Ubuntu 22.04, Linux kernels and me

This is a summary of interesting performance issues I have had with the HWE kernels (6.5.x) on Ubuntu 22.04, mostly this year. This is mostly a note to my future self.

  • Use ext4 instead of XFS
    • I have used XFS since pre-2010 because it avoided a few perf problems that show up in the ext family. But I finally found a reason to switch to ext4. The problem is that with 6.5 kernels (when HWE is used) there are perf problems and many error messages (WQ_UNBOUND, CPU hogging). Some details are here. I am curious if XFS needs a few updates to match whatever changed in the 6.5 kernels.
  • Switch from schedutil to performance CPU frequency governor
    • This issue hurt MyRocks a lot more than Postgres or InnoDB. I don't know the root cause but MyRocks perf was lousy with the schedutil CPU frequency governor. See here for more details from me, and this LWN post might be relevant.
  • readahead requests are dropped
    • RocksDB changed how readahead is done when O_DIRECT is not used and the change uses the readahead call. It looks like readahead requests are ignored when the request size is larger than the value of max_sectors_kb. Alas, to figure out the value of max_sectors_kb you must read /sys/block/$device/queue/max_sectors_kb and I prefer to avoid that (whether I have to do it, or RocksDB code does it). Surely there is a better way to handle this.


Comments

Post a Comment

Popular posts from this blog

Fixing bug 109595 makes MySQL almost 4X faster on the Insert Benchmark

Image
MySQL 8.0.35 includes a fix for bug 109595  and with that fix the QPS is almost 4X larger on the read+write benchmark steps compared to MySQL 8.0.34. Thank you to MySQL for fixing this quickly. I reported the bug in January of 2023. I have been aware of the performance problem for years, but didn't spend time debugging it until this year. I assume this problem was limited to InnoDB because I did not see problems with MyRocks. The chart below shows relative QPS which is (QPS for MySQL 8.0.35) / (QPS for MySQL 8.0.34). When the value is larger than 1.0 then MySQL 8.0.35 is faster than 8.0.34. And below the relative QPS is almost 4.0 on the read+write benchmark steps: q100, q500, q1000. The Insert Benchmark is explained here .
Read more

Postgres versions 11, 12, 13, 14, 15, and 16 vs sysbench with a medium server

Image
This provides additional results for Postgres versions 11 through 16 vs Sysbench on a medium server. My previous post is here . The goal is to document how performance changes over time with a focus on CPU overheads. tl;dr Postgres avoids performance regressions from old to new versions. This is excellent. Postgres 16.0 is mostly faster than 11.21 on read-only benchmark steps Postgres 16.0 and 11.21 have similar throughput on read-write and write-only benchmark steps Builds I compiled Postgres 11.21, 12.16, 13.12, 14.9, 15.2, 15.3, 15.4 and 16.0. The builds are named  o3_native_lto  which is shorthand for using: -O3 -march=native -mtune=native -flto. Benchmark I used sysbench and my usage is  explained here . Postgres was configured to cache all tables. This benchmark used a c2-standard-30 server from GCP with 15 cores, hyperthreads disabled, 120G of RAM, Ubuntu 22.04 and 1.5TB of NVMe SSD with XFS (SW RAID 0 over 4 local devices). The benchmark is run with 8 clients and 4 table with 2
Read more

Postgres vs MySQL: the impact of CPU overhead on performance

Image
This post documents how MySQL and Postgres performance has changed over time using sysbench. It compares QPS for Postgres 11.21 vs MySQL 5.6.21 and then Postgres 16.0 vs MySQL 8.0.34. The focus here is on CPU overhead and for that a workload that is low-concurrency, CPU-bound and cached is sufficient. However a full comparison would also include results for workloads with more concurrency and workloads that are IO-bound. The benchmark was explained in a series of posts - see  part 1 ,  part 2 ,  part 3 ,  part 4  and part 5  and the results here are from my small server. Usually I avoid direct comparisons between Postgres and MySQL but I am doing this because I care about the future of MySQL and the CPU regressions in it over time have become a problem. tl;dr Postgres 11.21 and MySQL 5.6.21 have similar QPS on most sysbench tests Postgres 16.0 gets much more QPS than MySQL 8.0.34 on most sysbench tests The problem for MySQL is new CPU overhead Results The spreadsheet with all of the nu
Read more