From PostgreSQL wiki
(Redirected from SyscallRemoval)
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.
- we used to do all disk IO with lseek+read/write, but we switched to pread/pwrite
- 3fd2a7932ef0708dda57369bb20c0499d905cc82 for portable interface (PG12)
- c24dcd0cfd949bdf245814c4c2b3df828ee7db36 for main data files and WAL (PG12)
- 0dc8ead46363fec6f621a12c7e1f889ba73b55a9 for WAL reader (PG13)
- 701a51fd4e01dbbd02067d8f01905a04bc571131 for miscellaenous other places (PG13)
- 2fd2effc50824a8775a088435a13f47b7a6f3b94 for base backup (PG14)
- e2b37d9e7cabc90633c4bd822e1bcfdd1bda44c4 for SLRU files (PG14)
- we use lseek(SEEK_END) to probe the size of relations
polling for unexpected postmaster exit
- we used to check if the postmaster had gone away every time through the recovery loop by reading from a pipe
- 9f09529952ac41a10e5874cba745c1c24e67ac79 to use a process exit signal instead, on Linux (PG12)
- f98b8476cd4a19dfc602ab95642ce08e53877d65 to use a process exit signal instead, on FreeBSD (PG12)
- TODO: We could do this on Windows, with help from the signal simulation thread
- TODO: A fallback solution that isn't quite as good but still pretty good for all other operating systems: thread
- we used to include a pipe in the poll() set we generally use for waiting
- we used to create and destroy temporary epoll/kqueue objects frequently
- 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)
- on FreeBSD, we used to call setproctitle() multiple times for every statement
- 1bc180cd2acc55e31b61c4cc9ab4b07670a2566e to switch to setproctitle_fast(), which has no system call (PG12)
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
- 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
- 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
- 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?
- In various places we walk a directory tree recursively, stating every entry to find out if it's a file or a directory. We should use common extensions to avoid the need for that.
- 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?