[Click] schedule tasks [ns-3 + click]
Sascha Alexander Jopen
jopen at informatik.uni-bonn.de
Wed Dec 21 05:50:27 EST 2011
Hey Eddie,
i didnt' test you changes yet, but after looking into the code i think
you accidently removed the newly introduced artificial time increase in
lib/routerthread.cc line 683. I think this should read
struct timeval nexttime = (Timestamp::now() +
Timestamp::make_usec(1)).timeval_ceil();
Regards,
Sascha
Am 20.12.2011 17:31, schrieb Eddie Kohler:
> Hi Sascha,
>
> I applied a different version of your patch, using a new function
> (Timestamp::timeval_ceil()) written for the purpose. This also
> slightly changed the active() case in ns3 scheduling. Take a look;
> does it work for you?
>
> Best,
> Eddie
>
>
> On Fri, Dec 16, 2011 at 5:40 PM, Sascha Alexander Jopen
> <jopen at informatik.uni-bonn.de> wrote:
>> Hey Eddie,
>>
>> i think my patch is still necessary.
>> The one integrated into ns3 is about rounding errors between double
>> and integer time representations, which led to similar endless loops
>> but on ns3 side.
>>
>> Regards,
>> Sascha
>>
>> Am 16.12.2011 15:20, schrieb Eddie Kohler:
>>> Hi Björn, Sascha,
>>>
>>> So, I'm a bit behind. Should I apply Sascha's patch, or some other
>>> version?
>>>
>>> Best, Eddie
>>>
>>>
>>> On 12/04/2011 10:25 AM, Björn Lichtblau wrote:
>>>> Hi, this flaw was already fixed on ns3 side (in ns3-dev):
>>>> http://code.nsnam.org/ns-3-dev/rev/0d04b625ea54
>>>>
>>>> Regarding Eddies change, it works but i had no time yet to test
>>>> it extensively. While it works i think it's a little work around
>>>> to let the simulator do artificial 1 us steps... however it
>>>> should not hurt in most cases and doing it cleaner my be hard..
>>>> i'll comment more detailed soon.
>>>>
>>>> Regards, Björn
>>>>
>>>> On 03.12.2011 23:42, Sascha Alexander Jopen wrote:
>>>>> Hey,
>>>>>
>>>>> after running some real simulations i found that scheduling
>>>>> timers may lead to infinite loops with nsclicks scheduling. On
>>>>> 64bit systems the nanosecond precision of a timestamp is mapped
>>>>> to a truncated microsecond precision timeval. The next ns event
>>>>> is scheduled at this microsecond timeval, but clicks
>>>>> run_timers() method doesn't run the timer, because its expiry
>>>>> is not reached. nsclick is then again scheduled for this timer,
>>>>> again some nanoseconds to early.
>>>>>
>>>>> The attached patch fixes this problem by scheduling nsclick to
>>>>> the next microsecond after the timer expires.
>>>>>
>>>>> Regards, Sascha
>>>>>
>>>>> On 11/09/11 18:56, Eddie Kohler wrote:
>>>>>> Sascha, Björn,
>>>>>>
>>>>>> Thanks for your patience with this problem. I took a look at
>>>>>> the patch, but decided to do it a different way that is less
>>>>>> intrusive and hopefully addresses all the cases. The checkin
>>>>>> is here:
>>>>>>
>>>>>> https://github.com/kohler/click/commit/e3c9f295ea1d5c5700e26f19afce873b0ce755f5
>>>>>>
>>>>>>
>>>>>>
>> Please let me know if this does not work (I have not tested it).
>>>>>>
>>>>>> Best, Eddie
>>>>>>
>>>>>>
>>>>>> On Fri, Nov 4, 2011 at 3:05 PM, Sascha Alexander Jopen
>>>>>> <jopen at informatik.uni-bonn.de> wrote:
>>>>>>> Hi,
>>>>>>>
>>>>>>> after rereading the stated problems, i found that my patch
>>>>>>> did not catch the case when a timer schedules a task. The
>>>>>>> attached patch should fix this. Both test scenarios from
>>>>>>> Björn seem to run as expected with this patch.
>>>>>>>
>>>>>>> However, i'm still not sure, how much time should pass
>>>>>>> between two driver runs.
>>>>>>>
>>>>>>> Regards, Sascha
>>>>>>>
>>>>>>>
>>>>>>> On 11/04/11 17:28, Sascha Alexander Jopen wrote:
>>>>>>>> Hey,
>>>>>>>>
>>>>>>>> i use a slightly different fix. After running the tasks,
>>>>>>>> i check if there is still at least one task scheduled. If
>>>>>>>> this is the case, i reschedule the router thread for
>>>>>>>> execution in ns-3 again with the smallest possible time
>>>>>>>> offset, which is one microsecond. This way it doesn't
>>>>>>>> matter if there are other timers to be executed or not.
>>>>>>>> For elements which do polling using tasks all the time,
>>>>>>>> this means that simulation time advances only one
>>>>>>>> microsecond per iteration which could lead to really long
>>>>>>>> running simulations. Using such elements is possible this
>>>>>>>> way, however.
>>>>>>>>
>>>>>>>> Regards, Sascha
>>>>>>>>
>>>>>>>> On 11/04/11 12:15, Björn Lichtblau wrote:
>>>>>>>>> Hi,
>>>>>>>>>
>>>>>>>>> i think you are experiencing what i described as
>>>>>>>>> problem A in
>>>>>>>>> http://pdos.csail.mit.edu/pipermail/click/2011-October/010357.html
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>> Tasks getting scheduled by a fired timer are not run immediatly, because
>>>>>>>>> run_timers() is behind run_tasks() and the
>>>>>>>>> routerthread-driver() loop is only run once at each
>>>>>>>>> call from the simulation. The fix i described there
>>>>>>>>> however was not correct, currently i'm working with
>>>>>>>>> this (but maybe incorrect too):
>>>>>>>>>
>>>>>>>>> diff --git a/lib/routerthread.cc b/lib/routerthread.cc
>>>>>>>>> index d118a91..7232b79 100644 ---
>>>>>>>>> a/lib/routerthread.cc +++ b/lib/routerthread.cc @@
>>>>>>>>> -640,14 +640,7 @@ RouterThread::driver() _oticks =
>>>>>>>>> ticks; #endif timer_set().run_timers(this, _master);
>>>>>>>>> -#if CLICK_NS - // If there's another timer,
>>>>>>>>> tell the simulator to make us - // run when
>>>>>>>>> it's due to go off. - if (Timestamp
>>>>>>>>> next_expiry = timer_set().timer_expiry_steady()) { -
>>>>>>>>> struct timeval nexttime = next_expiry.timeval(); -
>>>>>>>>> simclick_sim_command(_master->simnode(),
>>>>>>>>> SIMCLICK_SCHEDULE,&nexttime); - } -#endif + }
>>>>>>>>> while (0);
>>>>>>>>>
>>>>>>>>> // run operating system @@ -667,7 +660,20 @@
>>>>>>>>> RouterThread::driver() #if CLICK_NS || BSD_NETISRSCHED
>>>>>>>>> // Everyone except the NS driver stays in driver()
>>>>>>>>> until the driver is // stopped. - break; +
>>>>>>>>> if(task_begin() == task_end()){ +#if CLICK_NS +
>>>>>>>>> // If there's another timer, tell the simulator to make
>>>>>>>>> us + // run when it's due to go off. +
>>>>>>>>> if (Timestamp next_expiry =
>>>>>>>>> timer_set().timer_expiry_steady()) { +
>>>>>>>>> struct timeval nexttime = next_expiry.timeval(); +
>>>>>>>>> simclick_sim_command(_master->simnode(),
>>>>>>>>> SIMCLICK_SCHEDULE,&nexttime); + } +#endif +
>>>>>>>>> break; + + } #endif }
>>>>>>>>>
>>>>>>>>> So the driver loop only exits on task_begin() ==
>>>>>>>>> task_end() (means it will loop again if a timer
>>>>>>>>> scheduled a task), and we only tell the simulator to
>>>>>>>>> schedule a timer when the loop is exiting. While this
>>>>>>>>> is fine for me a colleague working with ns2 still has
>>>>>>>>> problems with ToSimDevice in some cases which is too
>>>>>>>>> much to describe now.
>>>>>>>>>
>>>>>>>>> Another potential problem i stumbled over in the
>>>>>>>>> documentation
>>>>>>>>> (http://www.read.cs.ucla.edu/click/doxygen/classTimer.html)
>>>>>>>>> is busy waiting: "Particularly at user level, there can
>>>>>>>>> be a significant delay between a Timer's nominal
>>>>>>>>> expiration time and the actual time it runs. Elements
>>>>>>>>> that desire extremely precise timings should combine a
>>>>>>>>> Timer with a Task. The Timer is set to go off a bit
>>>>>>>>> before the true expiration time (see
>>>>>>>>> Timer::adjustment()), after which the Task polls the
>>>>>>>>> CPU until the actual expiration time arrives."
>>>>>>>>>
>>>>>>>>> Elements doing busy waiting are a big headache in the
>>>>>>>>> sim environment, and no nice idea yet to fix such
>>>>>>>>> cases, because time will never advance without setting
>>>>>>>>> a timer...
>>>>>>>>>
>>>>>>>>> Regards, Björn
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> On 11/04/2011 11:29 AM, Giovanni Di Stasi wrote:
>>>>>>>>>> Hi everyone,
>>>>>>>>>>
>>>>>>>>>> I have developed an element which behaves like a
>>>>>>>>>> Queue. It has a pull output which is connected to a
>>>>>>>>>> ToSimDevice.
>>>>>>>>>>
>>>>>>>>>> Sometimes, when the pull gets called on the element,
>>>>>>>>>> it returns a NULL and sets a timer which expires
>>>>>>>>>> after a few milliseconds (e.g. 3). When the timer
>>>>>>>>>> expires, an empty_notifier is "activated"
>>>>>>>>>> (empty_not.wake()). I would expect, at this point,
>>>>>>>>>> the ToSimDevice task to be run, and the pull to be
>>>>>>>>>> called on my element.
>>>>>>>>>>
>>>>>>>>>> Unfortunately this does not happen. The Task schedule
>>>>>>>>>> function seems to be called after the notifier is
>>>>>>>>>> waken up, but the task is not run. Is this normal?
>>>>>>>>>> The task is sometimes run a few seconds later. So I
>>>>>>>>>> have two doubts: is it possible to schedule a task to
>>>>>>>>>> be run right away (maybe be putting it at the top of
>>>>>>>>>> the Click pending tasks)? If not, which delay should
>>>>>>>>>> I expect from the moment I schedule the Task and the
>>>>>>>>>> moment it gets executed?
>>>>>>>>>>
>>>>>>>>>> Thanks, Giovanni
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>
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