Re: 9.5: Better memory accounting, towards memory-bounded HashAgg

On 17.11.2014 18:04, Andres Freund wrote:
> Hi,
> 
> On 2014-11-16 23:31:51 -0800, Jeff Davis wrote:
>> *** a/src/include/nodes/memnodes.h
>> --- b/src/include/nodes/memnodes.h
>> ***************
>> *** 60,65 **** typedef struct MemoryContextData
>> --- 60,66 ----
>>       MemoryContext nextchild;    /* next child of same parent */
>>       char       *name;            /* context name (just for debugging) */
>>       bool        isReset;        /* T = no space alloced since last reset */
>> +     uint64        mem_allocated;    /* track memory allocated for this context */
>>   #ifdef USE_ASSERT_CHECKING
>>       bool        allowInCritSection;    /* allow palloc in critical section */
>>   #endif
> 
> That's quite possibly one culprit for the slowdown. Right now one 
> AllocSetContext struct fits precisely into three cachelines. After
> your change it doesn't anymore.

I'm no PPC64 expert, but I thought the cache lines are 128 B on that
platform, since at least Power6?

Also, if I'm counting right, the MemoryContextData structure is 56B
without the 'mem_allocated' field (and without the allowInCritSection),
and 64B with it (at that particular place). sizeof() seems to confirm
that. (But I'm on x86, so maybe the alignment on PPC64 is different?).

> Consider not counting memory in bytes but blocks and adding it
> directly after the NodeTag. That'd leave the size the same as right
> now.

I suppose you mean "kbytes", because the block size is not fixed.

regards
Tomas