Re: visibility maps

On Thu, Dec 11, 2008 at 5:01 PM, Zdenek Kotala <Zdenek.Kotala@sun.com> wrote:
>
>>
>
> IIRC, Memory reading/writing is atomic operation. Only one CPU(hw thread)
> can access to the same memory address(es)* in same time*. The question is
> how compiler compile C code to assembler.  But this code seems to me safe.

Yeah, I think the code is safe because we are just reading a bit.

BTW, I wonder if we need to acquire EXCLUSIVE lock while writing the
visibility map bit ? Since almost (8 * 8192) data blocks would map to
the same visibility map page, the lock can certainly become a hot
spot. I know we also update PageLSN during the set operation and that
would require EXLUSIVE lock, but is that required for consistency
given that the entire visibility map is just a hint ?

Thanks,
Pavan


-- 
Pavan Deolasee
EnterpriseDB     http://www.enterprisedb.com