Re: POC: Cache data in GetSnapshotData()

On Fri, Jan 15, 2016 at 11:23 AM, Mithun Cy <mithun(dot)cy(at)enterprisedb(dot)com>
wrote:

> On Mon, Jan 4, 2016 at 2:28 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
>
> > I think at the very least the cache should be protected by a separate
> > lock, and that lock should be acquired with TryLock. I.e. the cache is
> > updated opportunistically. I'd go for an lwlock in the first iteration.
>
> I tried to implement a simple patch which protect the cache. Of all the
> backend which
> compute the snapshot(when cache is invalid) only one of them will write to
> cache.
> This is done with one atomic compare and swap operation.
>
> After above fix memory corruption is not visible. But I see some more
> failures at higher client sessions(128 it is easily reproducible).
>
>
Don't you think we need to update the snapshot fields like count,
subcount before saving it to shared memory?

With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com