Re: Oversight in slab.c SlabContextCreate(), initial memory allocation size is not populated to context->mem_allocated

On Fri, Jul 29, 2022 at 12:43:45PM -0400, Reid Thompson wrote:
> slab.c
> does not in SlabContextCreate(). Is this intentional, it seems to be an
> oversight to me.
> 
>     /* Finally, do the type-independent part of context creation */   
>     MemoryContextCreate((MemoryContext) slab,                         
>                         T_SlabContext,                                
>                         &SlabMethods,                                 
>                         parent,                                       
>                         name);                                        
>
             
 
>     return (MemoryContext) slab;                                      
> }                      

IIUC this is because the header is tracked separately from the first
regular block, unlike aset.c.  See the following comment:

    /*
     * Allocate the context header.  Unlike aset.c, we never try to combine
     * this with the first regular block; not worth the extra complication.
     */

You'll also notice that the "reset" and "free" functions in aset.c and
generation.c have special logic for "keeper" blocks.  Here is a relevant
comment from AllocSetReset():

 * Actually, this routine has some discretion about what to do.
 * It should mark all allocated chunks freed, but it need not necessarily
 * give back all the resources the set owns.  Our actual implementation is
 * that we give back all but the "keeper" block (which we must keep, since
 * it shares a malloc chunk with the context header).  In this way, we don't
 * thrash malloc() when a context is repeatedly reset after small allocations,
 * which is typical behavior for per-tuple contexts.

-- 
Nathan Bossart
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