Investigate multicore performance drop

[krlmlr]

https://serverfault.com/q/1000690/95758

https://www.sqlite.org/wal.html

https://www.sqlite.org/pragma.html#pragma_synchronous

[DavorJ]

@krlmlr, this was on my todo list for a long time, but due to priorities....

Anyway, I executed your script (https://serverfault.com/q/1000690/95758) on windows with 2 small modifications: Sys.getpid() and 4 cores (since I only have 4 physical). Here are the plots:

Without gc()

With gc()

[DavorJ]

Here one with 2 processes: is a bit more clear:

They differ on each run:

[DavorJ]

Based on your unix graph

it seems that on unix, most of the time nothing happens. As if one process is locking the table, up until the "buffer" is full, then it bursts and unlocks it for an other process to do the same. On windows, you see one thread constantly writing while the others wait. Similar to unix with gc(), which also slows down the process much.