Regarding postgreSQL performance on DRAM

Hello,

I am a researcher in the Storage and Systems Lab at UT Austin. I am working
on analyzing the performance of PostgreSQL on Persistent Memory. To be more
specific, I am trying to analyze the performance of PostgreSQL-10.5 on a
ramdisk device mounted with the ext4 file system.

My workload is simple. I insert 1 million rows into a table with 100
columns, where each column is 256 bytes in length, and every 10 inserts are
batched into a transaction.

On running strace on all the PostgreSQL processes spawned by the server, I
am observing that the total time taken in system calls is less than 5% of
the total time it takes to do the 1 million inserts. I changed the
configuration file to make PostgreSQL as synchronous as possible, and even
then the time spent in system calls is very low.

When I run the perf tool to check where the majority time is being spent, I
see that there is a function called pglz_compress() where a lot of time is
getting spent and also there is a function called heap_compute_data_size()
where there is significant time spent.

Could I know why so much time is being spent in user space, and how I can
make PostgreSQL more I/O bound than it is now?

Thanks!
- Rohan