Features/MemoryDebuggingTools

= Memory Debugging Tools =

Summary
The gdb debugger has been extended with new commands that make it easier to track down and fix excessive memory usage within programs and libraries.

This functionality was created by Fedora contributor David Malcolm, and we believe it is unique to Fedora.

Owner

 * Name: Dave Malcolm
 * Email: 

Current status

 * Targeted release: Fedora 14
 * Last updated: 2010-09-16
 * Percentage of completion: 100%

TODO: This is "feature-complete", but some issues remain:
 * I need to blog about this and write better docs
 * Fix the bugs
 * Testing

You will need gdb-7.2-7.fc14 or later when attaching to a process (rather than starting the process within gdb), due to bug 634660

DONE:
 * Preparing upstream project for initial launch: https://fedorahosted.org/gdb-heap/
 * I've disabled C++ support for now, as the current implementation slows down other operations.

Initial version of code uploaded

Packaged and reviewed; available as an update to F14.

Upstream tickets:
 * Initial Release
 * Fedora 14 Goals

Detailed Description
The new "gdb-heap" package adds a new "heap" command to /usr/bin/gdb.

The command allows you to get a breakdown of how that process is using dynamic memory.

It allows for unplanned memory usage debugging: if a process unexpectedly starts using large amounts of memory you can attach to it with gdb, and use the heap command to figure out where the memory is going. You should also be able to use it on core dumps.

We believe this approach is entirely new, and is unique to Fedora 14.

Benefit to Fedora
This feature could be of great use to developers and system administrators: it provides a new way of analyzing how a process uses memory, without requiring advance planning.

It is unique to Fedora (it makes heavy use of the gdb/python integration we have in Fedora), and was developed by a Fedora contributor (who is a Red Hat engineer).

Scope
Code is isolated, as an extension to gdb, written in Python.


 * I'm tracking development of the code in the upstream tracker here:
 * Initial Release
 * Fedora 14 Goals
 * Package the code in RPM form, add it to Fedora
 * Ensure that it's available without the user needing excessive configuration; ideally, if the rpm is installed, then you get the command automagically
 * Add it to comps so that it's suggested for installed by default if gdb is installed.
 * Testing

How To Test
No special hardware is needed.

You will need to install the gdb-heap package (not yet packaged)

Exploratory testing

 * Pick a process on your system (either as root, or one of your own processes)
 * Use "gdb attach PID" to connect to it
 * Use  to register the "heap" command
 * Use the "heap" command and its various subcommands (as described on the upstream website)
 * Ensure that all results look correct, and that there are no Python tracebacks within gdb.

Ideally the amount of "uncategorized" data should not be a substantial proportion of the overall size of the dynamically-allocated memory; if it is, then that may be a bug.

Ideally the command should not take too long to run. The more blocks of memory that are "live" within a process, the longer it will take to analyze the usage. Crude timings suggest it can analyze about 5000 allocations per second, so if you have a process with 300,000 allocations, it could take a minute to analyze them.

Basic Operation
Having attached to a process with gdb [david@fedora-14] $ gdb attach $(pidof -x name-of-program) you should be able to use the "heap" command to get a breakdown of how that process is using memory.

You can also do this with core dumps: [david@fedora-14] $ gdb -c core.1976

In this example, I've attached gdb to a python process: (gdb) heap Domain                       Kind              Detail   Count  Allocated size - --  --  --  --       python                         str                       6,689         477,840 cpython          PyDictEntry table                         167         456,944 cpython          PyDictEntry table            interned       1         200,704 python                        str            bytecode     648          92,024 uncategorized                                       32 bytes   2,866          91,712 python                       code                         648          82,944 uncategorized                                     4128 bytes      19          78,432 python                   function                         609          73,080 python         wrapper_descriptor                         905          72,400 python                       dict                         247          71,200 uncategorized                                       72 bytes     852          61,344 (snipped)

As you can see, gdb-heap will attempt to categorize the chunks of dynamically-allocated memory that it finds. It shows you how many blocks of memory of each category it found, with the categories sorted by the number of bytes of RAM that they're using.

The categorization is divided into three parts:
 * domain: high-level grouping e.g. "python", "C++", etc
 * kind: type information, appropriate to the domain e.g. a class/type
 * detail: additional detail (e.g. the size of a buffer, or a note that this python string is actually bytecode)

Some domains:

You can see in the above example that much of the memory is taken up by python strings (the "str" type), but a considerable amount is also occupied by implementation details of python dictionaries (the "PyDictEntry tables").

There are numerous subcommands. heap is integrated into gdb's tab-completion, so that you can see the available commands with the TAB key: (gdb) heap [TAB pressed] all   diff   label  log    sizes  used

Here's a tour of what's available. Refer to the upstream documentation for more information.

Finding blocks of RAM (query language)
gdb-heap has a  subcommand, which provides a simple language for querying for blocks matching criteria.

For example, here's how to find all dynamically-allocated block of a given size: (gdb) heap select size == 1778224 Start                End         Domain  Kind         Detail                                                                             Hexdump -- --  -    -  -- 0x000000000360a810  0x00000000037bca3f  uncategorized        1778224 bytes  00 00 00 43 00 00 86 60 00 00 00 3f 00 00 00 07 00 00 80 fc |...C...`...?........| 0x00000000068596c0 0x0000000006a0b8ef  uncategorized        1778224 bytes  00 00 00 43 00 00 86 60 00 00 00 3f 00 00 00 07 00 00 80 fc |...C...`...?........|

You can query on any of 'domain', 'kind', 'detail', 'addr', 'start', 'size', and use equalities, inequalities and booleans.

Here's a query for all NUL-terminated C strings above a particular size (scroll the page right to see the hexdump) (gdb) heap select kind="string data" and size > 512 Blocks retrieved 10000 Start                End  Domain         Kind  Detail                                                                             Hexdump -- --  --  ---  --  -- 0x0000000000624070  0x000000000062430f       C  string data          41 20 63 6f 6e 74 65 78 74 20 6d 61 6e 61 67 65 72 20 74 68 |A context manager th| 0x0000000000627b50 0x0000000000627e8f       C  string data          41 20 64 65 63 6f 72 61 74 6f 72 20 69 6e 64 69 63 61 74 69 |A decorator indicati| 0x0000000000628b90 0x0000000000628e0f       C  string data          4d 65 74 61 63 6c 61 73 73 20 66 6f 72 20 64 65 66 69 6e 69 |Metaclass for defini| 0x0000000000661320 0x000000000066170f       C  string data          20 10 65 00 00 00 00 00 01 00 00 00 00 00 00 00 20 2e 78 05 | .e............. .x.| 0x00000000006a2410 0x00000000006a27ff       C  string data          20 13 66 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 | .f.................|

History
gdb-heap provides a "history" feature, somewhat analogous to a revision-control system such as git, although the data is merely stored within the gdb process (it does not persist to disk, and is lost when gdb exits)

allows you to take a named snapshot of the current state of the heap. (gdb) heap label "about to 'import sys'" Blocks retrieved 10000 2,626,416 allocated, in 18716 blocks (gdb) cont >>> import sys Ctrl+C (gdb) heap label "after 'import sys'" Blocks retrieved 10000 2,626,688 allocated, in 18720 blocks

shows you all such named snapshots: (gdb) heap log Label 2 ""after 'import sys'"" at 2010-09-16 18:01:17.879123 2,626,688 allocated, in 18720 blocks

+272 bytes, +4 blocks

Label 1 ""about to 'import sys'"" at 2010-09-16 17:55:17.030709 2,626,416 allocated, in 18716 blocks

allows you to compare dynamic memory allocations between two different states: either those in the log, or with the current state. (gdb) heap diff Blocks retrieved 10000 Changes from "about to 'import sys'" to current +272 bytes, +4 blocks

Free-d blocks: 0x0000000000602010 -> 0x000000000060206f      96 bytes Category(domain='uncategorized', kind='', detail='96 bytes') |00 00 00 00 00 00 00 00 a0 20 60 00 00 00 00 00 50 01 62 00 |......... `.....P.b.| 0x0000000000602070 -> 0x000000000060209f      48 bytes Category(domain='uncategorized', kind='', detail='48 bytes') |01 00 00 00 80 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |....................| (snipped)

You can use this in conjunction with breakpoints and stepping through the code to get a sense of how different parts of the program affect memory usage.

Hexdumps
In addition to, gdb-heap also provides a   command, to help you figure out what a block of data is being used for. (gdb) hexdump 0x000000000360a810 0x000000000360a810 -> 0x000000000360a82f 00 00 00 43 00 00 86 60 00 00 00 3f 00 00 00 07 00 00 80 fc 00 00 00 10 00 00 00 64 00 00 00 08 |...C...`...?...............d....| 0x000000000360a830 -> 0x000000000360a84f 00 00 00 00 00 00 00 01 00 00 03 e8 00 00 00 06 00 00 00 02 00 00 00 01 00 00 03 e9 00 00 00 06 |................................| 0x000000000360a850 -> 0x000000000360a86f 00 00 00 10 00 00 00 01 00 00 03 ea 00 00 00 06 00 00 00 16 00 00 00 01 00 00 03 ec 00 00 00 09 |................................| (snip)

The output shows a split view, showing address ranges, hexadecimal values, and ASCII values, where printable ("." elsewhere).

Showing all dynamic memory
"heap all" shows a detailed, low-level report on all dynamically-allocated chunks of memory. This is a simple loop through memory, typically showing you the large allocations first (implemented via "mmap"), then the smaller ones (implemented within the "sbrk" region).

It reports the start/end of each region, along with book-keeping information about the block.

This is likely to only be of use for debugging low-level problems. (gdb) heap all All chunks of memory on heap (both used and free) - 0: 0x00007ffff08cd000 -> 0x00007ffff090dfff inuse: 266240 bytes () 1: 0x00007ffff7ea7000 -> 0x00007ffff7ee7fff inuse: 266240 bytes () (copious output snipped)

Dependencies
There's a baseline of functionality that I'm developing on top of Fedora 13's gdb.

The gdb-heap code peeks around inside the internals of the glibc heap implementation, violating encapsulation (rather by definition for a debugger), so if that changes, corresponding changes will need to be made to gdb-heap.

Some features require additional work in gdb, which I've filed RFE bugs for. Naturally this will require coordination with gdb to ensure that they land in Fedora 14:
 * : RFE: please expose "info symbol ADDRESS" in the python API
 * : RFE: notification about changes in the inferior process

Contingency Plan
None necessary, simply remove the package

Documentation

 * See above, and at the project's website.

Release Notes

 * The gdb debugger has been extended with new commands that make it easier to track down and fix excessive memory usage within programs and libraries. This functionality was created by Fedora contributor David Malcolm, and we believe it is unique to Fedora 14.

Comments and Discussion

 * See Talk:Features/MemoryDebuggingTools