Here is a patch to reduce the size of PageFreeSpaceInfo on 64bit platform.
We will utilize maintenance_work_mem twice with the patch.

The sizeof(PageFreeSpaceInfo) is 16 bytes there because the type of 'avail'
is 'Size', that is typically 8 bytes and needs to be aligned in 8-byte bounds.
I changed the type of the field to uint32. We can store the freespace with
uint16 at smallest, but the alignment issue throws it away.

If we need to store freespace more compactly rather than calculation speed,
it might be good to use FSMPageData instead of PageFreeSpaceInfo, but I did
not change it in this patch.

Index: src/include/storage/freespace.h
--- src/include/storage/freespace.h     (head)
+++ src/include/storage/freespace.h     (fixed)
@@ -24,7 +24,7 @@
 typedef struct PageFreeSpaceInfo
        BlockNumber blkno;                      /* which page in relation */
-       Size            avail;                  /* space available on this page 
+       uint32          avail;                  /* space available on this page 
 } PageFreeSpaceInfo;

ITAGAKI Takahiro
NTT Open Source Software Center

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