RFR: 8011882: Replace spin loops as back off when suspending

[Rickard Bäckman]

David,

thanks for the input, I'll go back to the split versions and update the timing. 

/R

On Apr 16, 2013, at 1:27 AM, David Holmes wrote:

> PS. Also see the existing unpackTime and compute_abstime helper functions for dealing with pthread/POSIX absolute timed-waits. Better than using javaTimeMillis()
> 
> David
> 
> On 15/04/2013 10:50 PM, David Holmes wrote:
>> On 15/04/2013 10:07 PM, Rickard Bäckman wrote:
>>> David,
>>> 
>>> this is what the suggested semaphore.cpp/semaphore.hpp. Is that what
>>> you are looking for?
>> 
>> <sigh> I thought so till I saw it - far uglier and complicated than I
>> had hoped. Sadly the three separate versions wins for me.
>> 
>> By the way you can't do this:
>> 
>>  116 bool Semaphore::timedwait(unsigned int sec, int nsec) {
>>  117   struct timespec ts;
>>  118   jlong endtime = os::javaTimeNanos() + (sec * NANOSECS_PER_SEC) +
>> nsec;
>>  119   ts.tv_sec = endtime / NANOSECS_PER_SEC;
>>  120   ts.tv_nsec = endtime % NANOSECS_PER_SEC;
>> 
>> javaTimeNanos is not wall-clock time, but the POSIX sem_timewait
>> requires an absolute time - you need to use javaTimeMillis(). Which also
>> means the wait will be affected by changes to wall-clock time.
>> 
>> David
>> -----
>> 
>>> Webrev: http://cr.openjdk.java.net/~rbackman/webrev/
>>> 
>>> Thanks
>>> /R
>>> 
>>> On Apr 15, 2013, at 8:59 AM, David Holmes wrote:
>>> 
>>>> On 15/04/2013 4:55 PM, Rickard Bäckman wrote:
>>>>> David,
>>>>> 
>>>>> any new thoughts?
>>>> 
>>>> Not a new one but I think factoring into Semaphore.hpp/cpp and using
>>>> a few ifdefs is better than three versions of the Semaphore class.
>>>> The signal thread could use it also.
>>>> 
>>>> David
>>>> 
>>>>> Thanks
>>>>> /R
>>>>> 
>>>>> On Apr 12, 2013, at 8:06 AM, Rickard Bäckman wrote:
>>>>> 
>>>>>> 
>>>>>> On Apr 12, 2013, at 7:34 AM, David Holmes wrote:
>>>>>> 
>>>>>>> On 12/04/2013 3:01 PM, Rickard Bäckman wrote:
>>>>>>>> 
>>>>>>>> On Apr 12, 2013, at 1:04 AM, David Holmes wrote:
>>>>>>>> 
>>>>>>>>> On 11/04/2013 11:02 PM, Rickard Bäckman wrote:
>>>>>>>>>> On Apr 11, 2013, at 2:39 PM, David Holmes wrote:
>>>>>>>>>>> So what did you mean about pthread_semaphore (what is that
>>>>>>>>>>> anyway?) ??
>>>>>>>>>> 
>>>>>>>>>> Never mind, pthread condition variables.
>>>>>>>>> 
>>>>>>>>> Ah I see.
>>>>>>>>> 
>>>>>>>>>>> 
>>>>>>>>>>> I really, really, really don't like seeing three versions of
>>>>>>>>>>> this class :( Can't BSD and Linux at least share a POSIX
>>>>>>>>>>> version? (And I wonder if we can actually mix-n-match UI
>>>>>>>>>>> threads on Solaris with POSIX semaphores on Solaris?)
>>>>>>>>>> 
>>>>>>>>>> I don't like it either, our OS code isn't really helpful when
>>>>>>>>>> it comes do reusing things :) Not sure how I would layout
>>>>>>>>>> things to make them only available on BSD (Not Mac) and Linux.
>>>>>>>>>> I guess os_posix.hpp with lots of #ifdefs, but I'm not sure I"m
>>>>>>>>>> feeling that happy about that.
>>>>>>>>> 
>>>>>>>>> Why would the os_posix version need a lot of ifdefs?
>>>>>>>> 
>>>>>>>> Well, I guess we would need:
>>>>>>>> 
>>>>>>>> (in ifdef pseudo language)
>>>>>>>> 
>>>>>>>> #ifdef (LINUX || (BSD && !APPLE))
>>>>>>>> …
>>>>>>>> #endif
>>>>>>> 
>>>>>>> But if it isn't "posix" then we won't be building os_posix - right?
>>>>>> 
>>>>>> Linux, Solaris, Bsd & Mac builds and include os_posix. They are all
>>>>>> "implementing posix" we are just not using the same semaphore
>>>>>> implementation on all of them.
>>>>>> 
>>>>>>> 
>>>>>>>> The second interesting problem this will get us into is that
>>>>>>>> sem_t is not declared in this context. Where do we put the
>>>>>>>> #include <semaphore.h>? Impossible in os_posix.hpp since it is
>>>>>>>> included in the middle of a class definition. I could put it in
>>>>>>>> os.hpp in the #ifdef path that does the jvm_platform.h includes,
>>>>>>>> not sure if that is very pretty either.
>>>>>>> 
>>>>>>> Semaphores are already used by the signal handler thread -
>>>>>>> semaphore.h is included in os_linux.cpp etc, so why would os_posix
>>>>>>> be any different ?
>>>>>>> 
>>>>>>> But couldn't we just have a Semaphore.h/cpp with any needed ifdefs?
>>>>>>> 
>>>>>>>>> Do we really have four versions:
>>>>>>>>> - linux (posix)
>>>>>>>>> - BSD (posix)
>>>>>>>>> - Solaris
>>>>>>>>> - Mac (different to BSD?)
>>>>>>>>> 
>>>>>>>> 
>>>>>>>> 3:
>>>>>>>> 1) linux & bsd uses the sem_ interface
>>>>>>>> 2) solaris uses the sema_ interface
>>>>>>>> 3) mac uses the semaphore_ interface
>>>>>>> 
>>>>>>> Okay but if mac is BSD why can't we use bsd ie posix interface
>>>>>>> instead of the mach semaphore_ ?
>>>>>> 
>>>>>> Because apple decided not to implement sem_timedwait.
>>>>>> On Solaris we use sema_ because sem_ requires us to link with -lrt
>>>>>> which we currently don't (and I'm not really feeling like adding it)
>>>>>> 
>>>>>>> 
>>>>>>> 
>>>>>>> BTW I like the idea of using the semaphore, we're just haggling on
>>>>>>> the details. ;-)
>>>>>> 
>>>>>> I'm fine with that :)
>>>>>> 
>>>>>> /R
>>>>>> 
>>>>>>> 
>>>>>>> Thanks,
>>>>>>> David
>>>>>>> 
>>>>>>>> /R
>>>>>>>> 
>>>>>>>>> ??
>>>>>>>>> 
>>>>>>>>> David
>>>>>>>>> -----
>>>>>>>> 
>>>>>> 
>>>>> 
>>>