Re: Move PinBuffer and UnpinBuffer to atomics

On Mon, Mar 28, 2016 at 9:09 AM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> On 2016-03-28 11:48:46 +0530, Dilip Kumar wrote:
>> On Sun, Mar 27, 2016 at 5:48 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
>> > What's sizeof(BufferDesc) after applying these patches? It should better
>> > be <= 64...
>> >
>>
>> It is 72.
>
> Ah yes, miscalculated the required alignment. Hm. So we got to get this
> smaller. I see three approaches:
>
> 1) Reduce the spinlock size on ppc. That actually might just work by
> replacing "unsigned int" by "unsigned char"
> 2) Replace the lwlock spinlock by a bit in LWLock->state. That'd avoid
> embedding the spinlock, and actually might allow to avoid one atomic
> op in a number of cases.
> 3) Shrink the size of BufferDesc by removing buf_id; that'd bring it to
> 64byte.
>
> I'm a bit hesitant to go for 3), because it'd likely end up adding a bit
> of arithmetic to a number of places in bufmgr.c. Robert, what do you
> think?

I don't have a clear idea what's going to be better here.

--
Robert Haas
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