RFR: 8143157: Convert TraceVMOperation to Unified Logging

[Marcus Larsson]

Unified logging really only supports single line messages. Each such 
message is either output in its entirety, or not output at all. Multiple 
log calls can be interleaved by other log calls and also be 
disabled/enabled in between. In this case, having a "start" and "end" 
message makes perfect sense, and it shouldn't be a problem to see only 
one of these messages if you're reconfiguring the logging in the middle 
of the operation.

Marcus

On 2015-11-19 17:09, Max Ockner wrote:
> We do not always need to shut off logging immediately if the logging 
> configuration changes at runtime. In this case we have two error 
> logging sections surrounding a doit() section which may also contain 
> logging. If we allow logging to shut off in the middle of this 
> operation, then we may end up with half of a message. In the very real 
> scenario where someone is parsing brackets in the output, I can see 
> this as an issue.
>
> On 11/19/2015 7:23 AM, Marcus Larsson wrote:
>> Hi,
>>
>> On 2015-11-18 23:06, David Holmes wrote:
>>> Hi Rachel,
>>>
>>> On 19/11/2015 4:41 AM, Rachel Protacio wrote:
>>>> Hi,
>>>>
>>>> On 11/18/2015 12:32 PM, Max Ockner wrote:
>>>>> I think the issue is that doit() happens regardless of what is being
>>>>> logged. I don't think we can reorder the logging statements relative
>>>>> to doit(). In that case, we would be forced to split into 2
>>>>> conditionals for the logging.
>>>>>
>>>>> Still I don't think it is necessary to call log_is_enabled twice, and
>>>>> I don't think it is necessary to always define the outputStream. If
>>>>> this turns out to be a problem, we could flip things around a bit:
>>>>>
>>>>>   void VM_Operation::evaluate() {
>>>>>     ResourceMark rm;
>>>>> !   bool enabled = false;
>>>>> !   if (log_is_enabled(Debug, vmoperation)) {
>>>>> !     outputStream* debugstream = 
>>>>> LogHandle(vmoperation)::debug_stream();
>>>>> !     enabled = true;
>>>>> !   if (enabled) {
>>>>> !     print_on_error(debugstream);
>>>>> !     debugstream->cr();
>>>>>     }
>>>>>     doit();
>>>>> !   if (enabled) {
>>>>> !     print_on_error(debugstream);
>>>>> !     debugstream->print_cr("]");
>>>>>     }
>>>>>   }
>>>>>
>>>> I'm fine with moving the definition into a conditional (but I can't 
>>>> move
>>>> the declaration for scope reasons). It will look like this
>>>>
>>>>     outputStream* debugstream;
>>>>     if (log_is_enabled(Debug, vmoperation)) {
>>>>          debugstream = LogHandle(vmoperation)::debug_stream();
>>>
>>> You may run afoul of the static analyzer when you use debugstream in 
>>> the logging clause after the doit(). It won't know that 
>>> log_is_enabled has to return the same answer both times. The use of 
>>> the enabled flag would address that.
>>
>> The log configuration can change at runtime, so in this case the two 
>> log_is_enabled() tests might in fact give different answers.
>>
>>>
>>>> To be honest, I'm not sure whether evaluating log_is_enabled twice is
>>>> worse than adding a boolean check and cluttering the code and 
>>>> making it
>>>> less readable. The argument for changing log_is_enabled() to a boolean
>>>> could be extrapolated to say that it should be a boolean for every 
>>>> check
>>>> for every tag. And that might be a good idea. But I feel like the
>>>> problem with it here is that it comes across as confusing to add
>>>> "enabled", which only actually needs to be used once. If we want it
>>>> changed, maybe we should change the macro? Or put it outside the
>>>> function, even, to make it only evaluate once?
>>>
>>> I would argue that within a function log_is_enabled should only be 
>>> called once per tag/level combination. I don't see it impeding 
>>> readability at all - quite the opposite. (The new logging code in 
>>> the current example, looks worse to me, than the original, from a 
>>> clutter/readability perspective.)
>>>
>>>>> There really is nothing quite like typing code into an editor with
>>>>> misaligned columns.
>>>>> Anyway, the change looks pretty good to me.
>>>>> -Max
>>>>>
>>>>> On 11/18/2015 4:22 AM, David Holmes wrote:
>>>>>> Hi Rachel,
>>>>>>
>>>>>> On 18/11/2015 5:50 AM, Rachel Protacio wrote:
>>>>>>> Hi,
>>>>>>>
>>>>>>> Please review the following small logging enhancement.
>>>>>>>
>>>>>>> Summary: The former -XX:+TraceVMOperation flag is updated to the
>>>>>>> unified
>>>>>>> logging framework and is now replaced with -Xlog:vmoperation in 
>>>>>>> product
>>>>>>> mode.
>>>>>>>
>>>>>>> Open webrev: http://cr.openjdk.java.net/~rprotacio/8143157/
>>>>>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8143157
>>>>>>> Testing: Passes jtreg, JPRT, RBT quick tests, and refworkload
>>>>>>> performance tests.
>>>>>>
>>>>>> Meta-question: the logging framework is safe to be called when at a
>>>>>> safepoint isn't it?
>>>> I haven't seen any problems, but I also don't know what would cause it
>>>> to be unsafe. I'd appreciate any guidance in figuring it out!
>>>
>>> The main thing would be locking of course.
>>
>> Naturally, logging is and should be available for use at any time, 
>> including during safepoints.
>>
>>>
>>>>>>
>>>>>> ---
>>>>>>
>>>>>> src/share/vm/runtime/vm_operations.cpp
>>>>>>
>>>>>>   void VM_Operation::evaluate() {
>>>>>>     ResourceMark rm;
>>>>>> !   outputStream* debugstream = 
>>>>>> LogHandle(vmoperation)::debug_stream();
>>>>>> !   if (log_is_enabled(Debug, vmoperation)) {
>>>>>> !     debugstream->print("[");
>>>>>> !     print_on_error(debugstream);
>>>>>> !     debugstream->cr();
>>>>>>     }
>>>>>>     doit();
>>>>>> !   if (log_is_enabled(Debug, vmoperation)) {
>>>>>> !     print_on_error(debugstream);
>>>>>> !     debugstream->print_cr("]");
>>>>>>     }
>>>>>>   }
>>>>>>
>>>>>> Why are you calling print_on_error twice?
>>>> The problem with switching VMOperation to logging is that in the 
>>>> current
>>>> system, the output goes straight to tty and so can be concatenated.
>>>> However, since the output triggered by the flag needs to be printed 
>>>> in a
>>>> log stream (which is not necessarily tty and which has bracketed
>>>> decorations), the "[<VM operation info>" portion comes out in that
>>>> stream, which could then be broken be other output, making it 
>>>> confusing
>>>> to have a closing "]" sitting around later. Coleen and I agreed 
>>>> that it
>>>> would improve output readability to be able to match the closing 
>>>> bracket
>>>> by restating the VM operation info. In other words:
>>>>
>>>>     [VM_Operation (0x00007fa3b44ce700): G1CollectFull, mode: 
>>>> safepoint,
>>>>     requested by thread 0x00007fa3ac018000
>>>>     <code from doit() and not from TraceVMOperation>
>>>>     ]
>>>>
>>>> becomes
>>>>
>>>>     [0.257s][debug  ][vmoperation] [VM_Operation (0x00007f74e589c700):
>>>>     G1CollectFull, mode: safepoint, requested by thread 
>>>> 0x00007f74dc018000
>>>>     <code from doit() and not from TraceVMOperation>
>>>>     [0.272s][debug  ][vmoperation] VM_Operation (0x00007f74e589c700):
>>>>     G1CollectFull, mode: safepoint, requested by thread 
>>>> 0x00007f74dc018000]
>>>>
>>>> rather than
>>>>
>>>>     [0.257s][debug  ][vmoperation] [VM_Operation (0x00007f74e589c700):
>>>>     G1CollectFull, mode: safepoint, requested by thread 
>>>> 0x00007f74dc018000
>>>>     <code from doit() and not from TraceVMOperation>
>>>>     [0.272s][debug  ][vmoperation] ]
>>>>
>>>> It should be easier for users to parse this.
>>>
>>> Okay but in that case each part should have the trailing ]
>>>
>>> Or, perhaps better, given all the other decoration why not just drop 
>>> the [ ] bracketing altogether? The intent to was capture all output 
>>> that occurred while the VMOperation was active, but that is now much 
>>> clearer in the output anyway. Though perhaps a "start" and "end" 
>>> indicator would aid parsing - especially when nested VMOperations 
>>> might occur.
>>
>> I agree. Adding some sort of end message and skipping the brackets 
>> altogether seems clearer to me.
>>
>>>
>>>>>>
>>>>>> Why is the only logging level for this tag the "debug" level? I 
>>>>>> think
>>>>>> I may be missing part of the way UL works here - can you enable
>>>>>> logging both by explicit tag (ie vmoperation) and the level (ie 
>>>>>> debug)?
>>>>>>
>>>> You enable the logging with "-Xlog:vmoperation=debug". If you leave of
>>>> the "=<level>" portion, it is by default parsed as "=info". We don't
>>>> believe this vmoperation logging needs to come out by default in the
>>>> case where someone asks for "-Xlog:all", so we put it one level below.
>>>
>>> <sigh> So does -Xlog:all signify all tags at info level or ???
>>
>> Yes.
>>
>>>
>>> I think this is real conceptual problem with the UL framework. I 
>>> should be able to define the info/trace/debug levels for a given 
>>> tag, without having to think about how they interact with a "log 
>>> all" request.
>>
>> I believe focus should be on classifying each log message according 
>> to its component & type (tags) and verbosity & cost (level), and less 
>> about which users will get it using what command line.
>> The log configuration is very flexible to allow users full control 
>> over the logging they want, all we need to do on the development side 
>> is to keep the logging logically classified and ordered so that the 
>> users can easily select from it.
>>
>> Thanks,
>> Marcus
>>
>>>
>>>>>> And I know I sound like a broken record but I'm concerned about the
>>>>>> overhead of the logging actions when it is not enabled ie:
>>>>>>
>>>>>> outputStream* debugstream = LogHandle(vmoperation)::debug_stream();
>>>>>>
>>>>>> always gets executed - and we evaluate is_enabled twice.
>>>>>>
>>>> See my reply to Max above.
>>>>>> ---
>>>>>>
>>>>>> test/runtime/logging/VMOperationTestMain.java
>>>>>>
>>>>>> Can you add a comment explaining what the logic is attempting to do
>>>>>> please. I'm curious how many times the loop executes before you 
>>>>>> get a
>>>>>> safepoint-based GC (and how it varies for different GC's).
>>>>>>
>>>>>> Style nit: while(  -> while (
>>>> Yes, will fix.
>>>>>>
>>>>>>> A compatability request has been accepted with regard to this 
>>>>>>> change.
>>>>>>
>>>>>> I'll record my objections again to the conversion of develop 
>>>>>> flags to
>>>>>> product. <sigh>
>>>> Thanks for voicing your concern. The justification for this tag being
>>>> product is that it could be useful to helping diagnose user 
>>>> problems, it
>>>> does not slow performance when off, and it minimally increases the 
>>>> size
>>>> of the code. If any of these prove to be false in aggregated logging
>>>> options, we can move it to develop level during code freeze.
>>>
>>> You could make that argument for every piece of logging and end up 
>>> with a very simple flat logging system. :) It is all very subjective 
>>> of course.
>>>
>>> Thanks,
>>> David
>>>
>>>> Thanks,
>>>> Rachel
>>>>>>
>>>>>>
>>>>>> Thanks,
>>>>>> David
>>>>>>
>>>>>>> Thank you very much,
>>>>>>> Rachel
>>>>>
>>>>
>>
>