Re: Moving relation extension locks out of heavyweight lock manager

On Wed, May 17, 2017 at 1:30 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Sat, May 13, 2017 at 7:27 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>> On Fri, May 12, 2017 at 9:14 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>>> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>>>> On Wed, May 10, 2017 at 8:39 PM, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>>>>> ... I'd like to propose to change relation
>>>>> extension lock management so that it works using LWLock instead.
>>>
>>>> That's not a good idea because it'll make the code that executes while
>>>> holding that lock noninterruptible.
>>>
>>> Is that really a problem? We typically only hold it over one kernel call,
>>> which ought to be noninterruptible anyway.
>>
>> During parallel bulk load operations, I think we hold it over multiple
>> kernel calls.
>
> We do. Also, RelationGetNumberOfBlocks() is not necessarily only one
> kernel call, no? Nor is vm_extend.

Yeah, these functions could call more than one kernel calls while
holding extension lock.

> Also, it's not just the backend doing the filesystem operation that's
> non-interruptible, but also any waiters, right?
>
> Maybe this isn't a big problem, but it does seem to be that it would
> be better to avoid it if we can.
>

I agree to change it to be interruptible for more safety.

Regards,

--
Masahiko Sawada
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center