Trace object allocation flag proposal

Y. S. Ramakrishna y.s.ramakrishna at
Wed Dec 15 11:21:11 PST 2010

On 12/15/10 10:24, Xiaobin Lu wrote:
> One of the problems JVMTI has is that once it is enabled, it will fire 
> all kinds of events which will cause even more overheads. Correct me if 
> I am wrong here.

I didn't think JVMTI is "all or nothing" (if i understood yr statement).
But i will let the serviceability/JVMTI cognoscenti comment.

> DTrace provides obj-alloc probes which is exactly what many people like 
> to have on Linux. And that is exactly what I am proposing here.

Ah, i see. (Sorry for not reading through yr proposal before talking.)
But orthogonally to yr proposal, which is narrowly targeted to addressing
this specific issue related to object allocation,
why not use Linux system tap or other to mimic as much as possible the
Dtrace probes provided by the JVM, rather than making this point fix (if
i understand yr proposal) for just one aspect of what's offered by

Over to the serviceability folk; and out.
-- ramki

> Thanks,
> -Xiaobin
> On Wed, Dec 15, 2010 at 10:06 AM, Y. S. Ramakrishna 
> <y.s.ramakrishna at <mailto:y.s.ramakrishna at>> wrote:
>     Doesn't JVMTI already provide hooks (BCI?) for tracing allocation?
>     Perhaps that's found to be too obtrusive? Don't Java IDE's
>     use JVMTI to already deliver this kind of info to developers?
>     Pardon my naivette re JVMTI capabilities in this regard.
>     -- ramki
>     (java-serviceability-tools-illiterate)
>     On 12/15/10 10:00, Xiaobin Lu wrote:
>         Thanks for your feedback, David.
>         Let me try to clarify with some more background information.
>         One of the problems many application has is the object
>         allocation problem (Duh ...). Over releases, GC becomes more and
>         more frequent. It is pretty hard for one single team to find out
>         where the problem goes wrong. The existing tool such as jhat or
>         jmap is very hard to work with giant heap dump file. So the flag
>         I propose is mostly for diagnostic purpose, it won't be enabled
>         by default in production. Having said so, if we make it as a
>         manageable flag, we can turn it on when GC goes wild.
>         Another value this flag could provide is to find out why
>         sometime OOM occurs. For example, someone who wrote a search
>         application on top of Lucene framework suddenly found a crash
>         due to OOM. The stack trace points to Lucene code which is hard
>         to instrument, so this flag could provide insight to why the
>         allocation fails. Maybe it is due to a corrupted length value
>         passed to "new byte[length];" etc.
>         I like your idea on per-thread basis, however, for a lot of web
>         application, thread comes and go. It is pretty hard to pin point
>         on which thread I want to trace the object allocation.
>         To answer your last question, what I am really interested in
>         finding out is what type of object allocation makes GC happens
>         more frequent. Randomly taking snapshot of heap using jmap is
>         absolutely not an idea way to do so since not only it generates
>         a giant file which is difficult to work with, also it will pause
>         the application and cause negative impact to the application
>         throughput. After I get the type of hot allocation, if it
>         happens to be an application level object type such as
>, I can instrument the constructor to
>         dump the caller stack. People here also suggests it would be
>         nice if we could dump the allocation stack trace for some
>         particular hot types.
>         I could propose the diff for your folks to review.
>         Thanks,
>         -Xiaobin
>         On Tue, Dec 14, 2010 at 11:58 PM, David Holmes
>         <David.Holmes at <mailto:David.Holmes at>
>         <mailto:David.Holmes at
>         <mailto:David.Holmes at>>> wrote:
>            Hi Xiaobin,
>            The problem with tracing like this is that to be useful the
>         tracing
>            must be unobtrusive and be able to handle getting called
>         millions of
>            times (which is what will happen in those GC scenarios you
>            describe). The sheer volume of data generated as you suggest
>         below
>            would overwhelm the app and be pretty hard to work with.
>            Per-thread statistics of particular types (or of objects
>         larger than
>            a certain size) might be more useful, with a dump able to be
>            requested on-demand.
>            But I think you'd need to be able to combine this with heap dump
>            info to be useful.
>            It really depends on exactly what info you want to be able to
>            deduce: simple number of objects of given types, hot allocation
>            sites, hot threads, ...
>            Cheers,
>            David
>            Xiaobin Lu said the following on 12/15/10 17:07:
>                Hi folks,
>                I would like to propose a way to trace object allocation on
>                Linux. On Solaris, we have DTrace which is pretty nice.
>         But on
>                Linux, it is almost impossible to do so. Correct me if I am
>                wrong here.
>                So I am thinking to add a manageable VM flag and let's
>         call it
>                TraceObjectAllocation. When enabled, we can output
>         something like:
>                thread id: 10     class name: java/lang/reflect/Method  
>                          size: 80 bytes
>                thread id: 10     class name: [Ljava/lang/Class;        
>                           size: 16 bytes
>                thread id: 10     class name: [C                        
>                                                  size: 56 bytes
>                thread id: 10     class name: java/lang/reflect/Method  
>                         size: 80 bytes
>                thread id: 10     class name: [Ljava/lang/Class;        
>                          size: 16 bytes
>                As you could imagine, this could be very useful to keep
>         track of
>                object allocation behavior in the app. Some smart tool
>         can take
>                advantage of this to print a histogram (like top 10 hot
>                allocations) of object allocation. I would like to know your
>                thoughts and suggestions on this.
>                I have a detailed proposal on this attached in PDF file.
>                Thanks,
>                -Xiaobin

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