Evaluating Checkpointing in PostgreSQL

Continuing with the checkpointing topic I started a month ago with my blog post MongoDB Checkpointing Woes , this time I want to review how PostgreSQL performs in this area. After this, I will be taking a look at MySQL and MariaDB. If anything, it will be fair not only to complain about MongoDB but to review how other databases handle it, as well.

Benchmark

To evaluate PostgreSQL I will use a not identical but similar scenario: using sysbench-tpcc with 1000 Warehouses, and as with sysbench you can produce tpcc-like workload for PostgreSQL:

Sysbench-tpcc Supports PostgreSQL (No, Really This Time)

Tuning PostgreSQL for sysbench-tpcc

The hardware I use is:

System | Supermicro; SYS-F619P2-RTN; v0123456789 (Other)
   Platform | Linux
    Release | Ubuntu 18.04.4 LTS (bionic)
     Kernel | 5.3.0-42-generic
Architecture | CPU = 64-bit, OS = 64-bit
  Threading | NPTL 2.27
    SELinux | No SELinux detected
Virtualized | No virtualization detected
# Processor ##################################################
 Processors | physical = 2, cores = 40, virtual = 80, hyperthreading = yes
     Models | 80xIntel(R) Xeon(R) Gold 6230 CPU @ 2.10GHz
     Caches | 80x28160 KB
# Memory #####################################################
      Total | 187.6G

With the storage on SATA SSD INTEL SSDSC2KB960G8 (Intel Enterprise-grade SSD D3-S4510).

The PostgreSQL config is:

shared_buffers = '140GB'
work_mem = '4MB'
random_page_cost = '1'
maintenance_work_mem = '2GB'
 
wal_level = 'replica'
max_wal_senders = '3'
 
synchronous_commit = 'on'
seq_page_cost = '1'
synchronous_commit = 'on'
 
checkpoint_completion_target = '0.9'
checkpoint_timeout = '900'
 
max_wal_size = '20GB'
min_wal_size = '12GB'
 
autovacuum_vacuum_scale_factor = '0.4'
effective_cache_size = '200GB'
bgwriter_lru_maxpages = '1000'
bgwriter_lru_multiplier = '10.0'
logging_collector = 'ON'
wal_compression = 'ON'
log_checkpoints = 'ON'
archive_mode = 'OFF'
full_page_writes = 'ON'
fsync = 'ON'

The short settings overview:

• Data will totally fit into memory (The datasize is ~100GB, memory on the server is 188GB, and we allocate 140GB for PostgreSQL shared buffers.)
• The workload on storage will be mostly write-intensive (reads will be done from memory), with full ACID-compliant and data safe settings on PostgreSQL.
• I will vary log size from 1GB to 100GB, to see the effect of log sizes on checkpointing.

The benchmark command line is:

./tpcc.lua --pgsql-user=sbtest --pgsql-password=sbtest --pgsql-db=sbtest --time=3600 --threads=56 --report-interval=1 --tables=10 --scale=100 --use_fk=0 --trx_level=RC --db-driver=pgsql --report_csv=yes run 

This means that the benchmark will run for 1 hour, with reporting throughput every 1 sec.

Results

Let’s see what results I’ve got with this setup:

That’s an interesting pattern!

Although there are no drops to the floor, we see a saw-like pattern, where throughput raises to ~8000 tps and then drops to ~3000tps (that’s 2.6 times drop!).

It was suggested to check how PostgreSQL would perform with full_page_writes = 'OFF' (this is not a data-safe setting and I would not recommend to use it in production!)

Results with full_page_writes = ‘OFF’

This seems to improve the saw-like pattern, but there are micro-drops that are concerning.

If we zoom in only to 50GB WAL size, we can see it in detail:

I would be interested to hear ideas on how PostgreSQL results in 1-sec resolution can be improved! If you are interested in the raw results and notebooks, it is available here in GitHub .