Recent attacks, dubbed Meltdown and Spectre, have had a significant impact on various data processors and data security. In response, processors require modifications to the operating system; however, those modifications can be expensive and lead to potential slowdown in the applications and services. The following will explore the performance impact of the Meltdown workaround in the Linux kernel specific to MariaDB Server.

Test Environment

All tests were run on our benchmark machine: 2x 8 core Intel Xeon E5 v3 (Haswell) running Ubuntu Server LTS. We compared kernels 4.4.0-104 and 4.4.0-109. 4.4.0-104 is the last Ubuntu kernel in the 4.4.0 line without the KPTI patch. KPTI stands for Kernel Page Tabe Isolation and is meant as a remedy for the Meltdown attack.

The benchmark is sysbench 1.0 OLTP in read-only and read/write mode. The MariaDB Server version used for this benchmark is MariaDB Server 10.2.11 (GA). MariaDB Server was running mostly with defaults. The InnoDB Buffer Pool was configured to large (all reads coming from memory). The durability was for production (flush-log-at-trx-commit=1). Storage in this machine is SSD.

Since the performance often depends on the malloc implementation, a system function that allocates memory for the program, I ran two series of tests. One with the default glibc malloc and a second with tcmalloc (preloaded with the –malloc-lib option for mysqld_safe).

Results

The numbers shown below are queries per second. Note: higher is better.

OLTP read-only

The impact of KPTI seems to be much higher at high concurrency. Tcmalloc delivers not only better results, but also the impact of KPTI is somewhat lessened.

OLTP read/write

For read/write workload the KPTI impact is even higher at high thread counts. Again tcmalloc seems to deliver better results.

Conclusion

The Meltdown patch in the Linux kernel reduces the performance of MariaDB Server by 5% and, in some cases, more than 10%, depending on workload and concurrency. The performance impact is higher with high concurrency. In that case, it may help to try running MariaDB Server with a preloaded high-concurrency memory allocator like tcmalloc, jemalloc or other allocator of choice.

Additional Resources

There are some reports from third parties about KPTI regression for MySQL or general performance impact from Redhat that are worth reviewing. Their results are in line with what we found.