How dtrace have no impact on performance when probes are not in use.

dtrace probes can be used in any "C" applications. Slowly lots of server softwares on solaris are integrating dtrace probes in to their core functionality. Initial fear to a new user is that it may have a impact on performance. Dtrace replaces probes with "nop" instructions when not in use. When users typically uses these probes then it have some impact on performance. I wrote a simple test application to see what happens during runtime of a process. (testapp.c)

#include <stdio.h>
#include <unistd.h>
#include <sys/sdt.h>

void func()
{
    DTRACE_PROBE(firsttestapp, query__func);
}
int main()
{
    int i = 0;
    for(i = 0; i < 100; ++i)
    {
        func();
        sleep(2);
        printf("returned from func call\n");
        fflush(stdout);
    }
}
And a small provider (mytestapp.d):
provider firsttestapp {
probe query__func();
};

In this test application func will be called every 2 seconds. func has a dtrace
probe which doesn't do anything. I compiled my dtrace program using following
steps :
$ cc -c -xarch=generic -g testapp.c
$ dtrace -G -32 -s mytestapp.d testapp.o
$ cc -xarch=generic -g mytestapp.o testapp.o -o testapp

$ dbx ./testapp
(dbx) dis func
0x00011410: func       :        save     %sp, -96, %sp
0x00011414: func+0x0004:        nop
0x00011418: func+0x0008:        nop
0x0001141c: func+0x000c:        ret

Notice that function "func" doesn't have any instruction. It has two nop
instructions. Let us run the application and run a dtrace program from other
terminal which uses this probe and see what happens to the application during
runtime.
bash-3.00# dtrace -l | grep testapp
61026 firsttestapp26498           testapp                              func query-func
# dtrace -n 'firsttestapp*::func:query-func { @[probefunc] = count(); }'
dtrace: description 'firsttestapp*::func:query-func ' matched 1 probe

(dbx) dis func
0x00011410: func       :        save     %sp, -96, %sp
0x00011414: func+0x0004:        nop
0x00011418: func+0x0008:        ta       %icc,0x00000038
0x0001141c: func+0x000c:        ret
0x00011420: func+0x0010:        restore

Notice that 1 of the nop is now replaced with trap instruction "ta %icc,". So
when probes are used "nop" instruction is replaced with a trap instruction.
When probes are not in use, it is simply a nop instruction. This is the reason
that probes have no runtime impact on performance of applications when they are
not in use.




                    
        

            Posted at 
               05:51PM Jul 30, 2008
            by Basant Kukreja in technical  | 
                                        Comments[2]