Debugging with GDB


GDB is a source level debugger for C, C++ and more languages. It allows inspecting the internal state of a program as it is running as well the post-mortem inspection of chrashed programs.

You can attach GDB to a running process, run a process inside GDB or examine a coredump.

Starting GDB

The two most common usages of GDB for scylla is running a process inside it (e.g. a unit test):

gdb /path/to/executable

You can specify command-line arguments that gdb will forward to the executable:

gdb /path/to/executable --args arg1 arg2 arg3

Another prevalent usage is to examine coredumps:

gdb --core=/path/to/coredump /path/to/executable

You can also attach it to an already running process:

gdb -p $pid

Where $pid is the PID of the running process you wish to attach GDB to.

Using GDB

GDB has excellent online documentation that you can find here.

Some of the more important topics:

Debugging Scylla with GDB

In general Scylla is quite hard to debug in GDB due to its asynchronous nature. You will soon find that backtraces always lead to the reactor’s event loop and stepping through the code will not work as you expect as soon as you leave or enter an asynchronous function. That said GDB is an indispensable tool in debugging coredumps and when used right can be of great help.

Over the years we have collected a set of tools for helping with debugging scylla. These are collected in and are in the form of commands, conveninence functions and pretty printers. To load the file issue the following command (inside gdb):

(gdb) source /path/to/

You should be now ready to use all of the tools contained therein. To list all available commands do:

(gdb) help scylla

To read the documentation of an individual command do:

(gdb) help scylla $commandname

Some commands have self explanatory names, some have documentation, and some have neither :( (contributions are welcome).

To get the list of the available convenience functions do:

(gdb) help function

Note that this will list GDB internal functions as well as those added by Again, just like before, to see the documentation of an individual function do:

(gdb) help function $functionname

Tips and tricks

Tell GDB to not stop on signals used by seastar

When running scylla (or any seastar application for that matter) inside GDB it will get interrupted often due to catching some signals used by seastar internally. This makes debugging almost impossible. To avoid this, instruct GDB to not stop on these signals:

(gdb) handle SIG34 SIG35 SIGUSR1 nostop noprint pass

Enable extended python diagnostics

When using the facilities from it is very useful to know the full stack of a failure in some of the provided tools, so that you can fix it or report it. To enable this run:

(gdb) set python print-stack full

Helping GDB find the source code for the executable

Often you find yourself debugging an executable, whose internal source paths don’t match those where they can be found on your machine. There is an easy workaround for this:

(gdb) set substitute-path /path/to/src/in/executable /path/to/src/on/your/machine

Note that the pattern that you supply to set substitute-path just has to be a common prefix of the paths. Example: if the source location inside the executable to some file is /opt/src/scylla/database.hh and on your machine it is /home/joe/work/scylla/database.hh, you can make GDB find the sources on your machine via:

(gdb) set substitute-path /opt/src/scylla /home/joe/work/scylla

This method might not work if the sources do not have a prefix, e.g. they are relative to the source tree root directory. In this case you can use the set directories command to set the search path of sources for gdb:

(gdb) set directories /path/to/scylla/source/tree

Multiple directories can be listed, separated with :.


GDB supports writing arbitrary GDB commands in any file and sourcing it. One can use this to place commands that one would have to issue every time when debugging in a file, instead of typing them each time GDB is started. Conventionally this file is called .gdbinit and GDB in fact will look for it in you current directory, in your $HOME directory and some other places. You can always load it by hand if GDB refuses or fails to load it:

(gdb) source /path/to/your/.gdbinit


GDB has a terminal based GUI called TUI. This is extremely useful when you wish to see the source code while you are debugging. The TUI mode can be activated by passing -tui to GDB on the command line, or any time by executing the tui enable to activate it and tui disable to deactivate it respectively. By default the source window has the focus in TUI mode, meaning that command completion, searching history and line editing doesn’t work, e.g. if you use the up and down keys, you will scroll the source file up and down respectively, instead of moving in the command history. To focus the command window, issue focus cmd. To move the focus to the source window again, issue focus src.

Thread Local Storage (TLS) variables

Thread local variables are saved in a special area of memory, at a negative offset from $fs_base. Let’s look at an example TLS variable, given the following C++ code from seastar:

namespace seastar::internal {

current_scheduling_group_ptr() noexcept {
    // Slow unless constructor is constexpr
    static thread_local scheduling_group sg;
    return &sg;


Let’s have a look in GDB:

(gdb) p &'seastar::internal::current_scheduling_group_ptr()::sg'
$1 = (<thread local variable, no debug info> *) 0x7fc1f11e7c0c
(gdb) p/x $fs_base
$2 = 0x7fc1f11ff700
(gdb) p/x 0x7fc1f11e7c0c - $fs_base
$3 = 0xfffffffffffe850c
(gdb) p/x -0xfffffffffffe850c
$4 = 0x17af4

The variable sg is located at offset 0x17af4 beneath $fs_base. We can also calculate the offset (and hence address) of a known TLS variable in memory as follows:

$fs_offset = $tls_entry - $sizeof_TLS_header

$sizeof_TLS_header can be obtained by listing the program headers of the binary:

$ eu-readelf -l ./a.out
  Type           Offset    VirtAddr           PhysAddr           FileSiz  MemSiz   Flg Align
  TLS            0x31ead40 0x00000000033ecd40 0x00000000033ecd40 0x000058 0x017bf0 R   0x40

We are interested in the size of the TLS header, which is in the MemSiz column and is 0x017bf0 in this example. The value of the $tls_entry can be found in the process’ symbol table:

$ eu-readelf -s ./a.out

Symbol table [ 5] '.dynsym' contains 1288 entries:
 1 local symbol  String table: [ 9] '.dynstr'
  Num:            Value   Size Type    Bind   Vis          Ndx Name
 1282: 000000000000010c      4 TLS     LOCAL  HIDDEN        23 _ZZN7seastar8internal28current_scheduling_group_ptrEvE2sg

If we substitute these values in we can verify our theory:

(gdb) set $tls_entry = 0x000000000000010c
(gdb) set $sizeof_TLS_header = 0x017bf0
(gdb) p/x $tls_entry - $sizeof_TLS_header
$5 = 0xfffe851c
(gdb) p/x -($tls_entry - $sizeof_TLS_header)
$6 = 0x17ae4

We can also identify a TLS variable based on its address. We know the value of $sizeof_TLS_header and we can easily calculate $fs_offset. To identify the variable we need to calculate its $tls_entry based on which we can find the matching entry in the symbol table. Remaining with the above example of the address being 0x7fc1f11e7c0c, we can calculate this as:

$tls_entry = $sizeof_TLS_header + $fs_offset

Do note however that $fs_offset is negative so this is in effect a substituation:

$tls_entry = 0x017bf0 - 0x17ae4

This yields 0x10c which is exactly the value of the Value column in the matching symbol table entry. This should work also if you don’t have the address to the start of the object. In this case you have to locate the entry in the symbol table, whose value range includes the calculated value. This can be made easier by sorting the symbol table by the Value column.

Optimized-out variables

In release builds one will find that a significant portion of variables and function parameters are optimized out. This is very annoying but often one can find a way around to inspect the desired variables.

For non-local variables, there is a good chance that a few frames up one can find another reference that wasn’t optimized out. Or one can try to find another object, which is not optimized out, and which is also known to hold a reference to the variable.

If the variable is local, one can try to look at the registers (info registers) and try to identify which one holds its value. This is relatively easy for pointers to objects as heap pointers are easily identifiable (they start with 0x6 and are composed of 12 digits, e.g.: 0x60f146e3fbc0) and one can check the size of the object they point to with scylla ptr. If the pointed-to object is an instance of a class with virtual methods, the object can be easily identified by looking at its vtable pointer (x/1a $ptr).

Debugging coredumps

Up until release 3.0 we used to build and package Scylla separately for each supported distribution. Starting with 3.1 we moved to relocatable binaries. These are built with a common frozen toolchain and packages are bundled with all dependencies. This means that post 3.1 there is just one build across all supported distros and that the exact environment the binaries were built with is available in the form of a Docker image. This makes debugging cores generated from relocatable binaries much easier. As of now, all releases except 2019.1 ship via relocatable packages, so in this chapter we will focus on how to debug cores generated from relocatable binaries, with a subsection later explaining how to debug cores generated by 2019.1 binaries.

Relocatable binaries

Cores produced by relocatable binaries can be simply opened in the dbuild container they were built with. To do that, two things (apart from the core itself of course) are needed:

  1. The exact frozen toolchain (dbuild container).

  2. The exact relocatable package the binary was part of.

Obtaining the frozen toolchain

The frozen toolchain is obtained based on the commit id of the version of the scylla executable the core was produced with. The exact commit hash can be obtained by running:

$ scylla --version

The version can be divided into 4 parts:

  • The version identifier, in this case: 666; in case of a release this will be something like 4.2.

  • The build identifier, in this case: development-0.

  • The date, in this case: 20200630.

  • The commit hash, in this case: 28c3d4f8e.

Based on the latter, you can obtain the right frozen toolchain:

$ cd /path/to/scylla
$ git checkout $commit_hash
Obtaining the relocatable-package

Once we have the right toolchain, we have to obtain the relocatable package. This is obtained based on the build-id, which can be obtained from the coredump like this:

$ eu-unstrip -n --core $corefile

Or from the executable like this:

$ eu-unstrip -n --exec $executable

With the build-id you can find the relocatable using the scylla-pkg.git/scripts/scylla-s3-reloc (private repo) script.

Loading the core

Move the coredump and the unpackaged relocatable package into some dir $dir on your system, then:

(host)$ cd /path/to/scylla # with the right commit checked out
(host)$ ./tools/toolchain/dbuild -it -v $dir:/workdir:z -- bash -l
(dbuild)$ cd /workdir
(dbuild)$ cd unpackaged-relocatable-package && ./ && cd ..
(dbuild)$ gdb --core=$corefile /opt/scylladb/libexec/scylla

You might need to add

-ex 'set auto-load safe-path /opt/scylladb/libreloc'

to the command line, see No thread debugging.

Non-relocatable binaries (2019.1)

The first step in opening any core is (after obtaining the core itself) obtaining the exact executable and library versions it was produced with. This is very important, it is not enough to get a binary with the same version, or to build one with the same commit, the binaries and libraries have to be the very same ones the core was produced with. This is because neither distros nor use use reproducible builds, which means that each build will produce a slightly different binary, which might be enough to make debugging hard or downright impossible. A foolproof method to match cores to their matching executables and libraries is the build-id. This is automatically assigned to each binary when it is built and it uniquely identifies it.

Collecting libraries

As stated above, just installing Scylla on another machine with the same OS version is not enough to obtain the correct environment to load the core. Packages might have newer versions, or the customer might be using different packages. It is important that we obtain the exact packages installed on the system where the core was produced on.

A method that should work regardless of the used distro is zipping all libs:

eu-unstrip -n --core corefile | awk '{print $3}' | grep -v '[-.]$' | zip scylla-libs -@

This might however produce a rather large zip or zip might not be available on the node and it can’t be installed (yes really). So below we are going to look at methods that doesn’t involve copying a huge zip from the node, nor installing any new software.


CentOS and Redhat has means to obtain the binaries given a certain build-id. TODO: document how that works.


Run the following script to collect the version of each library used by Scylla:

function file_uri() {
    dpkg -S $1 | cut -d" " -f1 | cut -d: -f1,2 | xargs apt-get download --print-uris

for lib in $(ldd `which scylla` | cut -d'(' -f1 | awk '{print $3}'); do
    if [ -n "$lib" ]; then
        echo "${lib}: $(file_uri $lib)"

This will produce an output like:

/opt/scylladb/lib/ 'http://some.mirror.example/mirrors/scylla/versioned/2019100201/ubuntu/pool/main/s/scylla-gcc73/scylla-gcc73-libstdc++6_7.3.0-3ubuntu2~xenialppa1_amd64.deb' scylla-gcc73-libstdc++6_7.3.0-3ubuntu2~xenialppa1_amd64.deb 368128 SHA256:7aa085e85c2a6bbd5b1517985e84cf280b74839d7ccb313f823f201ad162fccc
/usr/lib/ 'http://some.mirror.example/mirrors/debian/versioned/2018060600/ubuntu/pool/universe/libc/libcrypto++/libcrypto++9v5_5.6.1-9_amd64.deb' libcrypto++9v5_5.6.1-9_amd64.deb 885184 SHA256:888ce5da554200dac297d97b376d27607515689a79df944295e0e43cd0d94d31
/lib/x86_64-linux-gnu/ 'http://some.mirror.example/mirrors/debian/versioned/2018060600/ubuntu/pool/main/g/glibc/libc6_2.23-0ubuntu10_amd64.deb' libc6_2.23-0ubuntu10_amd64.deb 2580230 SHA256:bd05c3487325a4386dee6abb02ad904e1f2d8d3d0adc0df8e8f29168fbe2b5bb

Usually the URLs are accessible and the packages can be simply downloaded. In some case however they are on some internal mirror that is only accessible from within the internal network of the cluster. In this case one has to obtain the exact name and version of the packages then download it themselves with apt-get. Given the file with the output of the above script containing the package versions (packages.txt from now on), you can use the below script to parse the package name and version from it and download the appropriate .deb package files:

while read l
    deb=$(echo $l | cut -f3 -d' ')
    pkg_name=$(echo $deb | cut -f1 -d_)
    pkg_version=$(echo $deb | cut -f2 -d_)
    apt-get download ${pkg_name}=${pkg_version}
done < /path/to/packages.txt

Note that in some cases the names of packages or their versions in packages.txt will contain encoded characters, like %3a. This will make apt-get fail to download the package. Be sure to decode any of these before attempting the run the downloading script. Also in some cases the exact version will not be available anymore. In this case try to locate the closest version to the desired one and download that one. You can use apt-cache to query available versions:

apt-cache show package-name | grep Version:
Opening the core on the same OS

The packages obtained can be installed on the system, and the core can be simply opened with GDB, which should find the libraries without issues. One thing to note is that usually these distros will have older GDB:s available, which might have problems handling Scylla symbols. For this reason we also build and package GDB for all supported distros. This can be installed via the scylla-gdb package, after adding the Scylla repositories. Run GDB via /opt/scylladb/bin/gdb.

Opening the core on another OS

If you don’t feel like struggling to get recent enough tools on a potentially very old distro, a viable alternative is to just debug on your development box. The first step is unpacking (not installing!) all the packages. This can be easily done with a docker image, e.g. for ubuntu:

$ docker run -it --privileged -v .:/workspace ubuntu:16.04 bash -l
(docker) $ for pkg in $(ls *.deb); do dpkg -x $pkg .; done

After running the above snippet the current directory will contain a linux / directory structure. To load the core in GDB:

$ gdb -q
(gdb) set sysroot .
(gdb) set solib-search-path ./lib64:./lib/x86_64-linux-gnu
(gdb) core /path/to/core
(gdb) file ./usr/bin/scylla
(gdb) set solib-search-path ./lib64:./lib/x86_64-linux-gnu

Note that although relative paths should work for set sysroot and set solib-search-path, sometimes I could only get this working with absolute path. If opening the core wasn’t successful (thread debugging doesn’t work) try using absolute paths. Also note that issuing the set solib-search-path command has to be done once before and once after the file command. Don’t ask me why. Another thing to keep in mind is that set solib-search-path has to contain all directories that contain libraries. In the case of ubuntu:16.04 this is /lib64 and /lib/x86_64-linux-gnu. Other distros could have less or more such paths. Multiple paths can be added separated with :.

You can check that GDB correctly loaded the libraries by using info sharedlibrary and ensuring all libraries are loaded from the directory where the packages were extracted to, and not from the host.


Namespace issues

GDB complaints that it can’t find namespace seastar or some other Scylla or Seastar symbol that you know exists. This usually happens when GDB is in the wrong context i.e. a frame is selected which is not in the Scylla executable but in some other library. A typical situation is opening a coredump and attempting to access Scylla symbols when the initial frame is in libc. Move up the stack, or select a frame which is a Scylla or Seastar function to fix.

No thread debugging

Unable to access thread-local variables. Example:

(gdb) p seastar::local_engine
Cannot find thread-local storage for LWP 22604, executable file /usr/lib/debug/usr/bin/scylla.debug:
Cannot find thread-local variables on this target

The first step in finding out why thread debugging doesn’t work is enabling additional information about why thread debugging is not working:

(gdb) set debug libthread-db 1

This has to be done right after starting GDB, before the core and the executable are loaded. You can do this by adding -iex "set debug libthread-db 1" to your gdb command line.

The usual cause is that GDB failed to find some libraries or that the library versions of those libraries GDB loaded don’t match those the core was generated with.

Of special note is the library, which is crucial for thread debugging to work. Common causes of failing to find or load this library are discussed below.

Loading denied by auto-load safe-path

You might see a message like this:

warning: File "/opt/scylladb/libreloc/" auto-loading has been declined by your `auto-load safe-path' set to "$debugdir:$datadir/auto-load"
thread_db_load_search returning 0

To declare the directory this library is found at as safe to load from, do:

set auto-load safe-path /opt/scylladb/libreloc

Use the path that is appropriate for your setup. Alternatively you can use / as the path to declare your entire file-system as safe to load stuff from. Note that is packaged together with libc. So if you have the build-id appropriate libc package, you can be sure you have the correct too.

Missing debug symbols for glibc

If you see a message like this:

warning: Expected absolute pathname for libpthread in the inferior, but got .gnu_debugdata for /lib64/
Trying host libthread_db library: /lib64/
td_ta_new failed: application not linked with libthread
thread_db_load_search returning 0

Installing debug symbols for glibc might solve this. This issue was seen on Fedora 34, for more details see the bug report. Debug symbols can be installed with:

sudo dnf debuginfo-install glibc-2.32-6.fc33.x86_64

Adjust for the exact version you have installed.

Missing critical shared libraries

If you ensured is present and is successfully loaded by GDB but thread debugging still doesn’t work, inspect the other libraries loaded by GDB:

(gdb) info sharedlibrary

The listing will contain the path of the loaded libraries. If a library wasn’t found by GDB that will also be visible in the listing. You can then use the file utility to obtain the build-id of the libraries:

file /path/to/

This build-id must match the one obtained from the core. The library build-ids from the core can be obtained with:

eu-unstrip -n --core=/path/to/core

In general you can get away some non-core libraries missing or having the wrong version, but the core libraries like,, and (often called something like etc. must have the correct version. Best to ensure all libraries are correct to minimize the chance of something not working. Also, make sure the build-id of the executable matches that the core was generated with. Again, you can use file to obtain the build-id of the executable, then compare it with the build-id obtained from the eu-unstrip listing. For more information on how to obtain the correct version of libraries and how to override the path GDB loads them from, see Collecting libraries and Opening the core on another OS.

GDB crashes when priting the backtrace or some variable

See Avoid (some) symbol parsing related crashes.

GDB has trouble with frames inlined into the outermost frame in a seastar thread, or any green threads in general – where the outermost frame is annotated with .cfi_undefined rip. See GDB#26387. To work around this, pass a limit to bt, such that it excludes the problematic frame. E.g. if bt prints 10 frames before GDB crashing, use bt 9 to avoid the crash.

GDB keeps stopping on some signals

See Tell GDB to not stop on signals used by seastar.

Debugging guides

Guides focusing on different aspects of debugging Scylla. These guides assume a release build of Scylla.

The seastar memory allocator

Seastar has its own memory allocator optimized for Seastar’s thread-per-core architecture. Memory, just like CPU, is sharded among threads, meaning that each shard has its equally-sized exclusive memory area.

The seastar allocator operates on three levels:

  • pages (4KB)

  • spans of pages (2^N pages, N ∈ [0, 31])

  • objects managed by small pools

Small allocations (<= 16KB) are served by small memory pools. There is exactly one pool per supported size, and there is a limited number of sizes available between 1B (8B in practice) and 16KB. Allocations are served by the pool with the closest, but larger than equal size to the requested allocation, but alignments complicate this. Pools allocate spans themselves for the space to allocate objects in.

Large allocations are served by allocating an entire span.

The allocator keeps a description of all the pages and pools in memory in thread-local variables. Using these, it is possible to arrive at the metadata describing any allocation with a few steps:

address -> page -> span -> pool?

This is exploited by the scylla ptr command which, given an address, prints the following information:

  • The allocation (small or large) this address is part of.

  • The offset of the address from the beginning of the allocation.

  • Is the object dead or live.


(gdb) scylla ptr 0x6000000f3830
thread 1, small (size <= 512), live (0x6000000f3800 +48)

It is possible to dump the state of the seastar allocator with the scylla memory command. This prints a report containing the state of all small pools as well as the availability of spans. It also prints other Scylla specific information.

Continuations chains

Continuation chains are everywhere in Scylla. Every execution flow takes the form of a continuation chain. This makes debugging very hard because the normal GDB commands for inspecting and controlling execution flow (backtrace, up, down, step, next, return, etc.) quickly fail to fulfill their purpose. One will quickly find that every backtrace just leads to the same event loop in the seastar reactor.

Continuation chains are formed by tasks. The tasks form an intrusive forward linked list in memory. Each tasks links to the task that depends on it. Example:

future<> bar() {
    return sleep(std::chrono::seconds(10)).then([] {

future<> foo() {
    return bar().then([] {

When foo() is called a continuation chain of 3 tasks will be created:

  • T0: sleep()

  • T1: bar()::lambda#1

  • T2: foo()::lambda#1

T1 depends on T0, and T2 depends on T1. In memory they form a forward linked list like:

T0 -> T1 -> T2

The links are provided by the promise-future pairs in these tasks. Each task contains a future half of one such pair and a promise half of another one. The future is for the value arriving from the previous task and the promise is for the value calculated in the local task, that the next task waits on.

The task object have the following interface:

class task {
    scheduling_group _sg;
    virtual void run_and_dispose() noexcept = 0;
    virtual task* waiting_task() noexcept = 0;
    scheduling_group group() const { return _sg; }
    shared_backtrace get_backtrace() const;

The only thing stored at a known offset is the scheduling group. Each task object also has an associated action that is executed when the task runs (run_and_dispose()), as well as pointer to the next task (returned by waiting_task()). However task being a polymorphic object the layout of the different kind of tasks is not known, so all we can say about them is that somewhere they contain a promise object, which has a pointer to the future object of the task that depends on them. Also note that continuations are just one kind of task, there are other kinds of tasks as well. Many seastar primites, like do_with(), repeat(), do_until(), etc. have their own task types.

Traversing the continuation chain forward

Or in other words finding out what are the continuations waiting on this one. This involves searching for all outbound references in the task and identifying the one which is also a task. As this is quite a labour-intensive task, there is a command in which automates it, called scylla fiber. Example usage:

(gdb) scylla fiber 0x60001a305910
Starting task: (task*) 0x000060001a305910 0x0000000004aa5260 vtable for seastar::continuation<...> + 16
#0  (task*) 0x0000600016217c80 0x0000000004aa5288 vtable for seastar::continuation<...> + 16
#1  (task*) 0x000060000ac42940 0x0000000004aa2aa0 vtable for seastar::continuation<...> + 16
#2  (task*) 0x0000600023f59a50 0x0000000004ac1b30 vtable for seastar::continuation<...> + 16

This is somewhat similar to a backtrace, in that it shows tasks that are waiting for this continuation to finish, similar to how upstream functions are waiting for the called function to finish before continuing their own execution. See help scylla fiber and scylla fiber --help for more information on usage.

Traversing the continuation chain backward

Or in other words find the continuations the current continuation is waiting on. This involves searching for all references to the task and identifying the one which is also a task. This is made quite easy with the scylla find command from By using the --resolve flag, the vtable symbol will be printed next to each inbound reference, making spotting the other task easy. Once found, repeat the same with the pointer of the other task, until a non-continuation future is found, e.g. a I/O, smp::submit(), etc.

Seastar threads

Seastar threads are a special kind of continuation. Each seastar thread hosts a stack but it can also be linked into a continuation chain. The stack of seastar threads is a regular stack and all the normal stack related GDB commands can be used in it. This can be used to inspect where exactly the seastar thread stopped when it was suspended to wait on some future. Local variables can be inspected too. The catch is how to make GDB context switch into the stack of the seastar stack. Unfortunately there is no method that works with GDB as of now, the scylla thread command crashes GDB and even if it didn’t, it’d only works in live processes. To get this working a patched GDB is needed, see for instructions on how to use.

Debugging assert failures

Assert failures are the easiest (easiest but not easy) coredumps to debug because we know the condition that failed, we know where and thus the investigation has a clear scope – to find out why. This is not always easy though, especially if the root cause happened much earlier, and thus the state the node was in at that time is not observable in the coredump. The root cause might even be on another node altogether. In this case we try to gather as much information as possible and write debug patches that hopefully catch the problem earlier, and try to reproduce with them, hoping to get a new coredump that has more information.

Debugging segmentation faults

Segmentation faults are usually caused by use-after-free, use-after-move, dangling pointer/reference or memory corruption. Unfortunately, coredumps often contain very little immediate information on what exactly was wrong. It is rare to find something as obvious as a null pointer trying to be dereferenced. So one has to dig a little to find out what exactly triggered the SEGFAULT. The most useful command in this is scylla ptr, as it allows determining whether the address the current function is working with belongs to a live object or not.

Once the immediate cause is found, only the “easy” part remains, finding out how it happened. In some cases this can be very difficult. For example in the case of a memory corruption overwriting memory belonging to another object, the overwrite could have happened much earlier, with no traces of what it was in the coredump. In this case the same method has to be used that was mentioned in the case of debugging assert failures: adding additional debug code and trying to reproduce, hoping to catch the perpetrator red-handed.

Debugging deadlocks

If the process that is stuck is known, start from there. Try to identify the continuation-chain that is stuck, then follow it to find the future that is blocking it all. There is no way to differentiate a stuck continuation chain from one that is making progress unfortunately, so there are not tried-and-proven methods here either.

Debugging Out Of Memory (OOM) crashes

OOM crashes are usually the hardest to debug issues. Not only one has to determine the immediate cause which is often already hard enough, as usual one also has to determine what lead to this state, how did it happen.

That said, finding the immediate cause has a pretty standard procedure. The first step is always issuing a scylla memory command and determining where the memory is. Lets look at a concrete example:

(gdb) scylla memory
Used memory:    7452069888
Free memory:      20082688
Total memory:   7472152576

  allocated:    1067712512
  used:         1065353216
  free:            2359296

  total:            393216
  used:             160704
  free:             232512

 total:          1067319296
  real dirty:    1064566784
  virt dirty:     811568656
  real dirty:        393216
  virt dirty:        393216
  real dirty:             0
  virt dirty:             0

  bg write bytes:         42133 B
  hints:                      0 B
  view hints:                 0 B
  00 "main"
    fg writes:              0
    bg writes:              0
    fg reads:               0
    bg reads:              -7
  05 "statement"
    fg writes:             14
    bg writes:              5
    fg reads:              94
    bg reads:            2352

  Read Concurrency Semaphores:
    user sstable reads:       84/100, remaining mem:     138033377 B, queued: 0
    streaming sstable reads:   0/ 10, remaining mem:     149443051 B, queued: 0
    system sstable reads:      0/ 10, remaining mem:     149443051 B, queued: 0
  Execution Stages:
    data query stage:
         Total                            0
    mutation query stage:
         Total                            0
    apply stage:
      02 "streaming"                      287
         Total                            287
  Tables - Ongoing Operations:
    pending writes phaser (top 10):
             12 cqlstress_lwt_example.blogposts
              2 system.paxos
             14 Total (all)
    pending reads phaser (top 10):
           1863 system.paxos
            809 cqlstress_lwt_example.blogposts
           2672 Total (all)
    pending streams phaser (top 10):
              0 Total (all)

Small pools:
objsz spansz    usedobj       memory       unused  wst%
    1   4096          0            0            0   0.0
    1   4096          0            0            0   0.0
    1   4096          0            0            0   0.0
    1   4096          0            0            0   0.0
    2   4096          0            0            0   0.0
    2   4096          0            0            0   0.0
    3   4096          0            0            0   0.0
    3   4096          0            0            0   0.0
    4   4096          0            0            0   0.0
    5   4096          0            0            0   0.0
    6   4096          0            0            0   0.0
    7   4096          0            0            0   0.0
    8   4096      15285       126976         4696   3.7
   10   4096          0         8192         8192  99.9
   12   4096        173         8192         6116  74.6
   14   4096          0         8192         8192  99.8
   16   4096      11151       184320         5904   1.0
   20   4096       3570        77824         6424   7.9
   24   4096      19131       462848         3704   0.4
   28   4096       2572        77824         5808   7.3
   32   4096      27021       868352         3680   0.4
   40   4096      14680       593920         6720   0.7
   48   4096       3318       163840         4576   2.4
   56   4096      12077       692224        15912   0.9
   64   4096      52719      3375104         1088   0.0
   80   4096      16382      1323008        12448   0.6
   96   4096      17045      1667072        30752   0.3
  112   4096       3402       397312        16288   2.5
  128   4096      17767      2281472         7296   0.3
  160   4096      17722      2912256        76736   0.3
  192   4096       8094      1585152        31104   0.4
  224   4096      17087      3891200        63712   0.1
  256   4096      77945     21274624      1320704   0.1
  320   8192      13232      4366336       132096   0.7
  384   8192       5571      2203648        64384   1.0
  448   4096       4290      1986560        64640   1.7
  512   4096       2830      1503232        54272   2.8
  640  12288        960       655360        40960   3.9
  768  12288       5751      4489216        72448   0.1
  896   8192        326       311296        19200   4.6
 1024   4096       4320      5677056      1253376   0.7
 1280  20480        251       425984       104704  22.2
 1536  12288       3818      6373376       508928   1.7
 1792  16384       2711      4980736       122624   0.9
 2048   8192        594      1343488       126976   9.5
 2560  20480        122       458752       146432  25.7
 3072  12288       6596     21823488      1560576   0.9
 3584  28672          6       294912       273408  91.1
 4096  16384       2039      8372224        20480   0.2
 5120  20480       7885     43220992      2849792   0.3
 6144  24576       8188     54099968      3792896   0.8
 7168  28672         30       622592       407552  53.0
 8192  32768       8091     66781184       499712   0.7
10240  40960      15058    165216256     11022336   0.4
12288  49152       7034     92471296      6037504   0.3
14336  57344       6815    111935488     14235648   0.2
16384  65536      14046    230555648       425984   0.2
Small allocations: 872148992 [B]
Page spans:
index      size [B]      free [B]     large [B] [spans]
    0          4096       1888256             0       0
    1          8192        663552             0       0
    2         16384             0             0       0
    3         32768         32768    2320334848   70811
    4         65536         65536    3031105536   46251
    5        131072        131072    1161822208    8864
    6        262144       3145728             0       0
    7        524288        524288             0       0
    8       1048576       1048576             0       0
    9       2097152             0       2097152       1
   10       4194304      12582912             0       0
   11       8388608             0             0       0
   12      16777216             0             0       0
   13      33554432             0             0       0
   14      67108864             0      67108864       1
   15     134217728             0             0       0
   16     268435456             0             0       0
   17     536870912             0             0       0
   18    1073741824             0             0       0
   19    2147483648             0             0       0
   20    4294967296             0             0       0
   21    8589934592             0             0       0
   22   17179869184             0             0       0
   23   34359738368             0             0       0
   24   68719476736             0             0       0
   25  137438953472             0             0       0
   26  274877906944             0             0       0
   27  549755813888             0             0       0
   28 1099511627776             0             0       0
   29 2199023255552             0             0       0
   30 4398046511104             0             0       0
   31 8796093022208             0             0       0
Large allocations: 6582468608 [B]

We can see a couple of things at glance here: free memory is very low, cache is fully evicted. These are sure signs of a real OOM. Note that free memory doesn’t have to be 0 in the case of an OOM. It is enough for a size pool to not be able to allocate more memory spans and thus fail a critical allocations we cannot recover from. Also cache is fully evicted doesn’t mean it has 0 memory, but when it has just a couple of KB, it is considered fully evicted. Cache is evicted by the seastar memory allocator’s memory reclamation mechanism, which is hooked up with the cache and will start trying to free up memory by evicting the cache, once memory runs low. The cause of the OOM in this case is too many reads (1863) on system.paxos. This can be seen in the replica section of the report. The Coordinator and replica sections contain high level stats of the state of the coordinator and replica respectively. These stats summarize the usual suspects. Sometimes just looking at these is enough to determine what is the cause of the OOM. If not, one has to look at the last section: the dump of the state of the small pools and the page spans. What we are looking for is a small pool or a span size that owns excessive amounts of memory. Once found (there can be more then one) the next task is to identify what the objects owning that memory are. Note that in the case of smaller allocations, the memory is usually occupied directly by some C++ object, why in the case of larger allocations, these are usually potentially fragmented buffers, owned by some other object.

If the scylla memory output alone is not enough to explain what exactly is eating up all the memory, there are some further usual suspects that should be examined.

Exploded task- and smp-queues and lots of objects

Look for exploded task- and smp-queues with:

scylla task-queues        # lists all task queues on the local shard
scylla smp-queues         # lists smp queues

Look for lots of objects with:

scylla task_histogram -a  # histogram of all objects with a vtable

A huge number in any of these reports can indicate problems of exploding concurrency, or a shard not being to keep up. This can easily lead to work accumulating, in the form of tasks and associated objects, to the point of OOM.

Expensive reads

Another usual suspect is the number of sstables. This can be queried via scylla sstables. A large number of sstables for a single table (in the hundreds or more) can cause an otherwise non-problematic amount of reads to use excessive amount of memory, potentially leading to OOM.

Reversed- and unpaged-reads (or both, combined) can also consume a huge amount of memory, to the point of a fiew of such reads causing OOM. The way to find these is to inspect readers in memory, trying to locate their partition slice and having a look at their respective options:

  • partition_slice::option::reversed is set for a reversed query

  • partition_slice::option::allow_short_read is cleared for an unpaged query

Note that scylla have protections against reverse queries since 4.0, and against unpaged queries since 4.3.

Other reasons

If none of the usual suspects are present then all bets are off and one has to try to identify who the objects of the exploded size-class or span-size belong to. Unfortunately there are no proven methods here: some try to inspect the memory patterns and try to make sense of it, some try to build an object graph out of these objects and make sense of that. For the latter, the following commands might be of help:

scylla small-objects         # lists objects from a small pool
scylla generate-object-graph # generates and visualizes an object graph

Good luck, you are off the charted path here.