Hi, On 2022-11-15 23:36:53 +1300, David Rowley wrote: > On Tue, 15 Nov 2022 at 11:11, Andres Freund <and...@anarazel.de> wrote: > > Couldn't we reduce the amount of over-allocation by a small amount by > > special > > casing the already-aligned case? That's not going to be relevant for page > > size > > aligne allocations, but for smaller alignment values it could matter. > > I don't quite follow this. How can we know the allocation is already > aligned without performing the allocation? To perform the allocation > we must tell palloc what size to allocate. So, we've already wasted > the space by the time we can tell if the allocation is aligned to what > we need.
What I mean is that we perhaps could over-allocate by a bit less than alignto + sizeof(MemoryChunk) If the value returned by the underlying memory context is already aligned to the correct value, we can just return it as-is. We already rely on memory context returning MAXIMUM_ALIGNOF aligned allocations. Adding the special case, I think, means that the we could safely over-allocate by "only" alignto + sizeof(MemoryChunk) - MAXIMUM_ALIGNOF Which would be a reasonable win for small allocations with a small > MAXIMUM_ALIGNOF alignment. But I don't think that'll be a very common case? > Aside from that, there's already a special case for alignto <= > MAXIMUM_ALIGNOF. But we know no palloc will ever return anything > aligned less than that in all cases, which is why that can work. Yep. Greetings, Andres Freund
