I don't have an objection, but I do wonder: can one (or perhaps a few) queries/workloads be provided where threading would be significantly beneficial?
(some material there could help get people on-board with the idea and potentially guide many of the smaller questions that arise along the way) On Mon, 5 Jun 2023 at 15:52, Heikki Linnakangas <hlinn...@iki.fi> wrote: > > I spoke with some folks at PGCon about making PostgreSQL multi-threaded, > so that the whole server runs in a single process, with multiple > threads. It has been discussed many times in the past, last thread on > pgsql-hackers was back in 2017 when Konstantin made some experiments [0]. > > I feel that there is now pretty strong consensus that it would be a good > thing, more so than before. Lots of work to get there, and lots of > details to be hashed out, but no objections to the idea at a high level. > > The purpose of this email is to make that silent consensus explicit. If > you have objections to switching from the current multi-process > architecture to a single-process, multi-threaded architecture, please > speak up. > > If there are no major objections, I'm going to update the developer FAQ, > removing the excuses there for why we don't use threads [1]. And we can > start to talk about the path to get there. Below is a list of some > hurdles and proposed high-level solutions. This isn't an exhaustive > list, just some of the most obvious problems: > > # Transition period > > The transition surely cannot be done fully in one release. Even if we > could pull it off in core, extensions will need more time to adapt. > There will be a transition period of at least one release, probably > more, where you can choose multi-process or multi-thread model using a > GUC. Depending on how it goes, we can document it as experimental at first. > > # Thread per connection > > To get started, it's most straightforward to have one thread per > connection, just replacing backend process with a backend thread. In the > future, we might want to have a thread pool with some kind of a > scheduler to assign active queries to worker threads. Or multiple > threads per connection, or spawn additional helper threads for specific > tasks. But that's future work. > > # Global variables > > We have a lot of global and static variables: > > $ objdump -t bin/postgres | grep -e "\.data" -e "\.bss" | grep -v > "data.rel.ro" | wc -l > 1666 > > Some of them are pointers to shared memory structures and can stay as > they are. But many of them are per-connection state. The most > straightforward conversion for those is to turn them into thread-local > variables, like Konstantin did in [0]. > > It might be good to have some kind of a Session context struct that we > pass everywhere, or maybe have a single thread-local variable to hold > it. Many of the global variables would become fields in the Session. But > that's future work. > > # Extensions > > A lot of extensions also contain global variables or other things that > break in a multi-threaded environment. We need a way to label extensions > that support multi-threading. And in the future, also extensions that > *require* a multi-threaded server. > > Let's add flags to the control file to mark if the extension is > thread-safe and/or process-safe. If you try to load an extension that's > not compatible with the server's mode, throw an error. > > We might need new functions in addition _PG_init, called at connection > startup and shutdown. And background worker API probably needs some changes. > > # Exposed PIDs > > We expose backend process PIDs to users in a few places. > pg_stat_activity.pid and pg_terminate_backend(), for example. They need > to be replaced, or we can assign a fake PID to each connection when > running in multi-threaded mode. > > # Signals > > We use signals for communication between backends. SIGURG in latches, > and SIGUSR1 in procsignal, for example. Those primitives need to be > rewritten with some other signalling mechanism in multi-threaded mode. > In principle, it's possible to set per-thread signal handlers, and send > a signal to a particular thread (pthread_kill), but I think it's better > to just rewrite them. > > We also document that you can send SIGINT, SIGTERM or SIGHUP to an > individual backend process. I think we need to deprecate that, and maybe > come up with some convenient replacement. E.g. send a message with > backend ID to a unix domain socket, and a new pg_kill executable to send > those messages. > > # Restart on crash > > If a backend process crashes, postmaster terminates all other backends > and restarts the system. That's hard (impossible?) to do safely if > everything runs in one process. We can continue have a separate > postmaster process that just monitors the main process and restarts it > on crash. > > # Thread-safe libraries > > Need to switch to thread-safe versions of library functions, e.g. > uselocale() instead of setlocale(). > > The Python interpreter has a Global Interpreter Lock. It's not possible > to create two completely independent Python interpreters in the same > process, there will be some lock contention on the GIL. Fortunately, the > python community just accepted https://peps.python.org/pep-0684/. That's > exactly what we need: it makes it possible for separate interpreters to > have their own GILs. It's not clear to me if that's in Python 3.12 > already, or under development for some future version, but by the time > we make the switch in Postgres, there probably will be a solution in > cpython. > > At a quick glance, I think perl and TCL are fine, you can have multiple > interpreters in one process. Need to check any other libraries we use. > > > [0] > https://www.postgresql.org/message-id/flat/9defcb14-a918-13fe-4b80-a0b02ff85527%40postgrespro.ru > > [1] > https://wiki.postgresql.org/wiki/Developer_FAQ#Why_don.27t_you_use_raw_devices.2C_async-I.2FO.2C_.3Cinsert_your_favorite_wizz-bang_feature_here.3E.3F > > -- > Heikki Linnakangas > Neon (https://neon.tech) > >
