Re: Changing shared_buffers without restart

> On Thu, Apr 17, 2025 at 02:21:07PM GMT, Konstantin Knizhnik wrote:
>
> 1. Performance of Postgres CLOCK page eviction algorithm depends on number
> of shared buffers. My first native attempt just to mark unused buffers as
> invalid cause significant degrade of performance

Thanks for sharing!

Right, but it concerns the case when the number of shared buffers is
high, independently from whether it was changed online or with a
restart, correct? In that case it's out of scope for this patch.

> 2. There are several data structures i Postgres which size depends on number
> of buffers.
> In my patch I used in some cases dynamic shared buffer size, but if this
> structure has to be allocated in shared memory then still maximal size has
> to be used. We have the buffers themselves (8 kB per buffer), then the main
> BufferDescriptors array (64 B), the BufferIOCVArray (16 B), checkpoint's
> CkptBufferIds (20 B), and the hashmap on the buffer cache (24B+8B/entry).
> 128 bytes per 8kb bytes seems to  large overhead (~1%) but but it may be
> quote noticeable with size differences larger than 2 orders of magnitude:
> E.g. to support scaling to from 0.5Gb to 128GB , with 128 bytes/buffer we'd
> have ~2GiB of static overhead on only 0.5GiB of actual buffers.

Not sure what do you mean by using a maximal size, can you elaborate.

In the current patch those structures are allocated as before, except
each goes into a separate segment -- without any extra memory overhead
as far as I see.

> 3. `madvise` is not portable.

The current implementation doesn't rely on madvise so far (it might for
shared memory shrinking), but yeah there are plenty of other not very
portable things (MAP_FIXED, memfd_create). All of that is mentioned in
the corresponding patches as a limitation.