[erlang-questions] Understanding etp-heapdump output in gdb

[Lukas Larsson]

Hello,

On Mon, Dec 18, 2017 at 8:30 PM, Roger Lipscombe <roger@REDACTED>
wrote:

> I'm attempting to figure out what binaries a particular process is
> holding onto. I have a core dump; I have a (seemingly) working gdb
> installation.
>

If it is off-heap binaries that you are interested in I would use
etp-offheapdump to get information from the process.

(gdb) etp-offheapdump $p->off_heap.first
[#Fun<0xa1e7c178,0xa20313a0,0x6,0>,
 #Fun<0xa1e7c2e0,0xa2031360,0x1,0>,
 #Fun<0xa1e7c268,0xa2031268,0x2,0>,
 #RefcBinary<0x5,0xa2031140,0xb53c0f60,0xb53c0f78,0>,
 #RefcBinary<0x3,0xa20310d0,0xb53c3468,0xb53c3480,0>,
 #Fun<0xb5b11900,0xa20310a0,0x2,0x3>,
 #Fun<0xb5b12350,0xa2031070,0x1,0>,
 #Fun<0xb5b11f18,0xa2030d08,0x1,0>,
 #RefcBinary<0x1,0xa2030c30,0xb53c1ac0,0xb53c1ad8,0>,
 #Fun<0xb5b11900,0xa2030cd8,0x2,0x3>,
 #Fun<0xb5b12350,0xa2030ca8,0x1,0>,
 #Fun<0xb5b11f18,0xa2048608,0x1,0>,
 #RefcBinary<0x1f,0xa20485a0,0xa1c405f0,0xa1c40608,0>,
 #Fun<0xb5b11900,0xa2048668,0x2,0x3>,
 #Fun<0xb5b12350,0xa2048638,0x1,0>,
 #Fun<0xb5b11f18,0,0x1,0>].

The second to last value of the #RefcBinary should be a char * to the
binary data.


> I've found the Process I care about:
>
>     (gdb) set $p = (Process *)0x7f6be3d3f560
>     (gdb) etp-heapdump $p
>
> When I run etp-heapdump, I get a boatload of output.
>
> It starts with:
>
> % heapdump (22077):
>  0x7f6b93bb8028: | H:   5-tuple | A:0x0003aa4b | B:0x93bb8072 |
> A:0x0002abcb | C:0x93bb8090 | A:0x001a09cb | H:   2-tuple |
> <0.1805.000>
>  0x7f6b93bb8068: | B:0x93bb80a2 | header     3 |   0x93bb8078 |
> 0x93bb8080 |   0x93bb8088 | B:0x93bb80ba | C:0x93bb80d8 | header     2
>  0x7f6b93bb80a8: |   0x93bb80a8 |   0x93bb80b0 | header     3 |
> 0x93bb80c0 |   0x93bb80c8 |   0x93bb80d0 | B:0x93bb80ea |    [] (NIL)
>  0x7f6b93bb80e8: | header     3 |   0x93bb80f0 |   0x93bb80f8 |
> 0x93bb8100 | H:   2-tuple |           ok | <0.20777.000> | H:
> 3-tuple
>
> How do I interpret this output?
>

You have to have a good understanding of how the erlang term format works
internally in order to understand what etp-heapdump prints. I believe that
https://github.com/happi/theBeamBook has a section describing how it works.

Lukas
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://erlang.org/pipermail/erlang-questions/attachments/20171219/52018d64/attachment.htm>