hprof format question

Thu Nov 1 22:08:52 UTC 2018

Thanks for the input Kirk, JC.

Kirk, do I assume that your code decodes the entire object graph? I admit
that mine is intended to be hyper-lean. It collects the strings, the
classes, and then simply counts object instances. But the upside of that is
that I can get a map of object count by class from a 20Gb heap in under two
minutes, on a machine that only has 16G RAM. I tried to read it using jhat
once on a 32G machine and it just ground into the dirt with all 8 cores
running flat out (clearly in the "I'm dying" GC mode, since I'm pretty sure
that jhat itself is single threaded!) But it's nice to know I'm not insane!

JC, I'm on the road right now, but will try to dig out the relevant segment
fwiw in the next few days. I will say that it was pretty clear that it was
somehow bogus.

Oh, while on the topic of what I learned from this exercise, I also believe
I discovered a bug in BufferedInputStream (don't laugh!). It fails
catastrophically after reading 2GB (I think I can guess why without even
looking at the code :). Everything was reading properly from an unadorned
FileInputStream, but then I needed to look at the byte sequence to work out
what was happening with these broken parts, and everything that was
previously working went crazy when I stuck the BIS on the front. I created
a replacement so that I could actually look at the failing bytes in a
debugger. Hmm, the point of that observation was to ask the off-topic
question of one should report a bug in a core API?

Thanks again,
Cheers,
Simon

On Thu, Nov 1, 2018 at 11:20 AM JC Beyler <jcbeyler at google.com> wrote:

> Hi Simon,
>
> I briefly looked at the code that does the dump (or seems to) and the code
> is written in a form of:
>                                           out.writeByte((byte)
> HPROF_GC_ROOT_JNI_LOCAL);
>                                            writeObjectID(oop);
>                                            out.writeInt(threadIndex);
>                                            out.writeInt(EMPTY_FRAME_DEPTH);
>
> (for the Java implementation). I quickly went through the various path but
> I don't see a case where it could just stop after having written the object
> ID, it seems that it would either throw a nice exception or would write
> those two integers afterward. The C++ implementation does the same
> void JNILocalsDumper::do_oop(oop* obj_p) {
>   // ignore null handles
>   oop o = *obj_p;
>   if (o != NULL) {
>     writer()->write_u1(HPROF_GC_ROOT_JNI_LOCAL);
>     writer()->write_objectID(o);
>     writer()->write_u4(_thread_serial_num);
>     writer()->write_u4((u4)_frame_num);
>   }
> }
>
> I'm making a lot of assumptions that the surrounding code is same, the
> writer does not get corrupted or messed up. But it does seem sane. What
> exactly are the few bytes at that 16th element that make you believe the
> next two 4-bytes could not be the thread serial number and frame number?
> Jc
>
>
> On Wed, Oct 31, 2018 at 11:34 AM Kirk Pepperdine <
> kirk.pepperdine at gmail.com> wrote:
>
>> Hi Simon,
>>
>> I’ve also started a small project to try and solve the we need to look at
>> very large heap problem. My solution is to load the data into Neo4J. You
>> can find the project on my GitHub account.
>>
>> So, I believe I’ve taken the same tactic in just abandoning the segment
>> for the moment. It would be useful to sort that out but I listed it as a
>> future…
>>
>> Kind regards,
>> Kirk
>> L
>>
>> On Oct 31, 2018, at 4:07 AM, Simon Roberts <
>> simon at dancingcloudservices.com> wrote:
>>
>> Hi all, I'm hoping this is the correct list for a question on the hprof
>> file format (1.0.2)?
>>
>> I found this information:
>> http://hg.openjdk.java.net/jdk6/jdk6/jdk/raw-file/tip/src/share/demo/jvmti/hprof/manual.html
>>
>> and have been working on a small project to read these files. (Yes, I
>> know that NetBeans/VisualVM and Eclipse both have such libraries, and a
>> number of other tools have been derived from those, but so far as I can
>> tell, they all are fundamentally built on the notion of fully decoding
>> everything, and creating memory representations of the entire heap. I want
>> to pull out only certain pieces of information--specifically object counts
>> by class--from a large, ~20Gb, dump file, and those tools just give up the
>> ghost on my systems.)
>>
>> Anyway, my code reads the file pretty well so far, except that the file I
>> want to analyze seems to contradict the specifications of the document
>> mentioned above. Specifically, after processing about five
>> HEAP_DUMP_SEGMENTS with around 1.5 million sub-records in each, I come
>> across some ROOT_JNI_LOCAL records. The first 15 follow the format
>> specified in the above document (one 8 byte "ID" and two four byte values.)
>> But the 16th omits the two four byte records (well, it might simply have
>> more, but visual analysis shows that after the 8 byte ID, I have a new
>> block tag, and a believable structure. I've actually noticed that several
>> of the record types defined in this "group" seem to diverge from the paper
>> I mentioned.
>>
>> My solution is that if my parser trips, it abandons that
>> HEAP_DUMP_SEGMENT from that point forward. It doesn't seem to matter much,
>> since I was looking for object data, and it appears that all of that has
>> already been handled. However, clearly this is not ideal!
>>
>> Is there any more detailed, newer, better, information? Or anything else
>> I should know to pursue this tool (or indeed a simple object frequency by
>> classname result) from an hprof 1.0.2 format file?
>>
>> (And yes, I'm pursuing a putative memory leak :)
>>
>> Thanks for any input (including "dude, this is the wrong list!")
>> Cheers,
>> Simon
>>
>>
>>
>> --
>> Simon Roberts
>> (303) 249 3613
>>
>>
>>
>> --
>> Simon Roberts
>> (303) 249 3613
>>
>>
>>
>
> --
>
> Thanks,
> Jc
>

-- 
Simon Roberts
(303) 249 3613
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