Re: LWLocks in DSM memory

On Thu, Jul 28, 2016 at 12:12 AM, Thomas Munro
<thomas.munro@enterprisedb.com> wrote:
> On Tue, Jul 26, 2016 at 6:00 AM, Andres Freund <andres@anarazel.de> wrote:
>> I think the better fix here would acutally be to get rid of a pointer
>> based list here, and a) replace the queue with integer offsets ...
>
> Here is a prototype of that idea.  It replaces that dlist with a
> proclist, a new specialised doubly-linked list for pgprocno values,
> using INVALID_PGPROCNO for termination.  It works with proclist_node
> objects inside PGPROC at an arbitrary offset which must be provided
> when you initialise the proclist.

Aside from the fact that this allows LWLocks inside DSM segments,
which I definitely want to support, this seems to have the nice
property of reducing the size of an LWLock by 8 bytes.  We need to
consider what to do about LWLOCK_MINIMAL_SIZE.  We could (a) decide to
still pad all arrays of LWLocks to 32 bytes per LWLock so as to reduce
false sharing, and rename this constant not to imply minimality; or
(b) alter the macro definition to allow for 16 bytes as a possible
result.

-- 
Robert Haas
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