On Tue, Oct 30, 2018 at 6:16 AM Andres Freund <and...@anarazel.de> wrote:
> Hi, > > Magnus cornered me at pgconf.eu and asked me whether I could prototype > the "barriers" I'd been talking about in the online checksumming thread. > > The problem there was to make sure that all processes, backends and > auxiliary processes have seen the new state of checksums being enabled, > and aren't currently in the process of writing a new page out. > > The current prototype solves that by requiring a restart, but that > strikes me as a far too large hammer. > > The attached patch introduces "global barriers" (name was invented in a > overcrowded hotel lounge, so ...), which allow to wait for such a change > to be absorbed by all backends. > > I've only tested the code with gdb, but that seems to work: > > p WaitForGlobalBarrier(EmitGlobalBarrier(GLOBBAR_CHECKSUM)) > > waits until all backends (including bgwriter, checkpointers, walwriters, > bgworkers, ...) have accepted interrupts at least once. Multiple such > requests are coalesced. > > I decided to wait until interrupts are actually process, rather than > just the signal received, because that means the system is in a well > defined state. E.g. there's no pages currently being written out. > > For the checksum enablement patch you'd do something like; > > EnableChecksumsInShmemWithLock(); > WaitForGlobalBarrier(EmitGlobalBarrier(GLOBBAR_CHECKSUM)); > > and after that you should be able to set it to a perstistent mode. > > > I chose to use procsignals to send the signals, a global uint64 > globalBarrierGen, and per-backend barrierGen, barrierFlags, with the > latter keeping track which barriers have been requested. There likely > seem to be other usecases. > > > The patch definitely is in a prototype stage. At the very least it needs > a high-level comment somewhere, and some of the lower-level code needs > to be cleaned up. > > One thing I wasn't happy about is how checksum internals have to absorb > barrier requests - that seems unavoidable, but I'd hope for something > more global than just BufferSync(). > > > Comments? > Finally getting around to playing with this one and it unfortunately doesn't apply anymore (0003). I think it's just a matter of adding those two rows though, right? That is, it's not an actual conflict it's just something else added in the same place? -- Magnus Hagander Me: https://www.hagander.net/ <http://www.hagander.net/> Work: https://www.redpill-linpro.com/ <http://www.redpill-linpro.com/>
