;
        us2 = tv2.tv_sec * 1000000 + tv2.tv_usec;
        printf("%ld\n", us2 - us1);

        return 0;
}