Re: PG15 beta1 sort performance regression due to Generation context change

В Вт, 24/05/2022 в 17:39 -0700, Andres Freund пишет:
> 
> A variation on your patch would be to only store the offset to the block
> header - that should always fit into 32bit (huge allocations being their own
> block, which is why this wouldn't work for storing an offset to the
> context). With a bit of care that'd allow aset.c to half it's overhead, by
> using 4 bytes of space for all non-huge allocations.  Of course, it'd increase
> the cost of pfree() of small allocations, because AllocSetFree() currently
> doesn't need to access the block for those. But I'd guess that'd be outweighed
> by the reduced memory usage.

I'm +1 for this.

And with this change every memory context kind can have same header:

    typedef struct MemoryChunk {
#ifdef MEMORY_CONTEXT_CHECKING
    Size   requested_size;
#endif
        uint32 encoded_size;    /* encoded allocation size */
        uint32 offset_to_block; /* backward offset to block header */
    }

Allocated size always could be encoded into uint32 since it is rounded
for large allocations (I believe, large allocations certainly rounded
to at least 4096 bytes):

    encoded_size = size < (1u<<31) ? size : (1u<<31)|(size>>12);
    /* and reverse */
    size = (encoded_size >> 31) ? ((Size)(encoded_size<<1)<<12) :
       (Size)encoded_size;

There is a glitch with Aset since it currently reuses `aset` pointer
for freelist link. With such change this link had to be encoded in
chunk-body itself instead of header. I was confused with this, since
there are valgrind hooks, and I was not sure how to change it (I'm
not good at valgrind hooks). But after thinking more about I believe
it is doable.


regards

-------

Yura Sokolov