RFR 8194085: Obsolete the deprecated SafepointSynchronize flags and remove related code

[Daniel D. Daugherty]

On 2/21/18 11:49 PM, David Holmes wrote:
> Hi Harold,
>
> On 22/02/2018 4:31 AM, harold seigel wrote:
>> Hi,
>>
>> Please review this JDK-11 change to obsolete the 
>> SafepointSpinBeforeYield, DeferThrSuspendLoopCount, and 
>> DeferPollingPageLoopCount options and related code.  With this 
>> change, these options are still accepted on the command line but have 
>> no affect other than to generate these warning messages:
>>
>>     Java HotSpot(TM) 64-Bit Server VM warning: Ignoring option
>>     SafepointSpinBeforeYield; support was removed in 11.0
>>     Java HotSpot(TM) 64-Bit Server VM warning: Ignoring option
>>     DeferThrSuspendLoopCount; support was removed in 11.0
>>     Java HotSpot(TM) 64-Bit Server VM warning: Ignoring option
>>     DeferPollingPageLoopCount; support was removed in 11.0
>
> Just for the record these were once useful tuning values for 
> investigating safepoint performance issues in the field.
>
>> Open Webrev: 
>> http://cr.openjdk.java.net/~hseigel/bug_8194085/webrev/index.html
>
> This one is a bit messier than I had envisaged: -Xconcurrentio ?? Wow! 
> :) Definitely overlooked that one. It should be deprecated in 11 for 
> certain.
>
> I have one problem with a change in safepoint.cpp, the removal of this:
>
>  312           if (SafepointMechanism::uses_global_page_poll() && 
> int(iterations) == DeferPollingPageLoopCount) {
>  313             guarantee (PageArmed == 0, "invariant") ;
>  314             PageArmed = 1 ;
>  315             os::make_polling_page_unreadable();
>  316           }
>
> This block triggers a guarantee failure if we fail to reach a 
> safepoint and the number of iterations overflows to reach -1 (the 
> value of DeferPollingPageLoopCount). That is the only thing that stops 
> the VM from just hanging when we have an issue reaching safepoints 
> (like all of the "counted loop" problems!) and is very valuable in 
> clearly identifying the problem. So I strongly recommend that this is 
> restored with an explicit check against -1.

Hmmmm... I guess I'm not seeing why the changes here don't
solve that problem:

>   190     if (SafepointMechanism::uses_global_page_poll()) {
>   191       // Make interpreter safepoint aware
>   192       Interpreter::notice_safepoints();
>   193
> 194 if (DeferPollingPageLoopCount < 0) {
>   195         // Make polling safepoint aware
>   196         guarantee (PageArmed == 0, "invariant") ;
>   197         PageArmed = 1 ;
>   198         os::make_polling_page_unreadable();
>   199       }
> 200 }

So L194 and L200 are deleted and the same block is always
run when SafepointMechanism::uses_global_page_poll() is true
and this happens before the "L221: while(still_running > 0) {"

Did I miss something?

Dan


>
> ---
>
> src/hotspot/share/runtime/safepoint.hpp
>
> +   static void set_throughput_mode() {
> +     _defer_thr_suspend_loop_count = 1;
> +   }
>
> First can you add a comment before this that it is only used for 
> -Xconcurrentio support.
>
> Second, the name seems odd - throughput mode? How about 
> set_concurrentio_mode() ? Or even just 
> set_defer_thr_suspend_loop_count() ?
>
> Thanks,
> David
> -----
>
>> JBS Bug: https://bugs.openjdk.java.net/browse/JDK-8194085
>>
>> The change was tested with Mach5 tiers 1 and 2 on all Mach5 platforms 
>> and tiers 3-5 in Linux-X64.
>>
>> Thanks, Harold