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

From: Tomas Vondra <tv(at)fuzzy(dot)cz>
To: pgsql-hackers(at)postgresql(dot)org
Subject: Re: 9.5: Better memory accounting, towards memory-bounded HashAgg
Date: 2014-11-17 18:42:25
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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 */
>> 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.


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