On Mon, 30 Sept 2024 at 16:49, Andres Freund <and...@anarazel.de> wrote: > On 2024-09-17 11:08:19 -0700, Noah Misch wrote: > > > - I am worried about the need for bounce buffers for writes of checksummed > > > buffers. That quickly ends up being a significant chunk of memory, > > > particularly when using a small shared_buffers with a higher than > > > default > > > number of connection. I'm currently hacking up a prototype that'd > > > prevent us > > > from setting hint bits with just a share lock. I'm planning to start a > > > separate thread about that. > > > > AioChooseBounceBuffers() limits usage to 256 blocks (2MB) per MaxBackends. > > Doing better is nice, but I don't consider this a blocker. I recommend > > dealing with the worry by reducing the limit initially (128 blocks?). Can > > always raise it later. > > On storage that has nontrivial latency, like just about all cloud storage, > even 256 will be too low. Particularly for checkpointer. > > Assuming 1ms latency - which isn't the high end of cloud storage latency - 256 > blocks in flight limits you to <= 256MByte/s, even on storage that can have a > lot more throughput. With 3ms, which isn't uncommon, it's 85MB/s.
FYI, I think you're off by a factor 8, i.e. that would be 2GB/sec and 666MB/sec respectively, given a normal page size of 8kB and exactly 1ms/3ms full round trip latency: 1 page/1 ms * 8kB/page * 256 concurrency = 256 pages/ms * 8kB/page = 2MiB/ms ~= 2GiB/sec. for 3ms divide by 3 -> ~666MiB/sec. Kind regards, Matthias van de Meent Neon (https://neon.tech)
