Re: heavily contended lwlocks with long wait queues scale badly

On Mon, Oct 31, 2022 at 5:19 PM Andres Freund <andres(at)anarazel(dot)de> wrote:

> Hi,
>
> On 2022-10-31 17:17:03 -0700, Zhihong Yu wrote:
> > On Mon, Oct 31, 2022 at 4:51 PM Andres Freund <andres(at)anarazel(dot)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.
>

Hi,
Thanks for the response.

If possible, it would be better to put your explanation in the code comment
(so that other people know the reasoning).