Re: Direct I/O

On 11/1/22 2:36 AM, Thomas Munro wrote:

> Hi,
>
> Here is a patch to allow PostgreSQL to use $SUBJECT. It is from the

This is exciting to see! There's two other items to add to the TODO list
before this would be ready for production:

1) work_mem. This is a significant impediment to scaling shared buffers
the way you'd want to.

2) Clock sweep. Specifically, currently the only thing that drives
usage_count is individual backends running the clock hand. On large
systems with 75% of memory going to shared_buffers, that becomes a very
significant problem, especially when the backend running the clock sweep
is doing so in order to perform an operation like a b-tree page split. I
suspect it shouldn't be too hard to deal with this issue by just having
bgwriter or another bgworker proactively ensuring some reasonable number
of buffers with usage_count=0 exist.

One other thing to be aware of: overflowing as SLRU becomes a massive
problem if there isn't a filesystem backing the SLRU. Obviously only an
issue if you try and apply DIO to SLRU files.