G Comments on porting to other systems

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G Comments on porting to other systems

A working Posix thread library is needed for the server. On Solaris 2.5 we use Sun PThreads (the native thread support in 2.4 and earlier versions are not good enough) and on Linux we use LinuxThreads by Xavier Leroy, Xavier.Leroy@inria.fr.

The hard part of porting to a new Unix variant without good native thread support is probably to port MIT-pthreads. See `mit-pthreads/README' and Programming POSIX Threads.

The MySQL distribution includes a patched version of Provenzano's Pthreads from MIT (see MIT Pthreads web page). This can be used for some operating systems that do not have POSIX threads.

It is also possible to use another user level thread package named FSU Pthreads (see FSU Pthreads home page). This implementation is being used for the SCO port.

See the `thr_lock.c' and `thr_alarm.c' programs in the `mysys' directory for some tests/examples of these problems.

Both the server and the client need a working C++ compiler (we use gcc and have tried SparcWorks). Another compiler that is known to work is the Irix cc.

To compile only the client use ./configure --without-server.

There is currently no support for only compiling the server, nor is it likly to be added unless someone has a good reason for it.

If you want/need to change any `Makefile' or the configure script you must get Automake and Autoconf. We have used the automake-1.2 and autoconf-2.12 distributions.

All steps needed to remake everything from the most basic files. 

/bin/rm */.deps/*.P
/bin/rm -f config.cache
aclocal
autoheader
aclocal
automake
autoconf
./configure --with-debug=full --prefix='your installation directory'

# The makefiles generated above need GNU make 3.75 or newer.
# (called gmake below)
gmake clean all install init-db

If you run into problems with a new port, you may have to do some debugging of MySQL! See section G.1 Debugging a MySQL server.

NOTE: Before you start debugging mysqld, first get the test programs mysys/thr_alarm and mysys/thr_lock to work. This will ensure that your thread installation has even a remote chance to work!

G.1 Debugging a MySQL server

If you are using some functionality that is very new in MySQL, you can try to run mysqld with the --skip-new (which will disable all new, potentially unsafe functionality) or with --safe-mode which disables a lot of optimization that may cause problems. See section A.4.1 What To Do If MySQL Keeps Crashing.

If mysqld doesn't want to start, you should check that you don't have any my.cnf files that interfere with your setup! You can check your my.cnf arguments with mysqld --print-defaults and avoid using them by starting with mysqld --no-defaults ....

If mysqld starts to eat up CPU or memory or if it ``hangs'', you can use mysqladmin processlist status to find out if someone is executing a query that takes a long time. It may be a good idea to run mysqladmin -i10 processlist status in some window if you are experiencing performance problems or problems when new clients can't connect.

The command mysqladmin debug will dump some information about locks in use, used memory and query usage to the mysql log file. This may help solve some problems. This command also provides some useful information even if you haven't compiled MySQL for debugging!

If the problem is that some tables are getting slower and slower you should try to optimize the table with OPTIMIZE TABLE or myisamchk. See section 4 MySQL Database Administration. You should also check the slow queries with EXPLAIN.

You should also read the OS-specific section in this manual for problems that may be unique to your environment. See section 2.6 Operating System Specific Notes.

G.1.1 Compiling MYSQL for debugging.

If you have some very specific problem, you can always try to debug MySQL. To do this you must configure MySQL with the --with-debug or the --with-debug=full option. You can check whether or not MySQL was compiled with debugging by doing: mysqld --help. If the --debug flag is listed with the options then you have debugging enabled. mysqladmin ver also lists the mysqld version as mysql ... --debug in this case.

If you are using gcc or egcs, the recommended configure line is: 

CC=gcc CFLAGS="-O2" CXX=gcc CXXFLAGS="-O2 -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --with-debug --with-extra-charsets=complex

This will avoid problems with the libstdc++ library and with C++ exceptions (many compilers have problems with C++ exceptions in threaded code) and compile a MySQL version with support for all character sets.

If you suspect a memory overrun error, you can configure MySQL with --with-debug=full, which will install a memory allocation (SAFEMALLOC) checker. Running with SAFEMALLOC is however quite slow, so if you get performance problems you should start mysqld with the --skip-safemalloc option. This will disable the memory overrun checks for each call to malloc and free.

If mysqld stops crashing when you compile it with --with-debug, you have probably found a compiler bug or a timing bug within MySQL. In this case you can try to add -g to the CFLAGS and CXXFLAGS variables above and not use --with-debug. If mysqld now dies, you can at least attach to it with gdb or use gdb on the core file to find out what happened.

When you configure MySQL for debugging you automatically enable a lot of extra safety check functions that monitor the health of mysqld. If they find something ``unexpected,'' an entry will be written to stderr, which safe_mysqld directs to the error log! This also means that if you are having some unexpected problems with MySQL and are using a source distribution, the first thing you should do is to configure MySQL for debugging! (The second thing, of course, is to send mail to mysql@lists.mysql.com and ask for help. Please use the mysqlbug script for all bug reports or questions regarding the MySQL version you are using!

In the Windows MySQL distribution, mysqld.exe is by default compiled with support for trace files.

G.1.2 Creating trace files

If the mysqld server doesn't start or if you can cause the mysqld server to crash quickly, you can try to create a trace file to find the problem.

To do this you have to have a mysqld that is compiled for debugging. You can check this by executing mysqld -V. If the version number ends with -debug, it's compiled with support for trace files.

Start the mysqld server with a trace log in `/tmp/mysqld.trace' (or `C:\mysqld.trace' on Windows):

mysqld --debug

On Windows you should also use the --standalone flag to not start mysqld as a service:

In a DOS window do: 

mysqld --debug --standalone

After this you can use the mysql.exe command line tool in a second DOS window to reproduce the problem. You can take down the above mysqld server with mysqladmin shutdown.

Note that the trace file will get very BIG! If you want to have a smaller trace file, you can use something like:

mysqld --debug=d,info,error,query,general,where:O,/tmp/mysqld.trace

which only prints information with the most interesting tags in `/tmp/mysqld.trace'.

If you make a bug report about this, please only send the lines from the trace file to the appropriate mailing list where something seems to go wrong! If you can't locate the wrong place, you can ftp the trace file, together with a full bug report, to ftp://support.mysql.com/pub/mysql/secret so that a MySQL developer can take a look a this.

The trace file is made with the DBUG package by Fred Fish. See section G.3 The DBUG package..

G.1.3 Debugging mysqld under gdb

On most system you can also start mysqld from gdb to get more information if mysqld crashes.

With some older gdb versions on Linux you must use run --one-thread if you want to be able to debug mysqld threads. In this case you can only have one thread active at a time.

When running mysqld under gdb, you should disable the stack trace with --skip-stack-trace to be able to catch segfaults within gdb.

It's very hard to debug MySQL under gdb if you do a lot of new connections the whole time as gdb doesn't free the memory for old threads. You can avoid this problem by starting mysqld with -O thread_cache_size= 'max_connections +1'. In most cases just using -O thread_cache_size=5' will help a lot!

If you want to get a core dump on Linux if mysqld dies with a SIGSEGV signal, you can start mysqld with the --core-file option. This core file can be used to make a backtrace that may help you find out why mysqld died: 

shell> gdb mysqld core
gdb>   backtrace full
gdb>   exit

See section A.4.1 What To Do If MySQL Keeps Crashing.

If you are using gdb 4.17.x or above on Linux, you should install a `.gdb' file, with the following information, in your current directory: 

set print sevenbit off
handle SIGUSR1 nostop noprint
handle SIGUSR2 nostop noprint
handle SIGWAITING nostop noprint
handle SIGLWP nostop noprint
handle SIGPIPE nostop
handle SIGALRM nostop
handle SIGHUP nostop
handle SIGTERM nostop noprint

If you have problems debugging threads with gdb, you should download gdb 5.x and try this instead. The new gdb version has very improved thread handling!

Here is an example how to debug mysqld: 

shell> gdb /usr/local/libexec/mysqld
gdb> run
...
backtrace full # Do this when mysqld crashes

Include the above output in a mail generated with mysqlbug and mail this to mysql@lists.mysql.com.

If mysqld hangs you can try to use some system tools like strace or /usr/proc/bin/pstack to examine where mysqld has hung. 

strace /tmp/log libexec/mysqld

If you are using the Perl DBI interface, you can turn on debugging information by using the trace method or by setting the DBI_TRACE environment variable. See section 8.2.2 The DBI Interface.

G.1.4 Using a stack trace

On some operating systems, the error log will contain a stack trace if mysqld dies unexpectedly. You can use this to find out where (and maybe why) mysqld died. See section 4.9.1 The Error Log. To get a stack trace, you should NOT compile mysqld with the -fomit-frame-pointer option to gcc. See section G.1.1 Compiling MYSQL for debugging..

If the error file contains something like the following: 

mysqld got signal 11;
The manual section 'Debugging a MySQL server' tells you how to use a
stack trace and/or the core file to produce a readable backtrace that may
help in finding out why mysqld died
Attemping backtrace. You can use the following information to find out
where mysqld died.  If you see no messages after this, something went
terribly wrong
stack range sanity check, ok, backtrace follows
0x40077552
0x81281a0
0x8128f47
0x8127be0
0x8127995
0x8104947
0x80ff28f
0x810131b
0x80ee4bc
0x80c3c91
0x80c6b43
0x80c1fd9
0x80c1686

you can find where mysqld died by doing the following:

  1. Copy the above numbers to a file, for example `mysqld.stack'.
  2. Make a symbol file for the mysqld server: 
    nm -n libexec/mysqld > /tmp/mysqld.sym
    Note that many MySQL binary distributions comes with the above file, named mysqld.sym.gz. In this case you must unpack this by doing: 
    gunzip < bin/mysqld.sym.gz > /tmp/mysqld.sym
  3. Execute resolve_stack_dump -s /tmp/mysqld.sym -n mysqld.stack. This will print out where mysqld died. If this doesn't help you find out why mysqld died, you should make a bug report and include the output from the above commend with the bug report. Note however that in most cases it will not help us to just have a stack trace to find the reason for the problem. To be able to locate the bug or provide a workaround, we would in most cases need to know the query that killed mysqld and preferable a test case so that we can repeat the problem! See section 1.2.22.3 How to Report Bugs or Problems.

G.1.5 Using log files to find cause of errors in mysqld

Note that before starting mysqld with --log you should check all your tables with myisamchk. See section 4 MySQL Database Administration.

If mysqld dies or hangs, you should start mysqld with --log. When mysqld dies again, you can examine the end of the log file for the query that killed mysqld.

If you are using --log without a file name, the log is stored in the database directory as 'hostname'.log In most cases it's the last query in the log file that killed mysqld, but if possible you should verify this by restarting mysqld and executing the found query from the mysql command line tools. If this works, you should also test all complicated queries that didn't complete.

You can also try the command EXPLAIN on all SELECT statements that takes a long time to ensure that mysqld is using indexes properly. See section 5.2.1 EXPLAIN Syntax (Get Information About a SELECT).

You can find the queries that take a long time to execute by starting mysqld with --log-slow-queries. See section 4.9.5 The Slow Query Log.

If you find the text mysqld restarted in the error log file (normally named `hostname.err') you have probably found a query that causes mysqld to fail. If this happens you should check all your tables with myisamchk (see section 4 MySQL Database Administration), and test the queries in the MySQL log files to see if one doesn't work. If you find such a query, try first upgrading to the newest MySQL version. If this doesn't help and you can't find anything in the mysql mail archive, you should report the bug to mysql@lists.mysql.com. Links to mail archives are available online at the MySQL documentation page.

If you have started mysqld with --with-myisam-recover, MySQL will automatically check and try to repair MyISAM tables if they are marked as 'not closed properly' or 'crashed'. If this happens, MySQL will write an entry in the hostname.err file 'Warning: Checking table ...' which is followed by Warning: Repairing table if the table needs to be repaired. If you get a lot of these errors, without mysqld having died unexpectedly just before, then something is wrong and needs to be investigated further. See section 4.1.1 mysqld Command-line Options.

It's of course not a good sign if mysqld did died unexpectedly, but in this case one shouldn't investigate the Checking table... messages but instead try to find out why mysqld died.

G.1.6 Making a test case when you experience table corruption

If you get corrupted tables or if mysqld always fails after some update commands, you can test if this bug is reproducible by doing the following:

You can also use the script mysql_find_rows to just execute some of the update statements if you want to narrow down the problem.

G.2 Debugging a MySQL client

To be able to debug a MySQL client with the integrated debug package, you should configure MySQL with --with-debug or --with-debug=full. See section 2.3.3 Typical configure Options.

Before running a client, you should set the MYSQL_DEBUG environment variable: 

shell> MYSQL_DEBUG=d:t:O,/tmp/client.trace
shell> export MYSQL_DEBUG

This causes clients to generate a trace file in `/tmp/client.trace'.

If you have problems with your own client code, you should attempt to connect to the server and run your query using a client that is known to work. Do this by running mysql in debugging mode (assuming you have compiled MySQL with debugging on): 

shell> mysql --debug=d:t:O,/tmp/client.trace

This will provide useful information in case you mail a bug report. See section 1.2.22.3 How to Report Bugs or Problems.

If your client crashes at some 'legal' looking code, you should check that your `mysql.h' include file matches your mysql library file. A very common mistake is to use an old `mysql.h' file from an old MySQL installation with new MySQL library.

G.3 The DBUG package.

The MySQL server and most MySQL clients are compiled with the DBUG package originally made by Fred Fish. When one has configured MySQL for debugging, this package makes it possible to get a trace file of what the program is debugging. See section G.1.2 Creating trace files.

One uses the debug package by invoking the program with the --debug="..." or the -#... option.

Most MySQL programs has a default debug string that will be used if you don't specify an option to --debug. The default trace file is usually /tmp/programname.trace on Unix and \programname.trace on Windows.

The debug control string is a sequence of colon separated fields as follows: 

<field_1>:<field_2>:...:<field_N>

Each field consists of a mandatory flag character followed by an optional "," and comma-separated list of modifiers: 

flag[,modifier,modifier,...,modifier]

The currently recognized flag characters are:

d Enable output from DBUG_<N> macros for the current state. May be followed by a list of keywords which selects output only for the DBUG macros with that keyword. An empty list of keywords implies output for all macros.
D Delay after each debugger output line. The argument is the number of tenths of seconds to delay, subject to machine capabilities. That is, -#D,20 is delay two seconds.
f Limit debugging and/or tracing, and profiling to the list of named functions. Note that a null list will disable all functions. The appropriate "d" or "t" flags must still be given, this flag only limits their actions if they are enabled.
F Identify the source file name for each line of debug or trace output.
i Identify the process with the pid or thread id for each line of debug or trace output.
g Enable profiling. Create a file called 'dbugmon.out' containing information that can be used to profile the program. May be followed by a list of keywords that select profiling only for the functions in that list. A null list implies that all functions are considered.
L Identify the source file line number for each line of debug or trace output.
n Print the current function nesting depth for each line of debug or trace output.
N Number each line of dbug output.
o Redirect the debugger output stream to the specified file. The default output is stderr.
O As O but the file is really flushed between each write. When needed the file is closed and reopened between each write.
p Limit debugger actions to specified processes. A process must be identified with the DBUG_PROCESS macro and match one in the list for debugger actions to occur.
P Print the current process name for each line of debug or trace output.
r When pushing a new state, do not inherit the previous state's function nesting level. Useful when the output is to start at the left margin.
S Do function _sanity(_file_,_line_) at each debugged function until _sanity() returns something that differs from 0. (Mostly used with safemalloc to find memory leaks)
t Enable function call/exit trace lines. May be followed by a list (containing only one modifier) giving a numeric maximum trace level, beyond which no output will occur for either debugging or tracing macros. The default is a compile time option.

Some examples of debug control strings which might appear on a shell command line (the "-#" is typically used to introduce a control string to an application program) are: 

-#d:t
-#d:f,main,subr1:F:L:t,20
-#d,input,output,files:n
-#d:t:i:O,\\mysqld.trace

In MySQL, common tags to print (with the d option) are: enter,exit,error,warning,info and loop.

G.4 Locking methods

Currently MySQL only supports table locking for ISAM/MyISAM and HEAP tables and page level locking for BDB tables. See section 5.3.1 How MySQL Locks Tables. With MyISAM tables one can freely mix INSERT and SELECT without locks (Versioning).

Starting in version 3.23.33, you can analyze the table lock contention on your system by checking Table_locks_waited and Table_locks_immediate environment variables.

Some database users claim that MySQL cannot support near the number of concurrent users because it lacks row-level locking. This may be true for some specific applications, but is not generally true. As always this depends totally on what the application does and what is the access/update pattern of the data.

Pros for row locking:

Cons:

Table locks are superior to page level / row level locks in the following cases:

Other options than row / page level locking:

Versioning (like we use in MySQL for concurrent inserts) where you can have one writer at the same time as many readers. This means that the database/table supports different views for the data depending on when one started to access it. Other names for this are time travel, copy on write or copy on demand.

Copy on demand is in many case much better than page or row level locking; The worst case does, however, use much more memory than when using normal locks.

Instead of using row level locks one can use application level locks. (Like get_lock/release_lock in MySQL). This works of course only in well-behaved applications.

In many cases one can do an educated guess which locking type is best for the application but generally it's very hard to say that a given lock type is better than another; Everything depends on the application and different part of the application may require different lock types.

Here are some tips about locking in MySQL:

On web application most applications do lots of selects, very few deletes, updates mainly on keys and inserts in some specific tables. The base MySQL setup is VERY tuned for this.

Concurrent users is not a problem if one doesn't mix updates and selects that needs to examine many rows in the same table.

If one mixes inserts and deletes on the same table then INSERT DELAYED may be of great help.

One can also use LOCK TABLES to speed up things (many updates within a single lock is much faster than updates without locks). Splitting thing to different tables will also helps.

If you get speed problems with the table locks in MySQL, you may be able to solve these to convert some of your tables to BDB tables. See section 7.5 BDB or Berkeley_DB Tables.

The optimization section in the manual covers a lot of different aspects of how to tune ones application. See section 5.2.11 Other Optimization Tips.

G.5 Comments about RTS threads

I have tried to use the RTS thread packages with MySQL but stumbled on the following problems:

They use an old version of a lot of POSIX calls and it is very tedious to make wrappers for all functions. I am inclined to think that it would be easier to change the thread libraries to the newest POSIX specification.

Some wrappers are already written. See `mysys/my_pthread.c' for more info.

At least the following should be changed:

pthread_get_specific should use one argument. sigwait should take two arguments. A lot of functions (at least pthread_cond_wait, pthread_cond_timedwait) should return the error code on error. Now they return -1 and set errno.

Another problem is that user-level threads use the ALRM signal and this aborts a lot of functions (read, write, open...). MySQL should do a retry on interrupt on all of these but it is not that easy to verify it.

The biggest unsolved problem is the following:

To get thread-level alarms I changed `mysys/thr_alarm.c' to wait between alarms with pthread_cond_timedwait(), but this aborts with error EINTR. I tried to debug the thread library as to why this happens, but couldn't find any easy solution.

If someone wants to try MySQL with RTS threads I suggest the following:

G.6 Differences between different thread packages

MySQL is very dependent on the thread package used. So when choosing a good platform for MySQL, the thread package is very important.

There are at least three types of thread packages:

In some systems kernel threads are managed by integrating user level threads in the system libraries. In such cases, the thread switching can only be done by the thread library and the kernel isn't really ``thread aware''.

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