Issue: size estimation for buffers (estimateMemoryForCompaction) seems to return too small values when called for small maps/vectors

[markusbattarbee]

No description provided.

[kstppd]

Trying to reproduce this and I cannot seem to get it to complain. Do you have any specific example either in Vlasiator or in the units tests?

[kstppd]

Any news on this?

[markusbattarbee]

After switching to the loop-interfaces for compactions, I no longer use this function. So I think the issue is still probably there.

It cropped up with small block counts, so that's where testing should go - also, I always initialize hashmaps with at least sizepower 7, so perhaps it's right down at that value.

In an older Vlasiator source file, I had these lines:

size_t bytesNeeded = split::tools::estimateMemoryForCompaction((size_t)std::pow(2,BlocksRequiredMap_sizepower+4));
const vmesh::LocalID nBlocksRequired = BlocksRequiredMap->extractAllKeys(*BlocksRequired,compaction_buffer[thread_id],bytesNeeded,stream,false);

where the +4 is exactly to get rid of this issue. This map was grown on-device, so it may have had a fill rate greater than 0.5.