Re: [COMMITTERS] pgsql: Move each SLRU's lwlocks to a separate tranche.

On March 25, 2016 1:04:13 PM GMT+01:00, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>On Fri, Mar 25, 2016 at 3:05 AM, Andres Freund <andres(at)anarazel(dot)de>
>wrote:
>> On 2015-11-12 19:59:54 +0000, Robert Haas wrote:
>>> Move each SLRU's lwlocks to a separate tranche.
>>>
>>> This makes it significantly easier to identify these lwlocks in
>>> LWLOCK_STATS or Trace_lwlocks output. It's also arguably better
>>> from a modularity standpoint, since lwlock.c no longer needs to
>>> know anything about the LWLock needs of the higher-level SLRU
>>> facility.
>>>
>>> Ildus Kurbangaliev, reviewd by Álvaro Herrera and by me.
>>
>> Before this commit the lwlocks were cacheline aligned, but that's not
>> the case anymore afterwards; afaics. I think that should be fixed? I
>> guess it'd be good to avoid duplicating the code for aligning, so
>maybe
>> we ought to add a ShmemAllocAligned or something?
>
>Does it actually matter? I wouldn't have thought the I/O locks had
>enough traffic for it to make any difference.
>
>But in any case I think the right solution is probably this:
>
>--- a/src/backend/storage/ipc/shmem.c
>+++ b/src/backend/storage/ipc/shmem.c
>@@ -173,7 +173,7 @@ ShmemAlloc(Size size)
> /*
> * ensure all space is adequately aligned.
> */
>- size = MAXALIGN(size);
>+ size = CACHELINEALIGN(size);
>
> Assert(ShmemSegHdr != NULL);
>
>It's stupid that we keep spending time and energy figuring out which
>shared memory data structures require alignment and which ones don't.
>Let's just align them *all* and be done with it. The memory cost
>shouldn't be more than a few kB.

Last time I proposed that it got shut down. I agree it'd be a good idea, it's really hard to find alignment issues.

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