[lldb-dev] Use of thread local storage in Timer

[Zachary Turner]

Anyway, you mentioned that it needs to work for threads other than those
created explicitly by us, so this approach doesn't work regardless.


On Mon, Sep 8, 2014 at 2:11 PM, Zachary Turner <zturner at google.com> wrote:

> No, what I'm saying is that if the thread routine returns normally and is
> not cancelled, you can just explicitly call the "destructor" at the end of
> the thread routine.  You don't need the OS to do it for you.
>
>
> On Mon, Sep 8, 2014 at 1:13 PM, Greg Clayton <gclayton at apple.com> wrote:
>
>>
>> > On Sep 8, 2014, at 11:27 AM, Zachary Turner <zturner at google.com> wrote:
>> >
>> > On Mon, Sep 8, 2014 at 10:39 AM, Greg Clayton <gclayton at apple.com>
>> wrote:
>> >
>> >>
>> >> This would solve the portability issue since TLS destructors would no
>> longer be needed, and making porting all of our TLS code to use llvm
>> support libraries very easy.
>> >>
>> >> Does this assessment seem correct?  Or am I missing something?
>> >
>> > The idea is you can place a "Timer" object anywhere in code and it can
>> some profiling by timing how long it takes. If another function makes
>> another "Timer" object, it will push itself on the stack, stop the previous
>> timer, wait until it destroys itself and pop itself back off the stack,
>> keep a long running total time spent doing the task, and then reactivate
>> the timer above it.
>> >
>> > So the TLS destructor are currently required. If TLS destructors aren't
>> supported on the platform, then this class should be disabled for that
>> platform (hollow it out and have it do nothing for platforms that don't
>> support it).
>> >
>> > Greg
>> >
>> > It sounds like you're talking about getting a function's exclusive
>> time?  As in, if function A calls B, and you create a timer in both A and
>> B, then the timer in A will compute timeof(A) - timeof(B)?
>>
>> It is timing from Timer construction time, to Timer destruction time, and
>> keeping a global map of all timer times using the constructors first string
>> value as a ConstString as the key. It does this on from all threads.
>>
>> > If not, and if it's sufficient to just time functions from start to end
>> along with all the child functions that execute, then llvm also has its own
>> timer class that we could use.  See llvm/Support/Timer.h.
>>
>> That just implements timing, not aggregation and history of total times.
>>
>> >
>> > As for the TLS destructors, it seems this is only necessary in the case
>> that a thread is cancelled?
>>
>> Why? You create a thread, it call some function that invokes a timer:
>>
>> int foo()
>> {
>>    {
>>        Timer a("parse something within foo", ...);
>>        bar();
>>    }
>>
>>    {
>>        Timer a("parse something else within foo", ...);
>>        bar();
>>    }
>> }
>>
>> Then in bar you have:
>>
>> int bar()
>> {
>>    Timer a("::bar()", ...);
>> }
>>
>> Any time a timer is constructed, it needs to get the TimerStack for the
>> current thread. It then pushes itself onto that stack and stops any other
>> timers already on that stack. When it destructs, it removes itself and
>> restarts the timers above. Total time spent and time between timers is kept
>> in a global repository where the key (the first string in the constructor)
>> is associated with the total and partial times.
>>
>> > Because if a thread routine runs to completion, we could just call
>> Timer::CleanupThreadSpecificData() or something.
>>
>> Then we need to guarantee every thread is created using our
>> Host::CreateThread() stuff so it can manage that. You can't guarantee that.
>> On MacOSX, somehow libdispatch() might get used and we don't control the
>> creation of those threads and they can call LLDB functions with contain
>> Timer objects.
>>
>> > Could we use pthread_cleanup_cancel() for the same purpose, and then
>> have our thread trampoline cleanup any timer-specific TLS data before it
>> returns to catch the non-cancellation case?
>>
>> I don't care how it happens, but the cleanup of deleting the TimerStack
>> for each thread must happen for any thread regardless of how it was started
>> (by us using Host::CreateThread(), or by the OS for any other reason).
>>
>> >
>> > My main concern is that I don't want to expose a generic thread-local
>> class that supports TLS destructors if TLS destructors aren't generic
>> enough to be available on all platforms.  It makes it too easy to make use
>> of them in generic code, breaking other platforms.
>>
>> How can you have thread specific data on ANY platform without being able
>> to clean it up? Are we expected to only store integers and POD types in the
>> TLS data? That would surprise me very much if that is the case.
>>
>> Greg
>>
>>
>
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