Trace object allocation flag proposal

[Xiaobin Lu]

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
com.Xyz.search.IndexReader, 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 oracle.com>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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