Re: Query regarding 2.6.335 RT[Ingo's] and Non-RT performance

Re: Query regarding 2.6.335 RT[Ingo's] and Non-RT performance

[Manikandan Ramachandran]

> ------------------------------------------------------
> > Date: Thu, 12 Aug 2010 13:53:51 -0400
> > From: Jeff Angielski <jeff@theptrgroup.com>
> > To: linuxppc-dev@lists.ozlabs.org
> > Subject: Re: Query regarding 2.6.335 RT[Ingo's] and Non-RT performance
> > Message-ID: <4C64352F.4090005@theptrgroup.com>
> > Content-Type: text/plain; charset=3DISO-8859-1; format=3Dflowed
> >
> > On 08/11/2010 06:18 PM, Manikandan Ramachandran wrote:
> > > Hello All,
> > > =A0 =A0 =A0I created a very simple program which has higher priority =
than
> > > normal tasks and runs a tight loop. Under same test environment I ran
> > > this program on both non-rt and rt 2.6.33.5 kernel. =A0To my suprise =
I see
> > > that performance of non-RT kernel is better than RT. non-RT kernel to=
ok
> > > 3 sec and 366156 usec while RT kernel took about 3 sec and 418011
> > > usec.Can someone please explain why the performance of non-rt kernel =
is
> > > better than rt kernel? From the face of the test result, I feel RT ha=
s
> > > more overhead,Is there any configuration that I could do to bring dow=
n
> > > the overhead?
> >
> > Your "surprise" is due to your definition of "performance".
> >
> > The purpose of the -rt kernels is to reduce the kernel latency. =A0This=
 is
> > important for servicing hardware. =A0Normal users find the -rt useful f=
or
> > audio/video applications. =A0Engineering and scientific users find the =
-rt
> > beneficially for servicing hardware like sensors or control systems.
> >
> > If you are just trying to run calculations as fast as you can in user
> > space, you'd be better off using the non-rt variants.
> >
> >
> > --
> > Jeff Angielski
> > The PTR Group
> > www.theptrgroup.com



 Thanks for your response.

On one hand I hear that RT-kernel is meant for reducing kernel latency on
other hand I see that there is RT-kernel overhead. So what really RT-kernel
brings to system performance?

Actually I see that latency for higher priority is more or less same for
non-rt system.

One more thing, since irqs being threaded in RT, and with CFS scheduler in
2.6.33, wouldn't we bring down system performance as CFS is O(log(n)) =A0in
nature?
 --
 Thanks,
 Manik

Query regarding 2.6.335 RT[Ingo's] and Non-RT performance

[Manikandan Ramachandran]

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Hello All,

    I created a very simple program which has higher priority than normal
tasks and runs a tight loop. Under same test environment I ran this
program on both non-rt and rt 2.6.33.5 kernel.  To my suprise I see that
performance of non-RT kernel is better than RT. non-RT kernel took 3 sec and
366156 usec while RT kernel took about 3 sec and 418011 usec.Can someone
please explain why the performance of non-rt kernel is better than rt
kernel? From the face of the test result, I feel RT has more overhead,Is
there any configuration that I could do to bring down the overhead?

Processor:
----------------
processor       : 0
cpu             : 7448
clock           : 996.000000MHz
revision        : 2.2 (pvr 8004 0202)
bogomips        : 83.10
processor       : 1
cpu             : 7448
clock           : 996.000000MHz
revision        : 2.2 (pvr 8004 0202)
bogomips        : 83.10

CFS optimization:
--------------------------
# cat /proc/sys/kernel/sched_rt_runtime_us
1000000
# cat /proc/sys/kernel/sched_rt_period_us
1000000
# cat /proc/sys/kernel/sched_compat_yield
1

Test Program:
---------------------

main()
{

    int sched_rr_min,sched_rr_max;
    struct sched_param scheduling_parameters;
    struct timeval tv,late_tv;
    suseconds_t usec_diff,avg_usec = 0;
    time_t sec_diff, avg_sec = 0;
    int i;
    long count = 1;

    sched_rr_min = sched_get_priority_min(SCHED_RR);
    sched_rr_max = sched_get_priority_max(SCHED_RR);
    scheduling_parameters.sched_priority = sched_rr_min+4;
    sched_setscheduler(0, SCHED_RR, &scheduling_parameters);// Run the
process with the given priority


    for(i = 0 ; i < 150 ; i++) {
       gettimeofday(&tv, NULL);
       while(count > 0){
        //printf(".");
        count++;
       }
       gettimeofday(&late_tv, NULL);
       count = 1;
       sec_diff = (late_tv.tv_sec - tv.tv_sec);
       avg_sec += sec_diff;
       usec_diff = ( (late_tv.tv_usec > tv.tv_usec) ? (late_tv.tv_usec -
tv.tv_usec) : ( tv.tv_usec - late_tv.tv_usec));
       avg_usec += usec_diff;
       printf("Iteration #%d sec %x usec %x\n",i,(sec_diff),(usec_diff));
    }
       printf("Average of #%d sec %x usec %x\n",i,(avg_sec/i),(avg_usec)/i);
}

Partial Result of non-rt kernel:
-------------------------------------------

Iteration #140 sec 3 usec 3aef8
Iteration #141 sec 3 usec 3aefe
Iteration #142 sec 3 usec 3aee4
*Iteration #143 sec 4 usec b935b  [Why there is this periodic bump ??]
[Scheduler at work??]*
Iteration #144 sec 3 usec 3aef2
Iteration #145 sec 3 usec 3aef0
Iteration #146 sec 3 usec 3aef4
*Iteration #147 sec 4 usec b934b*
Iteration #148 sec 3 usec 3aeed
Iteration #149 sec 3 usec 3aef9

 Partial Result of rt kernel:
-------------------------------------------
Iteration #135 sec 3 usec 47328
*Iteration #136 sec 4 usec ac4fd
*Iteration #137 sec 3 usec 48b0b
Iteration #138 sec 3 usec 4738c
Iteration #139 sec 4 usec ac4d5
Iteration #140 sec 3 usec 483cb
Iteration #141 sec 3 usec 48500
*Iteration #142 sec 4 usec acc49
*Iteration #143 sec 3 usec 47c1f
Iteration #144 sec 3 usec 478c2
Iteration #145 sec 3 usec 47e48
Iteration #146 sec 4 usec ac9b5
Iteration #147 sec 3 usec 48de4
Iteration #148 sec 3 usec 46fbe
Iteration #149 sec 4 usec ac52e
Average of #150 sec 3 usec 660db

Thanks,
Mani


-- 
Thanks,
Manik

Think twice about a tree before you take a printout

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Re: Query regarding 2.6.335 RT[Ingo's] and Non-RT performance

[Jeff Angielski]

On 08/11/2010 06:18 PM, Manikandan Ramachandran wrote:
> Hello All,
>      I created a very simple program which has higher priority than
> normal tasks and runs a tight loop. Under same test environment I ran
> this program on both non-rt and rt 2.6.33.5 kernel.  To my suprise I see
> that performance of non-RT kernel is better than RT. non-RT kernel took
> 3 sec and 366156 usec while RT kernel took about 3 sec and 418011
> usec.Can someone please explain why the performance of non-rt kernel is
> better than rt kernel? From the face of the test result, I feel RT has
> more overhead,Is there any configuration that I could do to bring down
> the overhead?

Your "surprise" is due to your definition of "performance".

The purpose of the -rt kernels is to reduce the kernel latency.  This is 
important for servicing hardware.  Normal users find the -rt useful for 
audio/video applications.  Engineering and scientific users find the -rt 
beneficially for servicing hardware like sensors or control systems.

If you are just trying to run calculations as fast as you can in user 
space, you'd be better off using the non-rt variants.


-- 
Jeff Angielski
The PTR Group
www.theptrgroup.com

Re: Query regarding 2.6.335 RT[Ingo's] and Non-RT performance

[Xianghua Xiao]

On Thu, Aug 12, 2010 at 12:53 PM, Jeff Angielski <jeff@theptrgroup.com> wro=
te:
> On 08/11/2010 06:18 PM, Manikandan Ramachandran wrote:
>>
>> Hello All,
>> =C2=A0 =C2=A0 I created a very simple program which has higher priority =
than
>> normal tasks and runs a tight loop. Under same test environment I ran
>> this program on both non-rt and rt 2.6.33.5 kernel. =C2=A0To my suprise =
I see
>> that performance of non-RT kernel is better than RT. non-RT kernel took
>> 3 sec and 366156 usec while RT kernel took about 3 sec and 418011
>> usec.Can someone please explain why the performance of non-rt kernel is
>> better than rt kernel? From the face of the test result, I feel RT has
>> more overhead,Is there any configuration that I could do to bring down
>> the overhead?
>
> Your "surprise" is due to your definition of "performance".
>
> The purpose of the -rt kernels is to reduce the kernel latency. =C2=A0Thi=
s is
> important for servicing hardware. =C2=A0Normal users find the -rt useful =
for
> audio/video applications. =C2=A0Engineering and scientific users find the=
 -rt
> beneficially for servicing hardware like sensors or control systems.
>
> If you are just trying to run calculations as fast as you can in user spa=
ce,
> you'd be better off using the non-rt variants.
>
>
> --
> Jeff Angielski
> The PTR Group
> www.theptrgroup.com
> _______________________________________________
> Linuxppc-dev mailing list
> Linuxppc-dev@lists.ozlabs.org
> https://lists.ozlabs.org/listinfo/linuxppc-dev
>

true, in most cases non-rt will have better performance/throughput,
while rt's major goal is to have better latency for high priority
tasks. also true is that, rt kernel will have more overhead.

xianghua