Re: Let's make PostgreSQL multi-threaded

Hi,

On 6/7/23 23:37, Andres Freund wrote:
> I think we're starting to hit quite a few limits related to the process model,
> particularly on bigger machines. The overhead of cross-process context
> switches is inherently higher than switching between threads in the same
> process - and my suspicion is that that overhead will continue to
> increase. Once you have a significant number of connections we end up spending
> a *lot* of time in TLB misses, and that's inherent to the process model,
> because you can't share the TLB across processes.

Another problem I haven't seen mentioned yet is the excessive kernel
memory usage because every process has its own set of page table entries
(PTEs). Without huge pages the amount of wasted memory can be huge if
shared buffers are big.

For example with 256 GiB of used shared buffers a single process needs
about 256 MiB for the PTEs (for simplicity I ignored the tree structure
of the page tables and just took the number of 4k pages times 4 bytes
per PTE). With 512 connections, which is not uncommon for machines with
many cores, a total of 128 GiB of memory is just spent on page tables.

We used non-transparent huge pages to work around this limitation but
they come with plenty of provisioning challenges, especially in cloud
infrastructures where different services run next to each other on the
same server. Transparent huge pages have unpredictable performance
disadvantages. Also if some backends only use shared buffers sparsely,
memory is wasted for the remaining, unused range inside the huge page.

--
David Geier
(ServiceNow)