system profilers and incomplete stacks
serguei.spitsyn at oracle.com
serguei.spitsyn at oracle.com
Tue Jun 17 07:11:18 UTC 2014
Brendan,
Thank you for the details, especially, about the perf_events!
I was not aware about the issue on Linux.
I agree, the Solaris jstack issue was not that bad back in 2005.
It needs to be fixed cooperatively with the OS.
We have an idea how to fix it, this work is at the prototyping stage now.
Thanks,
Serguei
On 6/16/14 11:52 PM, Brendan Gregg wrote:
> G'Day Serguei,
>
> On Mon, Jun 16, 2014 at 10:45 PM, serguei.spitsyn at oracle.com
> <mailto:serguei.spitsyn at oracle.com> <serguei.spitsyn at oracle.com
> <mailto:serguei.spitsyn at oracle.com>> wrote:
>
> Hi Brendan,
>
> We are aware of these issues and work with the Solaris team to fix
> them in JDK 9.
> One is the frame pointer is used by the server compiler as a
> general purpose register on intel.
> Another is about the virtual (or inlined) frames.
>
> There are a couple of related bugs:
> https://bugs.openjdk.java.net/browse/JDK-6617153
> https://bugs.openjdk.java.net/browse/JDK-6276264
>
> There can be more issues filed on this.
>
>
> Ah, thanks, it's JDK-6276264.
>
> As Tom Rodriguez said at the time (2005): "The server VM uses the
> frame pointer as an allocatable register and there's no way to turn
> that off." I was really hoping there was a way to turn that off, like
> -fno-omit-frame-pointer.
>
> This also means DTrace jstack() has never worked fully. For the
> applications I tried it on, 50% of stacks were incomplete. Perhaps it
> wasn't that bad in 2005. I've been getting more mileage today from
> Java profilers.
>
> Please, note, that the jstack action is not implemented on Linux yet.
>
>
> Linux doesn't have DTrace jstack(), no, but its perf_events does has
> support for loading an auxiliary file of symbols, which can created
> via a Java agent for that purpose (eg,
> https://github.com/jrudolph/perf-map-agent). But that hasn't been
> working fully for the same reason - incomplete stacks.
>
> Brendan
>
>
> Thanks,
> Serguei
>
>
>
> On 6/16/14 5:14 PM, Brendan Gregg wrote:
>> Thanks but no, I'm aware of that bug and workarounds (I'm using
>> the LD_AUDIT_64=/usr/lib/dtrace/64/libdtrace_forceload.so
>> workaround, which isn't mentioned in the bug comments, but
>> probably should be). That bug is about missing symbols, but the
>> stacks shown in that bug still go all the way to thread_start. My
>> stacks often don't.
>>
>> For simple programs, the stacks are complete. But something
>> complex (eg, vert.x with event loops), and the stacks are often
>> incomplete, one frame only. Very much like what I see with
>> -fomit-frame-pointer, although this is hotspot, not gcc. Such
>> incomplete stacks are seen using either DTrace or perf_events.
>>
>> It was suggested to me to email the hotspot developers, because
>> this may well be a hotspot optimization they are familiar with.
>> It may also be something really obvious, like that the JVM breaks
>> native stacks due to optimized frames / green threads / etc, and
>> there is absolutely no way around it (no way to disable it). If
>> that's true, it may also mean that the DTrace jstack() action has
>> always had this issue. I'm still reading the source...
>>
>> Brendan
>>
>>
>>
>> On Mon, Jun 16, 2014 at 4:04 AM, Staffan Larsen
>> <staffan.larsen at oracle.com <mailto:staffan.larsen at oracle.com>> wrote:
>>
>> I think this is the bug you are looking at:
>> https://bugs.openjdk.java.net/browse/JDK-7187999, but I’ll
>> defer to someone else to confirm.
>>
>> /Staffan
>>
>>
>> On 16 jun 2014, at 12:47, Roland Westrelin
>> <roland.westrelin at oracle.com
>> <mailto:roland.westrelin at oracle.com>> wrote:
>>
>>> Forwarding to serviceability alias where this question
>>> belongs I think.
>>>
>>> Begin forwarded message:
>>>
>>>> *From: *Brendan Gregg <brendan.d.gregg at gmail.com
>>>> <mailto:brendan.d.gregg at gmail.com>>
>>>> *Subject: **system profilers and incomplete stacks*
>>>> *Date: *June 12, 2014 at 7:15:54 PM GMT+2
>>>> *To: *hotspot-compiler-dev at openjdk.java.net
>>>> <mailto:hotspot-compiler-dev at openjdk.java.net>
>>>>
>>>> G'Day,
>>>>
>>>> Is there a way to run hotspot so that a system profiler
>>>> (eg, DTrace, or Linux perf_events) can measure complete
>>>> stacks? I often get incomplete, partial stacks, with one or
>>>> a few frames only. I'm not worried about symbols right now,
>>>> what I'd like is to walk stacks all the way down to thread
>>>> start.
>>>>
>>>> I've been browsing the hotspot code, but haven't found out
>>>> how yet. I suspect it's related to Java optimized frames,
>>>> and has ditched the frame pointer. I was looking for an
>>>> equivalent -fno-omit-frame-pointer option.
>>>>
>>>> Here's an example:
>>>>
>>>> # dtrace -n 'profile-99 /execname == "java"/ {
>>>> @[jstack(100, 8000)] = count(); }'
>>>> [...]
>>>> org/mozilla/javascript/
>>>> ScriptableObject.createSlot(Ljava/lang/String;II)Lorg/mozilla/javascript/ScriptableObject$Slot;*
>>>> 0x884acce8200002da
>>>> 1
>>>>
>>>> sun/nio/ch/SocketChannelImpl.read(Ljava/nio/ByteBuffer;)I*
>>>> 0xffffffff20007f4b
>>>> 1
>>>>
>>>> org/mozilla/javascript/ScriptRuntime.newObjectLiteral([Ljava/lang/Object;[Ljava/lang/Object;[ILorg/mozilla/javascript/Context;Lorg/mozilla/javascript/Scriptable;)Lorg/mozilla/javascript/Scriptable;*
>>>> 0xa20000041
>>>> 1
>>>> [...]
>>>>
>>>> I see similar incomplete stacks with Linux perf_events.
>>>> Oracle JDKs from 6 to 8, and OpenJDK.
>>>>
>>>> thanks,
>>>>
>>>> Brendan
>>>> --
>>>> http://www.brendangregg.com <http://www.brendangregg.com/>
>>>
>>
>>
>>
>>
>> --
>> http://www.brendangregg.com
>
>
>
>
> --
> http://www.brendangregg.com
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