Re: heavily contended lwlocks with long wait queues scale badly

Hi,

On 2022-10-31 17:17:03 -0700, Zhihong Yu wrote:
> On Mon, Oct 31, 2022 at 4:51 PM Andres Freund <andres@anarazel.de> wrote:
>
> > Hi,
> >
> > On 2022-10-31 16:21:06 +0530, Bharath Rupireddy wrote:
> > > BTW, I've seen a sporadic crash (SEGV) with the patch in bg writer
> > > with the same set up [1], I'm not sure if it's really because of the
> > > patch. I'm unable to reproduce it now and unfortunately I didn't
> > > capture further details when it occurred.
> >
> > That's likely because the prototype patch I submitted in this thread missed
> > updating LWLockUpdateVar().
> >
> > Updated patch attached.
> >
> > Greetings,
> >
> > Andres Freund
> >
>
> Hi,
> Minor comment:
>
> +   uint8       lwWaiting;      /* see LWLockWaitState */
>
> Why not declare `lwWaiting` of type LWLockWaitState ?

Unfortunately C99 (*) doesn't allow to specify the width of an enum
field. With most compilers we'd end up using 4 bytes.

Greetings,

Andres Freund

(*) C++ has allowed specifying this for quite a few years now and I think C23
will support it too, but that doesn't help us at this point.