Syscall Reduction

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In the past few releases we've done a lot of work to remove unnecessary system calls made by PostgreSQL, but there are plenty more opportunities. Here is a log and todo list about that.

lseek

polling for unexpected postmaster exit

epoll/kqueue setup/teardown

shm_open

  • for parallel query, we allocate and free large chunks of temporary shared memory using POSIX shmem facilities
    • 84b1c63ad41872792d47e523363fce1f0e230022 to preallocate a region up front and recycle that, rather and creating and destroying memory for every parallel query, though it's not enabled by default (PG14)

process titles

socket wait in request/response protocol

  • currently we often do an extra non-blocking recvfrom() that fails with EAGAIN, followed by epoll_wait()/kevent() after sending a response and then waiting for the next query to arrive
    • can we get rid of that extra system call? go straight to wait, if we predict that is most likely?
    • with a local benchmark and a smallish number of threads, often the recvfrom() succeeds due to good timing, but in the read world with many threads and context switches or remote clients it's usually EAGAIN and then sleep -- hence desire for something adaptive

setitimer

  • the statement_timeout, the deadlock detector and various other things use SIGALARM, but setting that up for every statement is known to cost several percent performance in benchmarks
    • TODO: can we skip resetting the timer if there is already a shorter one installed, and then reset it when that one expires? then perhaps we can call itimer() very infrequently while using the statement_timeout feature and other things like that
    • discussion and draft patch

signals/latches

  • we make SetLatch(), WaitLatch() more efficient?
    • not sending signals when the other side isn't even waiting avoids many signals (12% on make check)
    • we don't need the self-pipe trick on modern systems
    • prototype patches exist

fsync

  • TODO: when SLRU files (CLOG, ...) are written back to disk, they are always fsync'd, causing synchronous I/O stalls; move that work to the checkpointer, so the fsyncs are not in a foreground process affecting query latency, and are deferred and collapsed?

stat

disk IO

  • Synchronous block-at-a-time IO should be replaced with async scatter/gather, but that's a larger architectural project that doesn't belong on this list of micro-optimisation scale improvements. Work is in progress...
  • Relation extension is currently done by writing zeroes; would fallocate() be better?