Difference between revisions of "Syscall Reduction"

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m (Macdice moved page SyscallRemoval to Syscall Reduction)
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** {{PgCommitURL|701a51fd4e01dbbd02067d8f01905a04bc571131}} for miscellaenous other places (PG13)
 
** {{PgCommitURL|701a51fd4e01dbbd02067d8f01905a04bc571131}} for miscellaenous other places (PG13)
 
** {{PgCommitURL|2fd2effc50824a8775a088435a13f47b7a6f3b94}} for base backup (PG14)
 
** {{PgCommitURL|2fd2effc50824a8775a088435a13f47b7a6f3b94}} for base backup (PG14)
** TODO: Some cases remain in the SLRU code [https://www.postgresql.org/message-id/flat/CA%2BhUKGJ%2BoHhnvqjn3%3DHro7xu-YDR8FPr0FL6LF35kHRX%3D_bUzg%40mail.gmail.com thread]
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** {{PgCommitURL|e2b37d9e7cabc90633c4bd822e1bcfdd1bda44c4}} for SLRU files (PG14)
 
* we use lseek(SEEK_END) to probe the size of relations
 
* we use lseek(SEEK_END) to probe the size of relations
 
** {{PgCommitURL|c5315f4f44843c20ada876fdb0d0828795dfbdf5}} to cache that instead of checking on every block referenced in recovery (PG14)
 
** {{PgCommitURL|c5315f4f44843c20ada876fdb0d0828795dfbdf5}} to cache that instead of checking on every block referenced in recovery (PG14)
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** It sounds like this idea would involve removing the timing race by blocking SIGUSR1, and then atomically unblocking it on Linux with epoll_pwait(), and handling on *BSD/macOS with EVFILT_SIGNAL; you'd probably need the self-pipe fallback path for other systems?
 
** It sounds like this idea would involve removing the timing race by blocking SIGUSR1, and then atomically unblocking it on Linux with epoll_pwait(), and handling on *BSD/macOS with EVFILT_SIGNAL; you'd probably need the self-pipe fallback path for other systems?
 
** SHOW US THE CODE
 
** SHOW US THE CODE
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 +
= fsync =
 +
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* 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? [https://www.postgresql.org/message-id/flat/CA%2BhUKGLJ%3D84YT%2BNvhkEEDAuUtVHMfQ9i-N7k_o50JmQ6Rpj_OQ%40mail.gmail.com thread]
  
 
= disk IO =
 
= disk IO =

Revision as of 13:06, 1 August 2020

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

itimer

  • 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 thread

signals/latches

  • can we make SetLatch(), WaitLatch() more efficient?
    • Andres Freund has described an alternative design that doesn't require the self-pipe trick (writing a byte in the signal handler, reading it later), and doesn't require signals to be sent at all if the recipient isn't sleeping
    • It sounds like this idea would involve removing the timing race by blocking SIGUSR1, and then atomically unblocking it on Linux with epoll_pwait(), and handling on *BSD/macOS with EVFILT_SIGNAL; you'd probably need the self-pipe fallback path for other systems?
    • SHOW US THE CODE

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? thread

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?