Re: [PoC] Improve dead tuple storage for lazy vacuum

On Thu, Dec 22, 2022 at 10:00 PM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com>
wrote:

> If the value is a power of 2, it seems to work perfectly fine. But for
> example if it's 700MB, the total memory exceeds the limit:
>
> 2*(1+2+4+8+16+32+64+128) = 510MB (72.8% of 700MB) -> keep going
> 510 + 256 = 766MB -> stop but it exceeds the limit.
>
> In a more bigger case, if it's 11000MB,
>
> 2*(1+2+...+2048) = 8190MB (74.4%)
> 8190 + 4096 = 12286MB
>
> That being said, I don't think they are not common cases. So the 75%
> threshold seems to work fine in most cases.

Thinking some more, I agree this doesn't have large practical risk, but
thinking from the point of view of the community, being loose with memory
limits by up to 10% is not a good precedent.

Perhaps we can be clever and use 75% when the limit is a power of two and
50% otherwise. I'm skeptical of trying to be clever, and I just thought of
an additional concern: We're assuming behavior of the growth in size of new
DSA segments, which could possibly change. Given how allocators are
typically coded, though, it seems safe to assume that they'll at most
double in size.

--
John Naylor
EDB: http://www.enterprisedb.com