system profilers and incomplete stacks

[Brendan Gregg]

G'Day Serguei,

On Mon, Jun 16, 2014 at 10:45 PM, serguei.spitsyn at oracle.com <
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
> > 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>
>> wrote:
>>
>>  Forwarding to serviceability alias where this question belongs I think.
>>
>>  Begin forwarded message:
>>
>>  *From: *Brendan Gregg <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
>>
>>  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
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