Thursday, October 28, 2010

Impact of changes in the run-time environment of MySQL 5.5

When experimenting with the Google patches a few years ago I found that tcmalloc had a fairly large impact on performance of the MySQL Server. So the question I asked myself was obviously whether the libc malloc have regained some of the lost territory (also had regressions of major drop in performance in certain libc versions). Using tcmalloc used to have a 5-10% positive impact on performance, the matter of the fact is that this gain remains. I lost 8-10% in performance on all thread counts tested (16, 32, 64, 128 and 256) by not using tcmalloc in running Sysbench RW.

The experiments are performed on a fairly high-end x86 box with 4 sockets. I run the sysbench program on the same machine as the MySQL Server runs on. So this means that it's interesting to check whether I get better performance by locking the MySQL Server to 3 of the 4 sockets and let sysbench use its own socket compared to not control CPU usage at all.

What I discovered is a mixed picture. Performance when locking to CPU's was much more stable although top performance was better without locking. Performance at 16 threads improved 3% and at 32 threads it improved 7%. But at higher thread counts the performance was better for the locked scenario, 10% at 64 threads and 4% at 256 threads. I used the Linux feature taskset to lock the MySQL Server and Sysbench to certain CPUs.

So the conclusion is that locking to CPUs gives a more stable environment. When the number of threads increases the scheduler is allowed to use more CPUs than what is beneficial for MySQL execution. I've seen this also in other experiments that making sure that MySQL reuses the CPU caches as much as possible is very important for performance. Thus when MySQL competes with other programs on use of CPUs and there are many concurrent MySQL threads it's usually not beneficial to performance since the CPU caches will be too cold.

Using Unix sockets instead of TCP/IP sockets is very beneficial for MySQL performance still. I haven't made any recent experiments in this area but the difference is definitely significant. I have also seen OS bottlenecks sometimes appear when using TCP/IP sockets. This is an area for further investigation which I have had on my TODO list for a while. It's also interesting to experiment with different communication mechanisms when the Sysbench program and MySQL runs on different computers. However this is for future testing.

Finding the optimum configuration of MySQL 5.5 running Sysbench

Sysbench is a commonly used benchmark tool to discover ways to improve MySQL performance. It is certainly not representative for every application, but it's still a useful tool for finding bottlenecks in the MySQL code.

In MySQL 5.5 a great number of new scalability improvements have been developed. Some of these will always be active and some of them requires using new configuration parameters.

In order to assist users of MySQL I am performing a fairly extensive benchmark series where I test the various configuration parameters that have an effect on running MySQL/InnoDB using Sysbench.

The parameters can be categorized into:
1) Changes of run-time environment
2) Compile time parameters (including choice of compiler)
3) Configuration parameters for MySQL
4) Configuration parameters for InnoDB

Finally there is also a set of parameters one can use to affect the execution of sysbench itself to have it behave differently. It's possible to have Sysbench use secondary index instead of primary key index, it's possible to let the table be partitioned and it's possible to run sysbench using multiple tables instead of only one (it's very syntetic to only have one table in a system which gets all the queries).

The parameters that I have found to be important to consider for performance of MySQL 5.5 when running sysbench are:
1.1) Use of tcmalloc
1.2) Use of taskset (lock MySQL to certain CPUs)
1.3) Affecting memory allocation by use of numactl
1.4) Connecting to MySQL through socket or using TCP/IP
2.1) Use of gcc compiler with platform-specific optimisations
2.2) Use --with-fast-mutexes flag when compiling MySQL
2.3) Choice of compiler (gcc, Open64, icc, SunStudio, ..)
2.4) Using feedback compilation in compilation
2.5) Compiling sysbench itself with platform specific optimisations
3.1) Choice of transaction isolation (READ COMMITTED or REPEATABLE READ)
3.2) Use of large pages through setting --large-pages=ON
3.3) Compiling with Performance Schema activated
3.4) Running with Performance Schema activated through --perfschema
3.5) Deactivating Query Cache completely through --query_cache_size=0
and --query_cache_type=0
4.1) Using new file format Barracuda or old format Antelope through
4.2) Setting --innodb_stats_on_metadata to ON/OFF
4.3) Deactivating InnoDB doublewrite through --skip-innodb_doublewrite
4.4) Setting innodb-change-buffering to none/insert/all and so forth
4.5) Setting number of buffer pool instances using innodb-buffer-pool-instances=x
4.6) Setting InnoDB log file size using innodb-log-file-size=x
4.7) Setting InnoDB log buffer size using innodb-log-buffer-size=x
4.8) Setting InnoDB flush method (default fsync, O_DIRECT, O_DSYNC) using
4.9) Setting InnoDB to use file per table using --innodb_file_per_table
4.10) Deactivating InnoDB adaptive hash index using
4.11) Adapting read-ahead using --innodb_read_ahead=x
4.12) Using InnoDB max purge lag through --innodb_max_purge_lag=x
4.13) Using InnoDB purge thread through --innodb_purge_thread=1
4.14) Changing behaviour of InnoDB spin loops by changing
--innodb_sync_spin_loops=x and --innodb_spin_wait_delay=x
4.15) Changing InnoDB IO capacity by setting --innodb-io-capacity=x
4.16) Setting InnoDB buffer pool size through --innodb-buffer-pool-size=x
4.17) Setting InnoDB dirty pages percent through --innodb_dirty_pages_pct=x
4.18) Setting InnoDB old blocks percentage through --innodb_old_blocks_pct=x
4.19) Activating support for InnoDB XA through --innodb_support_xa=FALSE/TRUE
4.20) Activating InnoDB thread concurrency using
4.21) Setting InnoDB commit mechanism through
4.22) Setting number of read and write IO threads through
--innodb-read-io-threads=x and --innodb-write-io-threads=x

This makes for a total of 36 parameters that can be tuned to affect sysbench performance. Most of these parameters have only a limited impact on performance but some of them can have a large impact and there is also a number of them where most reasonable values are ok, but if a bad value is used it can have a major impact on performance.

In addition the problem of finding the optimal values is a multi-dimensional search. So changing one parameter might very well affect the impact of other parameters. So my approach will be to start with a reasonable baseline configuration, then try each parameter and see how it affects the outcome, come to a new baseline by changing the most important parameter. Next step is to restart tests varying all the parameters which have had a measurable impact on performance. Using this method we should be able to find an optimum to a reasonable degree. Finding the absolute optimum is probably more or less practically impossible.

It's important here to understand that the MySQL defaults can sometimes have really bad values for performance. The reason is that MySQL defaults have been choosen mainly to make MySQL run on any HW platform and use very small resources. Remember that the normal use of MySQL isn't running sysbench on a 32-core server :) So it is important to consider all of these parameters to get the optimal performance of the MySQL Server.

I plan to write about these variables and how they affect sysbench performance in a number of upcoming blogs. I might also do a similar run with MySQL using the DBT2 benchmark. So hopefully after finishing these test runs I have found a reasonable optimum configuration to run sysbench and probably also the configuration parameters that needs changing when running DBT2 instead.